SEARCH

SEARCH BY CITATION

Keywords:

  • diet resistance;
  • mitochondria;
  • obesity;
  • oxidative phosphorylation;
  • proton leak;
  • uncoupling;
  • weight loss

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

Understanding the metabolic factors that contribute to obesity development and weight loss success are critical for combating obesity and obesity-related disorders. This review provides an overview of energy metabolism with a particular focus on mitochondrial function in health and in obesity. Mitochondrial proton leak contributes significantly to whole body energy expenditure and the potential role of energy uncoupling in weight loss success is discussed. We provide evidence to support the hypothesis that differences in energy efficiency are important regulators of body weight and weight loss success.


Abbreviations
BAT

brown adipose tissue

BMR

basal metabolic rate

FA

fatty acid

IMF

intermyofibrilar

ODR

obese diet resistant

ODS

obese diet sensitive

OP1

optic atrophy 1

RMR

resting metabolic rate

ROS

reactive oxygen species

SNS

sympathetic nervous system

SS

subsarcolemmal

T2DM

type 2 diabetes mellitus

TDEE

total daily energy expenditure

UCP

uncoupling protein

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

Obesity is a complex multifactorial disease and is a major risk factor for other chronic diseases, including type 2 diabetes mellitus (T2DM), cardiovascular disease, and cancer [1, 2]. Obesity has reached epidemic proportions in many parts of the world; its prevalence is highest in the Americas, where it is estimated that 62% of adults are overweight or obese [3]. Understanding the myriad of biological and environmental factors that contribute to obesity is paramount for the development of personalized preventative and treatment strategies.

The development of obesity is a complex issue involving the interaction of numerous environmental and biological factors. Caloric intake and energy expenditure, primarily through physical activity, are considered to be major environmental determinants in the development of obesity. Although not discussed in detail in the present review, other environmental factors that have been associated with obesity development include (but are not limited to) socioeconomic status, education, stress and poor sleeping habits. Genetic predisposition and other biological factors such as thyroid status, which is known to alter energy efficiency, may predispose individuals to obesity and/or affect their capacity to lose weight [4-6]. An emerging area of research has also linked the gut microflora population with obesity development [7]. All of these factors, as well as others that are not listed, can contribute to energy intakes that exceed overall energy expenditure.

This review discusses the biological factors that are considered to contribute to obesity susceptibility and affect the response to therapeutic strategies. In particular, the focus is on factors that affect the efficiency of energy metabolism. There is abundant evidence that the efficiency of ATP production during fuel oxidation is variable (i.e. the amount of ATP produced per unit oxygen consumed by cells and tissues varies). We examine the relevance of this physiologic fact with respect to obesity susceptibility and inter-individual variability in weight loss during hypocaloric treatment strategies with a focus on mitochondrial function in the skeletal muscle.

Partitioning of energy expenditure at the whole body level

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

Obesity usually develops gradually, and occurs when energy intake is in excess of total body energy expenditure. Energy is expended by all cells and tissues of the body to support a wide variety of fuel-consuming processes. Before focusing on the variable efficiency with which energy is used, we briefly partition out the various components of total body energy expenditure. Total daily energy expenditure (TDEE) of adults living under fairly standard conditions can be separated broadly into three major components. The largest contributor to TDEE is basal metabolic rate (BMR), which can account for 60–75% of TDEE in a sedentary individual [8]. BMR is defined as the energy expended when an individual is in a resting and post-absorptive state in a thermoneutral environment (22–27 °C in adult humans); measurements are made in individuals resting in a supine position in the morning following sleep [9-11]. Resting metabolic rate (RMR) is much more commonly measured than BMR because it is measured in individuals at complete rest, at any time of the day, in the post-absorptive state. When measured in individuals in a supine rested state, at normal room temperature (i.e. 22–23 °C), RMR is usually within 10% of BMR [12]. BMR is dependent on many physical and biological factors. It is increased with body weight, male sex, stress, thyroid hormones, pregnancy and lactation [13-16] and is decreased with weight loss, calorie restriction, starvation and ageing [16-20]. The major determinant of BMR is fat free mass [21].

Cellular reactions that contribute to BMR include ATP-demanding processes, such as the maintenance of ion gradients across cell membranes; RNA and protein synthesis; as well as other energy demanding reactions involved in cellular turnover and repair [22]. Another important processes is mitochondrial proton leak, which, unlike the aforementioned reactions, does not consume ATP; instead, it decreases the efficiency of ATP synthesis [23]. There is evidence that mitochondrial proton leak is altered in different metabolic conditions; this is discussed below, as are the implications of proton leak to the development and treatment of obesity.

Another important contributor to TDEE is the thermic effect of food, which is the increase in metabolic rate following a meal. The increase in metabolic rate is a result of the energy used for the digestion, absorption and metabolism of dietary nutrients, as well as sympathetic nervous system activation to increase blood flow to digestive organs and to peripheral tissue to promote nutrient clearance [24-26]. Dietary protein causes the largest thermogenic response, followed by carbohydrate and then fat, which has a very low thermogenic effect. The thermogenic response to eating accounts for approximately 10% of TDEE [27]. There is evidence that the thermogenic response to food is impaired in obesity [28, 29].

The energy expended as a result of physical activity is also a major component of TDEE and is the most variable amongst individuals [12]. This includes both volitional exercise and energy expended as a result of non-exercising thermogenesis such as that expended during standing, talking, walking, fidgeting, daily living, spontaneous muscular contractions or shivering [8, 30]. There is evidence that spontaneous movement, such as fidgeting, contributes a small but significant amount to differences in TDEE [12], and this may mitigate obesity development [31]. However, a much more significant contributor to TDEE is volitional exercise.

The contributions of various tissues to overall metabolic rate have been estimated in rodents and humans [22]. The major contributors are skeletal muscle (20%), brain (20%), liver (17%), heart (11%) and gastrointestinal tract (10%). Although the metabolic rate per gram of tissue is quite low for resting skeletal muscle, it is one of the major contributors to basal metabolic rate, by virtue of its size, which is approximately 40% of adult human body mass. The brain, liver and gastrointestinal tract all account for < 2% of total body mass but are estimated to account for approximately 40% of basal metabolic rate[22, 32, 33].

Oxidative phosphorylation

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

The oxidation of dietary energy substrates, mainly carbohydrate and fatty acids, results in the production of reducing equivalents in the form of NADH + H+ and FADH2. The latter are transferred through a series of shuttles and oxidation-reduction reactions in the mitochondrial electron transport chain to the final electron acceptor: molecular oxygen. The movement of electrons through the electron transport chain is coupled to proton pumping from the mitochondrial matrix to the intermembrane space, thus generating an electrochemical gradient across the inner mitochondrial membrane, referred to as the protonmotive force. The return of protons to the matrix through the ATP-synthase complex drives ATP production. The importance of these reactions is obvious when considering the fact that most adults produce approximately 65 kg of ATP per day [34].

Mitochondrial proton leak

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

At such a high volume of ATP synthesis at the whole body level, it is easy to conceive of the potential impact of even moderate uncoupling of mitochondrial proton return from ATP synthesis; uncoupling of oxidative phosphorylation essentially means that dietary energy is not captured as ATP. Mitochondrial energy metabolism is considered as coupled or efficient when all or most of the oxygen consumed by mitochondria is associated with ATP production. Coupling efficiency approaches 90% when ADP is present, as a result of rapid ATP synthesis [35, 36]. Coupling efficiency varies across different cell types and has been estimated to be 75–80% in rats [37]. Uncoupled respiration is the situation in which oxygen is consumed in the absence of ATP production. The respiration that occurs when ATP synthesis is not occurring is referred to as ‘state 4 respiration’, a metabolic state in which protons leak back across the mitochondrial inner membrane to the matrix, bypassing ATP synthase. In such uncoupled states, energy substrates [e.g. fatty acids (FA)] are oxidized, and oxygen is consumed but no ATP is synthesized. In perfused rat skeletal muscle, proton leak has been estimated to account for approximately 52% of resting oxygen consumption [33]. Considering that skeletal muscle accounts for upwards of 30% of RMR in rats [38], skeletal muscle proton leak likely contributes considerably to TDEE. Indeed, it is estimated that proton leak accounts to 20–30% of BMR in rats [22, 33, 37].

There are considered to be two classes of mitochondrial proton leak: basal and inducible. Basal proton leak occurs in mitochondria of all tissues and is a process that does not appear to be acutely controlled by specific factors. It is associated with adenine nucleotide translocase [39], and the novel uncoupling proteins (UCP1–3) [36]. Although the physiological purpose of basal uncoupling is unknown, it has been hypothesized that a basal level of electron movement through the electron transport chain facilitates movement to higher energy state in times of energy demand by increasing sensitivity and decreasing the response time to changes in cellular ATP utilization [22]. Basal proton leak may also function to mitigate oxidative stress [22].

Inducible proton leak is catalyzed by the uncoupling proteins UCP1, UCP2, and UCP3 and can be acutely controlled [36]; adenine nucleotide translocase may also be acutely induced to perform proton leak reactions [40, 41]. UCP1 is expressed exclusively in brown adipose tissue and is considered to be the classic uncoupling protein. UCP1 is activated in response to sympathetic nervous system (SNS) activation and is necessary for brown adipose tissue (BAT) thermogenesis. In response to SNS stimulation of brown adipocytes, fatty acids are considered to relieve purine nucleotide inhibition of UCP1, thereby activating UCP1 proton leak [42]. UCP1 is an abundant mitochondrial protein (approximately 10%) and BAT is dense with mitochondria [42]. In rodent models, UCP1 activation can increase RMR up to four-fold [43] and protects against diet-induced obesity [44]. In humans, BAT is present in infants [45] and functions to maintain body temperature [46]. Recent evidence has unequivocally demonstrated that UCP1-expressing BAT is present in adults and is activated with cold exposure [47]. Indeed, BAT, identified based on histological staining of mitochondrial enzymes and multilocular tissue, had previously been detected in outdoor workers in cold climates [48].

UCP2 is expressed widely in tissues, including the stomach, thymus, spleen, macrophages, hypothalamus and pancreatic β cells. UCP3 is primarily expressed in skeletal muscle but is also found in BAT and sometimes in heart [49]. The role and mechanism of activation of UCP2 and UCP3 have been extensively debated ever since their discovery in the late 1990s. Evidence from the Brand laboratory first demonstrated that UCP1–3 are activated in the presence of reactive oxygen species (ROS). This elicited the ‘uncoupling to survive hypothesis’, which holds that proton leak acts to decrease excessive mitochondrial superoxide production to prevent oxidative damage [50]. Whether or not this is indeed the physiological functions of UCP2 and UCP3 are still under investigation. It has been noted that macrophages of UCP2−/− mice have increased ROS production and mice are protected from parasitic infection compared to wild-type mice [51]. Moreover, muscle mitochondria of UCP3−/− mice produce more ROS and have more evidence of oxidative damage compared to wild-type [52, 53] and UCP3 overexpression lowers age-associated increases in mitochondrial ROS emission [54]. UCP3 transgenic mice are less metabolically efficient [55, 56] and also are protected against diet-induced obesity [55-57] and high-fat diet-induced insulin resistance [58]. Accordingly, UCP1 and UCP3 protein expression is higher in the BAT and skeletal muscle mitochondria of obesity-resistant compared to obesity-prone mice [59]. UCP3−/− mice are not obese, although they demonstrate higher levels of oxidative stress compared to their wild-type littermates [52, 53]. After 8 and 26 weeks of high-fat feeding, UCP3−/− mice demonstrate similar body weights and mitochondrial respiration but higher ROS production compared to wild-type mice [60].

Several polymorphisms in the UCP1–3 genes identified in humans have been associated with obesity phenotypes, increased weight gain, weight loss resistance and T2DM [61-64] and this has been reviewed extensively [65, 66]. The most commonly studied UCP1 polymorphisms include the A1766G and A-112C at the 5′ flanking region, A3826G SNP in the promoter region and the A64T polymorphism in exon 2 [65]. Although controversial, the A3826G polymorphism is the most widely studied and has been associated with obesity, weight gain and reduced weight loss success [62-65]. In the UCP2 gene, polymorphisms at the 866 position in the promoter region have been associated with obesity and T2DM risk [66]. Carriers of the –866A allele demonstrate modest reductions in obesity rates [67] but a decreased insulin response to glucose and thus increased T2DM risk [68, 69]. Carriers of the –866G allele demonstrate a higher body mass index and fat mass and an increased risk for obesity [66, 67]. The UCP3 gene polymorphism –55T allele has been associated with higher rates of UCP3 mRNA expression [70] and resting energy expenditure [71], whereas heterozygosity (C/T) is associated with decreased obesity [72] and T2DM risk [73].

Physiological factors affecting mitochondrial proton leak

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

The most widely recognized endocrine factors affecting mitochondrial proton leak are the thyroid hormones. Thyroid hormones are also the major endocrine regulators of BMR. In hypothyroidism or following thyroidectomy, BMR is decreased, whereas it is increased in hyperthyroidism [74, 75]. Most of the thyroid hormone effects mitochondrial respiration are mediated by genomic mechanisms. Thyroid hormone receptors have been located in the nucleus and mitochondria in various tissues [76-78]. Major effects of thyroid hormones on proton leak and energy metabolism appear to be related to mitochondrial biogenesis. Specifically, thyroid hormones increase the expression of proteins regulating mitochondrial biogenesis, namely peroxisome-proliferator-activated receptor γ coactivator α and nuclear respiratory factor-1 [79-81]. Thyroid hormones also increase oxidative phosphorylation gene expression [81] and modulate mitochondrial respiration [82]. Mitochondrial respiration is increased and decreased in mitochondria and hepatocytes isolated from hyper- and hypothyroid rats, respectively, and approximately 50% of these differences in respiration occur as a result of changes in proton leak [83-87]. Finally, in situations of elevated thyroid hormones, resting energy expenditure, substrate oxidation and skeletal muscle mitochondrial proton leak are all increased [88].

Short- and long-term calorie restrictions have consistently been shown to decrease proton leak in rodent liver and skeletal muscle [89-92]. However, there is less consensus on the effects of high-fat feeding on mitochondrial coupling in response to a high-fat diet; the effects may depend on the tissue studied or the model used. High-fat feeding has been shown to decrease [93] or not change [94] mitochondrial coupling in isolated mitochondria from liver of high-fat fed rats. Uncoupling protein expression and mitochondrial proton leak were assessed in isolated mitochondria of BAT and hindlimb muscle in obesity-prone and obesity-resistant mice [59]. In that study, UCP1 and UCP3 expression and proton leak were higher in BAT of obesity-resistant compared to obesity-prone mice on a normal chow diet. Interestingly, when challenged with a high fat diet, BAT proton leak was increased in obesity-prone mice to levels comparable with obesity-resistant mice. In hindlimb muscle, UCP1 but not UCP3 protein expression is higher in obesity-resistant mice. This was previously reported to be the result of BAT tissue in the hindlimb of this strain of mouse (129 S6/SvEvTac strain) [95]. Despite this, there was no difference in skeletal muscle proton leak between the two groups when on a control or high-fat diet [59]. Mitochondrial proton leak was also reduced in the skeletal muscle of individuals with T2DM following the initiation of intensive insulin therapy, which is consistent with improved mitochondrial efficiency [96]. In situations of acute substrate oversupply, mitochondrial uncoupling was increased in saponin permeabilized fibres from hearts perfused with glucose and fatty acids [97].

Exercise training is known to increase mitochondrial oxidative capacity and mitochondrial content; however, the effect of exercise training on muscle mitochondrial coupling efficiency is not completely understood, with studies showing it to be both increased [98, 99] and decreased [100] in active individuals. Recent research demonstrates that mitochondrial content and ATP flux are reduced in sedentary compared to active individuals, despite similar rates of O2 flux, which is consistent with the conclusion that there is an increased uncoupled respiration in sedentary individuals [99].

In summary, mitochondrial proton leak contributes significantly to energy expenditure and is altered under a number of physiological conditions. It is thus feasible that alterations in mitochondrial proton leak could contribute to obesity development or weight loss success.

Energy balance in response to weight gain or loss

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

Resting metabolic rate is higher in obesity, primarily as a result of an increase in body mass. When weight loss occurs, resting energy expenditure is decreased [19, 101-103] and this persists beyond the weight reduction phase [19]. Equations have been developed to estimate energy expenditure based on that measured in healthy individuals. These equations typically use sex, age, height and weight to estimate energy expenditure [104-106]. Although controversial, resting energy expenditure [18, 102, 103, 107, 108] and exercise energy expenditure [104] in response to weight loss have been shown to be less than predicted. The reduction in resting energy expenditure following weight loss has often been attributed to a loss in lean mass. However, a recent study showed that RMR and TDEE were still significantly reduced to values that were lower than predicted in individuals who had predominantly maintained lean mass after drastic weight reduction following a demanding combination of diet and exercise [103].

It has been suggested that reductions in RMR and TDEE in response to weight loss may be a result of adaptive mechanisms to protect body mass [109]. Such adaptive mechanisms are complex and poorly understood but may include hormonal, SNS signalling and behavioural changes. Increased appetite and food intake [110] have been observed in response to decreased energy intake. Increased food cravings [111], calories to reach satiation and meal size [112, 113] have been reported following weight loss. Following weight reduction, increased neural activity in reward centres of the brain and in areas responsible for processing of food-related stimuli and decreased activity in areas of the brain responsible for food restrain have been observed in humans [114]. Interestingly, post-weight loss leptin administration largely reverses the weight loss-induced changes in neural activity [114, 115], and increases post-meal satiation [112]. Non-exercise activity thermogenesis is also decreased and increased in response to energy restriction and overfeeding, respectively [31, 107], and may also be a mechanism to protect body mass.

Circulating hormones are also altered in situations of energy imbalance and may play an important role in weight regulation. For example, circulating thyroid hormones and 24-h urine catecholamine output are increased and decreased following weight gain and loss, respectively, and are associated with changes in resting energy expenditure [116]. Decreases in circulating leptin have been associated with weight loss-induced decreases in energy expenditure and whole body fat oxidation [117], and may contribute to weight regain post-weight loss [118]. Restoring circulating leptin levels to pre-weight loss values with leptin administration prevents weight loss-induced reductions in energy expenditure and circulating thyroid hormones, and also restores sympathetic tone [112, 114, 118].

Hormonal and metabolic changes that occur in obesity and weight loss can also affect mitochondrial function and efficiency, which would contribute to changes in energy expenditure. Indeed, diet-induced weight loss has been shown to reduce mitochondrial oxidative capacity as measured in permeabilized muscle fibres, despite no changes in mitochondrial content [119]. In summary, it appears that there are numerous physiological, metabolic and behavioural adaptations that occur, in an attempt to protect body weight, during energy imbalance that would ultimately affect weight loss success.

The biological basis of weight loss variability

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

There is considerable evidence demonstrating variability in response to positive or negative energy balance. For example, the biological basis for weight gain variability has been demonstrated in highly controlled, overfeeding studies in monozygotic twins. This research demonstrated a greater between-pair variability than within-pairs with respect to weight gain and changes in RMR and fat mass gain [4].

Research conducted by our group has investigated the metabolic and genetic basis for weight loss variability in humans. The weight loss programme at the Ottawa Hospital Weight Management Clinic is a highly regimented meal replacement weight loss programme. Individuals enrolled in the programme consume 900 kcal·day−1 in the form of a meal replacement for 6 or 12 weeks. Meal replacement is accompanied by weekly visits with physicians and information sessions. Food is slowly re-introduced at 12 weeks. In highly-adherent subjects, we have observed a 10-fold difference in weight loss during the first 6 weeks of weight loss. When factors known to effect metabolic rate are taken into account (age, weight, sex, thyroid hormone status), a two-fold difference in weight loss remains unexplained [120]. Metabolic and genomic studies have been conducted in patients in the upper quintile for rate of weight loss (obese diet sensitive; ODS) and lower quintile for rate of weight loss (obese diet resistant; ODR) (Fig. 1). To address the impact of possible differences in physical activity between groups, planned physical activity and work-related activity were compared and found not to be different between the ODS and ODR groups [121]. This suggests that there may be biological factors contributing to the observed differences in rate of weight loss. Indeed, in isolated mitochondria of rectus femoris, proton leak was 51% higher in ODS compared to ODR. Considering that muscle energy expenditure accounts for approximately 30% of RMR in humans [22], the observed differences in muscle mitochondrial proton leak could reflect a 5% difference in RMR between ODS and ODR individuals [6]. Further investigations revealed distinct differences in the skeletal muscle of ODS and ODR individuals. Skeletal muscle (vastus lateralis) from ODS has more type I oxidative muscle fibers and exhibited fiber hypertrophy compared to ODR individuals, whereas type IIa (oxidative-glycolytic) fibres were higher in ODR than ODS individuals and IIx fibers were higher in ODR compared to lean individuals [121]. Type IIx fiber expression has also been shown to correlate with insulin resistance in obesity and T2DM [122]. Overfeeding studies in monozygotic twins have demonstrated that those individuals with higher proportions of type I and lower type IIa fibers gain less fat mass in response to overfeeding [123], which follows a pattern that is similar to that observed in the impaired weight loss response in our studies [121].

image

Figure 1. Rate of weight loss in ODS and ODR women. Weight loss data are shown for the time of entry into the weight management programme (week 0) and over the subsequent 32 weeks. The dashed line indicates the point at which rate of weight loss is calculated (6 weeks) (n = 12 per group). Reproduced with permission [6].

Download figure to PowerPoint

Skeletal muscle (rectus femoris) of ODS subjects also demonstrated higher mRNA expression of UCP3 and the genes involved in oxidative metabolism compared to muscle of ODR subjects [121]. Interestingly, this difference in gene expression pattern (e.g. ox phos gene transcripts) has also been observed in blood samples of ODS and ODR individuals before the weight loss programme; hence, blood-based predictor tests may be available in the future [124]. Taken together, this research suggests that the skeletal muscle of ODS individuals has a greater oxidative capacity, although oxidative phosphorylation is less efficient and this likely contributes to the observed differences in weight loss success (Fig. 2).

image

Figure 2. Schematic representation of proposed mechanism of weight loss variability. ODS individuals demonstrate a higher expression of oxidative phosphorylation genes, type I oxidative muscle fiber and increased levels of proton leak compared to ODR individuals. Collectively, this results in higher rates of weight loss success [6, 121]. ANT, adenine nucleotide translocase.

Download figure to PowerPoint

Mitochondrial dysfunction in obesity and T2DM

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

Numerous studies have demonstrated that skeletal muscle metabolism and mitochondrial function are impaired in obesity. Specifically, skeletal muscle from obese humans is characterized by increased FA uptake, lipid accumulation and oxidative stress [125-128]. The accumulation of reactive lipid species, namely diacyglycerol and ceramide, are associated with impaired insulin sensitivity in this tissue [129-132]. Many studies have also demonstrated that FA oxidation is reduced in obesity and T2DM [133-136], which may be a contributing factor in lipid accumulation and oxidative stress. Other studies, however, have demonstrated that FA oxidation is either moderately increased or not different compared to lean controls in rodents and humans [137-139]. The observed differences in mitochondrial oxidation may be a result of differences in mitochondrial content [137, 140]. Mitochondrial content has been shown to be reduced in the skeletal muscle of obese individuals and individuals with T2DM compared to lean controls [141]. This suggests that mitochondrial dysfunction may in part be the result of a reduced mitochondrial mass. In accordance with this, the levels of mitochondrial proteins and their (predominantly nuclear) genes are reduced in the skeletal muscle in obesity and T2DM [142, 143] and also in individuals with an impaired weight loss response [121, 124]. Some studies have demonstrated that FA oxidation is incomplete in the skeletal muscle of obese rodents and humans and following a high-fat diet [144-146]. Incomplete FA oxidation has been observed in obesity [144] and can lead to the accumulation of by products of metabolism, namely acylcarnitines. Acylcarnitines have been proposed to cause mitochondrial dysfunction and insulin resistance [145, 146]; however, the metabolic effects and mechanisms of action of these metabolites remain unknown.

In skeletal muscle, mitochondria form a reticulum and are categorized into two distinct compartments: subsarcolemmal (SS) and intermyofibrilar (IMF). SS mitochondria are predominantly responsible for energy demands at the cell surface, such as substrate transport, ion exchange and cellular signalling, whereas IMF are important for muscle contraction [147, 148]. Studies have demonstrated that SS and IMF mitochondria respond differently to various metabolic perturbations and there may be a different pattern of response in rodents compared to humans [141, 149-151]. In obese Zucker rats, FA oxidation and mitochondrial content (number, width, density) are increased in SS but not IMF mitochondria compared to lean controls [151]. By contrast to rodent models, IMF but not SS mitochondrial content is decreased in obese insulin-resistant and obese T2DM individuals compared to lean insulin-sensitive individuals. IMF mitochondrial content was also positively associated with insulin sensitivity [150]. The electron transport chain activity is also reduced in SS and IMF mitochondria of obese individuals with T2DM compared to lean controls [141]. It is worth noting that physical activity levels were reportedly higher in the lean population, which may impact upon the observed findings [141]. In summary, the role of subcellular locations of mitochondria in mitochondrial dysfunction in humans under different metabolic states (obesity, diabetes, insulin resistance, weight loss) is not completely understood and warrants further investigation.

As discussed above, fiber type may play an important role in skeletal muscle function and weight loss success in obesity. Type IIx glycolytic fibre expression is higher in the skeletal muscle of diabetic individuals [122] and type IIa fibre expression is positively correlated with the weight gain response to overfeeding [123]. Type II fibres have a reduced capacity to oxidize fat [152] and may also have a a reduced capacity to deal with oxidative stress [153], thus potentially contributing to the increases oxidative stress observed in obesity and T2DM.

Changes in mitochondrial morphology have also been observed in obesity and T2DM. Higher rates of mitochondrial fission have been implicated with diabetic neuropathy [154, 155]. Shorter, rounder mitochondria and increases in mitochondrial fission proteins dynamin-related protein 1 and fission protein 1 have been observed in the skeletal muscle of ob/ob and high-fat fed mice, and palmitate-treated C2C12 cells [156]. The fusion protein, mitofusin-1 is reduced in the skeletal muscle of obese rodents and humans, and this is associated with smaller mitochondria and a fragmented mitochondrial network [157]. Lower levels of the fusion proteins mitofusin 1 and optic atrophy 1 have also been observed in the skeletal muscle of individuals with T2DM [158].

Exercise, weight loss maintenance and mitochondrial function

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

Exercise training has been shown to improve mitochondrial function and content in obesity and T2DM [159-163]. Exercise training stimulates mitochondrial biogenesis resulting in increased mitochondrial size and content [159] and increases the expression of proteins involved in oxidative phosphorylation and FA utilization [164-166]. In addition, exercise training increases FA oxidation, promotes more complete FA oxidation and increases oxidative phosphorylation [100, 145, 167]. Diet-induced weight loss decreases skeletal muscle oxidative metabolism [119] and exercise may prevent this [160, 167, 168]. In addition, weight loss from diet and exercise demonstrates increased mitochondrial density and capacity (NADH-oxidase activity) compared to diet alone [169]. In addition to the beneficial effects of exercise-induced weight loss on mitochondrial function and insulin sensitivity, there is evidence to suggest that regular exercise is important for weight loss maintenance [170, 171].

In summary, the mechanism of the development of muscle mitochondrial dysfunction in obesity and T2DM is as yet poorly understood. Mitochondrial dysfunction combined with decreases in mitochondrial content apparently increases the risk for the development of obesity and obesity-related disease (e.g. muscle insulin resistance). However, the latter is the subject of continuing debate [172, 173].

Concluding remarks

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References

Energy intake and expenditure, and thus body weight, are regulated by a complex interplay between of physiological and behavioural factors, which are only beginning to be understood. Research suggests that mitochondrial function and/or content is altered in obesity, and likely contributes to the development of the disease. It appears that mitochondrial efficiency is an important factor in human energy expenditure and body weight regulation, with higher rates of muscle mitochondrial proton leak in those individuals who lose weight much more quickly than others. However, more research is needed to determine the contribution of mitochondrial efficiency to body weight regulation and energy expenditure. Understanding mitochondrial dysfunction associated with obesity and weight loss success is important for developing appropriate prevention and treatment strategies.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Partitioning of energy expenditure at the whole body level
  5. Oxidative phosphorylation
  6. Mitochondrial proton leak
  7. Physiological factors affecting mitochondrial proton leak
  8. Energy balance in response to weight gain or loss
  9. The biological basis of weight loss variability
  10. Mitochondrial dysfunction in obesity and T2DM
  11. Exercise, weight loss maintenance and mitochondrial function
  12. Concluding remarks
  13. References
  • 1
    Rossen L & Rossen EA (2012) Obesity 101. Springer Publishing Co., LLC., New York.
  • 2
    Vucenik I & Stains JP (2012) Obesity and cancer risk: evidence, mechanisms, and recommendations. Ann N Y Acad Sci 1271, 3743.
  • 3
    World Health Organization (2012) World Health Statistics 2012.
  • 4
    Bouchard C, Tremblay A, Despres JP, Nadeau A, Lupien PJ, Theriault G, Dussault J, Moorjani S, Pinault S & Fournier G (1990) The response to long-term overfeeding in identical twins. N Engl J Med 322, 14771482.
  • 5
    Bouchard C, Tremblay A, Despres JP, Theriault G, Nadeau A, Lupien PJ, Moorjani S, Prudhomme D & Fournier G (1994) The response to exercise with constant energy intake in identical twins. Obes Res 2, 400410.
  • 6
    Harper ME, Dent R, Monemdjou S, Bezaire V, Van Wyck L, Wells G, Kavaslar GN, Gauthier A, Tesson F & McPherson R (2002) Decreased mitochondrial proton leak and reduced expression of uncoupling protein 3 in skeletal muscle of obese diet-resistant women. Diabetes 51, 24592466.
  • 7
    Burcelin R (2012) Regulation of metabolism: a cross talk between gut microbiota and its human host. Physiology 27, 300307.
  • 8
    Ravussin E, Lillioja S, Anderson TE, Christin L & Bogardus C (1986) Determinants of 24-hour energy expenditure in man. Methods and results using a respiratory chamber. J Clin Invest 78, 15681578.
  • 9
    Dubois EF, Ebaugh FG Jr & Hardy JD (1952) Basal heat production and elimination of thirteen normal women at temperatures from 22 degrees C. to 35 degrees C. J Nutr 48, 257293.
  • 10
    Wilkerson JE, Raven PB & Horvath SM (1972) Critical temperature of unacclimatized male Caucasians. J Appl Physiol 33, 451455.
  • 11
    Henry CJ (2005) Basal metabolic rate studies in humans: measurement and development of new equations. Public Health Nutr 8, 11331152.
  • 12
    Levine JA (2004) Nonexercise activity thermogenesis (NEAT): environment and biology. Am J Physiol Endocrinol Metab 286, E675E685.
  • 13
    Johnstone AM, Murison SD, Duncan JS, Rance KA & Speakman JR (2005) Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Am J Clin Nutr 82, 941948.
  • 14
    Harper ME & Seifert EL (2008) Thyroid hormone effects on mitochondrial energetics. Thyroid 18, 145156.
  • 15
    Butte NF, Hopkinson JM, Mehta N, Moon JK & Smith EO (1999) Adjustments in energy expenditure and substrate utilization during late pregnancy and lactation. Am J Clin Nutr 69, 299307.
  • 16
    Grande F, Anderson JT & Keys A (1958) Changes of basal metabolic rate in man in semistarvation and refeeding. J Appl Physiol 12, 230238.
  • 17
    Fukagawa NK, Bandini LG & Young JB (1990) Effect of age on body composition and resting metabolic rate. Am J Physiol 259, E233E238.
  • 18
    Leibel RL, Rosenbaum M & Hirsch J (1995) Changes in energy expenditure resulting from altered body weight. N Engl J Med 332, 621628.
  • 19
    Rosenbaum M, Hirsch J, Gallagher DA & Leibel RL (2008) Long-term persistence of adaptive thermogenesis in subjects who have maintained a reduced body weight. Am J Clin Nutr 88, 906912.
  • 20
    Martin CK, Heilbronn LK, de Jonge L, DeLany JP, Volaufova J, Anton SD, Redman LM, Smith SR & Ravussin E (2007) Effect of calorie restriction on resting metabolic rate and spontaneous physical activity. Obesity (Silver Spring) 15, 29642973.
  • 21
    Ravussin E & Bogardus C (1992) A brief overview of human energy metabolism and its relationship to essential obesity. Am J Clin Nutr 55, 242S245S.
  • 22
    Rolfe DF & Brown GC (1997) Cellular energy utilization and molecular origin of standard metabolic rate in mammals. Physiol Rev 77, 731758.
  • 23
    Harper ME, Green K & Brand MD (2008) The efficiency of cellular energy transduction and its implications for obesity. Annu Rev Nutr 28, 1333.
  • 24
    Muller AF, Fullwood L, Hawkins M & Cowley AJ (1992) The integrated response of the cardiovascular system to food. Digestion 52, 184193.
  • 25
    Sidery MB & Macdonald IA (1994) The effect of meal size on the cardiovascular responses to food ingestion. Br J Nutr 71, 835848.
  • 26
    van Baak MA (2008) Meal-induced activation of the sympathetic nervous system and its cardiovascular and thermogenic effects in man. Physiol Behav 94, 178186.
  • 27
    Tappy L (1996) Thermic effect of food and sympathetic nervous system activity in humans. Reprod Nutr Dev 36, 391397.
  • 28
    Astrup A, Andersen T, Christensen NJ, Bulow J, Madsen J, Breum L & Quaade F (1990) Impaired glucose-induced thermogenesis and arterial norepinephrine response persist after weight reduction in obese humans. Am J Clin Nutr 51, 331337.
  • 29
    Jung RT, Shetty PS, James WP, Barrand MA & Callingham BA (1979) Reduced thermogenesis in obesity. Nature 279, 322323.
  • 30
    Garland T Jr, Schutz H, Chappell MA, Keeney BK, Meek TH, Copes LE, Acosta W, Drenowatz C, Maciel RC, van Dijk G et al. (2011) The biological control of voluntary exercise, spontaneous physical activity and daily energy expenditure in relation to obesity: human and rodent perspectives. J Exp Biol 214, 206229.
  • 31
    Levine JA, Eberhardt NL & Jensen MD (1999) Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science 283, 212214.
  • 32
    Field J, Belding HS & Martin A (1939) An analysis of the relation between basal metabolism and summated tissue respiraton in the rat. J Cell Comp Physiol 14, 143155.
  • 33
    Rolfe DF & Brand MD (1996) Contribution of mitochondrial proton leak to skeletal muscle respiration and to standard metabolic rate. Am J Physiol 271, C1380C1389.
  • 34
    Rich P (2003) Chemiosmotic coupling: the cost of living. Nature 421, 583.
  • 35
    Brand MD, Harper ME & Taylor HC (1993) Control of the effective P/O ratio of oxidative phosphorylation in liver mitochondria and hepatocytes. Biochem J 291, 739748.
  • 36
    Divakaruni AS & Brand MD (2011) The regulation and physiology of mitochondrial proton leak. Physiology 26, 192205.
  • 37
    Rolfe DF, Newman JM, Buckingham JA, Clark MG & Brand MD (1999) Contribution of mitochondrial proton leak to respiration rate in working skeletal muscle and liver and to SMR. Am J Physiol 276, C692C699.
  • 38
    Zurlo F, Larson K, Bogardus C & Ravussin E (1990) Skeletal muscle metabolism is a major determinant of resting energy expenditure. J Clin Invest 86, 14231427.
  • 39
    Brand MD, Pakay JL, Ocloo A, Kokoszka J, Wallace DC, Brookes PS & Cornwall EJ (2005) The basal proton conductance of mitochondria depends on adenine nucleotide translocase content. Biochem J 392, 353362.
  • 40
    Echtay KS, Esteves TC, Pakay JL, Jekabsons MB, Lambert AJ, Portero-Otin M, Pamplona R, Vidal-Puig AJ, Wang S, Roebuck SJ et al. (2003) A signalling role for 4-hydroxy-2-nonenal in regulation of mitochondrial uncoupling. EMBO J 22, 41034110.
  • 41
    Andreyev A, Bondareva TO, Dedukhova VI, Mokhova EN, Skulachev VP & Volkov NI (1988) Carboxyatractylate inhibits the uncoupling effect of free fatty acids. FEBS Lett 226, 265269.
  • 42
    Nicholls DG & Rial E (1999) A history of the first uncoupling protein, UCP1. J Bioenerg Biomembr 31, 399406.
  • 43
    Himms-Hagen J (1970) Regulation of metabolic processes in brown adipose tissue in relation to nonshivering thermogenesis. Adv Enzyme Regul 8, 131151.
  • 44
    Li B, Nolte LA, Ju JS, Han DH, Coleman T, Holloszy JO & Semenkovich CF (2000) Skeletal muscle respiratory uncoupling prevents diet-induced obesity and insulin resistance in mice. Nat Med 6, 11151120.
  • 45
    Heaton JM (1972) The distribution of brown adipose tissue in the human. J Anat 112, 3539.
  • 46
    Cannon B & Nedergaard J (2004) Brown adipose tissue: function and physiological significance. Physiol Rev 84, 277359.
  • 47
    Virtanen KA, Lidell ME, Orava J, Heglind M, Westergren R, Niemi T, Taittonen M, Laine J, Savisto NJ, Enerback S et al. (2009) Functional brown adipose tissue in healthy adults. N Engl J Med 360, 15181525.
  • 48
    Huttunen P, Hirvonen J & Kinnula V (1981) The occurrence of brown adipose tissue in outdoor workers. Eur J Appl Physiol Occup Physiol 46, 339345.
  • 49
    Azzu V & Brand MD (2010) The on-off switches of the mitochondrial uncoupling proteins. Trends Biochem Sci 35, 298307.
  • 50
    Brand MD (2000) Uncoupling to survive? The role of mitochondrial inefficiency in ageing Exp Gerontol 35, 811820.
  • 51
    Arsenijevic D, Onuma H, Pecqueur C, Raimbault S, Manning BS, Miroux B, Couplan E, Alves-Guerra MC, Goubern M, Surwit R et al. (2000) Disruption of the uncoupling protein-2 gene in mice reveals a role in immunity and reactive oxygen species production. Nat Genet 26, 435439.
  • 52
    Vidal-Puig AJ, Grujic D, Zhang CY, Hagen T, Boss O, Ido Y, Szczepanik A, Wade J, Mootha V, Cortright R et al. (2000) Energy metabolism in uncoupling protein 3 gene knockout mice. J Biol Chem 275, 1625816266.
  • 53
    Seifert EL, Bezaire V, Estey C & Harper ME (2008) Essential role for uncoupling protein-3 in mitochondrial adaptation to fasting but not in fatty acid oxidation or fatty acid anion export. J Biol Chem 283, 2512425131.
  • 54
    Nabben M, Hoeks J, Briede JJ, Glatz JF, Moonen-Kornips E, Hesselink MK & Schrauwen P (2008) The effect of UCP3 overexpression on mitochondrial ROS production in skeletal muscle of young versus aged mice. FEBS Lett 582, 41474152.
  • 55
    Costford SR, Chaudhry SN, Salkhordeh M & Harper ME (2006) Effects of the presence, absence, and overexpression of uncoupling protein-3 on adiposity and fuel metabolism in congenic mice. Am J Physiol Endocrinol Metab 290, E1304E1312.
  • 56
    Costford SR, Seifert EL, Bezaire V & Gerrits MF, Bevilacqua L Gowing A & Harper ME (2007) The energetic implications of uncoupling protein-3 in skeletal muscle. Appl Physiol Nutr Metab 32, 884894.
  • 57
    Son C, Hosoda K, Ishihara K, Bevilacqua L, Masuzaki H, Fushiki T, Harper ME & Nakao K (2004) Reduction of diet-induced obesity in transgenic mice overexpressing uncoupling protein 3 in skeletal muscle. Diabetologia 47, 4754.
  • 58
    Costford SR, Chaudhry SN, Crawford SA, Salkhordeh M & Harper ME (2008) Long-term high-fat feeding induces greater fat storage in mice lacking UCP3. Am J Physiol Endocrinol Metab 295, E1018E1024.
  • 59
    Fink BD, Herlein JA, Almind K, Cinti S, Kahn CR & Sivitz WI (2007) Mitochondrial proton leak in obesity-resistant and obesity-prone mice. Am J Physiol Regul Integr Comp Physiol 293, R1773R1780.
  • 60
    Nabben M, Hoeks J, Moonen-Kornips E, van Beurden D, Briede JJ, Hesselink MK, Glatz JF & Schrauwen P (2011) Significance of uncoupling protein 3 in mitochondrial function upon mid- and long-term dietary high-fat exposure. FEBS Lett 585, 40104017.
  • 61
    Ukkola O, Tremblay A, Sun G, Chagnon YC & Bouchard C (2001) Genetic variation at the uncoupling protein 1, 2 and 3 loci and the response to long-term overfeeding. Eur J Clin Nutr 55, 10081015.
  • 62
    Fumeron F, Durack-Bown I, Betoulle D, Cassard-Doulcier AM, Tuzet S, Bouillaud F, Melchior JC, Ricquier D & Apfelbaum M (1996) Polymorphisms of uncoupling protein (UCP) and beta 3 adrenoreceptor genes in obese people submitted to a low calorie diet. Int J Obes Relat Metab Disord 20, 10511054.
  • 63
    Kogure A, Yoshida T, Sakane N, Umekawa T, Takakura Y & Kondo M (1998) Synergic effect of polymorphisms in uncoupling protein 1 and beta3-adrenergic receptor genes on weight loss in obese Japanese. Diabetologia 41, 1399.
  • 64
    Oppert JM, Vohl MC, Chagnon M, Dionne FT, Cassard-Doulcier AM, Ricquier D, Perusse L & Bouchard C (1994) DNA polymorphism in the uncoupling protein (UCP) gene and human body fat. Int J Obes Relat Metab Disord 18, 526531.
  • 65
    Jia JJ, Tian YB, Cao ZH, Tao LL, Zhang X, Gao SZ, Ge CR, Lin QY & Jois M (2010) The polymorphisms of UCP1 genes associated with fat metabolism, obesity and diabetes. Mol Biol Rep 37, 15131522.
  • 66
    Jia JJ, Zhang X, Ge CR & Jois M (2009) The polymorphisms of UCP2 and UCP3 genes associated with fat metabolism, obesity and diabetes. Obes Rev 10, 519526.
  • 67
    Esterbauer H, Schneitler C, Oberkofler H, Ebenbichler C, Paulweber B, Sandhofer F, Ladurner G, Hell E, Strosberg AD, Patsch JR et al. (2001) A common polymorphism in the promoter of UCP2 is associated with decreased risk of obesity in middle-aged humans. Nat Genet 28, 178183.
  • 68
    Krempler F, Esterbauer H, Weitgasser R, Ebenbichler C, Patsch JR, Miller K, Xie M, Linnemayr V, Oberkofler H & Patsch W (2002) A functional polymorphism in the promoter of UCP2 enhances obesity risk but reduces type 2 diabetes risk in obese middle-aged humans. Diabetes 51, 33313335.
  • 69
    Gable DR, Stephens JW, Cooper JA, Miller GJ & Humphries SE (2006) Variation in the UCP2-UCP3 gene cluster predicts the development of type 2 diabetes in healthy middle-aged men. Diabetes 55, 15041511.
  • 70
    Schrauwen P, Xia J, Walder K, Snitker S & Ravussin E (1999) A novel polymorphism in the proximal UCP3 promoter region: effect on skeletal muscle UCP3 mRNA expression and obesity in male non-diabetic Pima Indians. Int J Obes Relat Metab Disord 23, 12421245.
  • 71
    Kimm SY, Glynn NW, Aston CE, Damcott CM, Poehlman ET, Daniels SR & Ferrell RE (2002) Racial differences in the relation between uncoupling protein genes and resting energy expenditure. Am J Clin Nutr 75, 714719.
  • 72
    Lindholm E, Klannemark M, Agardh E, Groop L & Agardh CD (2004) Putative role of polymorphisms in UCP1-3 genes for diabetic nephropathy. J Diabetes Complications 18, 103107.
  • 73
    Bulotta A, Ludovico O, Coco A, Di Paola R, Quattrone A, Carella M, Pellegrini F, Prudente S & Trischitta V (2005) The common -866G/A polymorphism in the promoter region of the UCP-2 gene is associated with reduced risk of type 2 diabetes in Caucasians from Italy. J Clin Endocrinol Metab 90, 11761180.
  • 74
    DuBois E (1936) Basal Metabolism in Health and Disease. Lea and Febiger, Philadelphia.
  • 75
    Tomasi TE (1991) Utilization rates of thyroid hormones in mammals. Comp Biochem Physiol A Comp Physiol 100, 503516.
  • 76
    Ardail D, Lerme F, Puymirat J & Morel G (1993) Evidence for the presence of alpha and beta-related T3 receptors in rat liver mitochondria. Eur J Cell Biol 62, 105113.
  • 77
    Wrutniak C, Cassar-Malek I, Marchal S, Rascle A, Heusser S, Keller JM, Flechon J, Dauca M, Samarut J, Ghysdael J et al. (1995) A 43-kDa protein related to c-Erb A alpha 1 is located in the mitochondrial matrix of rat liver. J Biol Chem 270, 1634716354.
  • 78
    Morrish F, Buroker NE, Ge M, Ning XH, Lopez-Guisa J, Hockenbery D & Portman MA (2006) Thyroid hormone receptor isoforms localize to cardiac mitochondrial matrix with potential for binding to receptor elements on mtDNA. Mitochondrion 6, 143148.
  • 79
    Weitzel JM, Iwen KA & Seitz HJ (2003) Regulation of mitochondrial biogenesis by thyroid hormone. Exp Physiol 88, 121128.
  • 80
    Irrcher I, Adhihetty PJ, Sheehan T, Joseph AM & Hood DA (2003) PPARgamma coactivator-1alpha expression during thyroid hormone- and contractile activity-induced mitochondrial adaptations. Am J Physiol Cell Physiol 284, C1669C1677.
  • 81
    Scheller K & Sekeris CE (2003) The effects of steroid hormones on the transcription of genes encoding enzymes of oxidative phosphorylation. Exp Physiol 88, 129140.
  • 82
    Seitz HJ, Muller MJ & Soboll S (1985) Rapid thyroid-hormone effect on mitochondrial and cytosolic ATP/ADP ratios in the intact liver cell. Biochem J 227, 149153.
  • 83
    Rebuffe-Scrive M, Anderson B, Olbe L & Bjorntorp P (1990) Metabolism of adipose tissue in intraabdominal depots in severely obese men and women. Metabolism 39, 10211025.
  • 84
    Hafner RP & Brand MD (1988) Hypothyroidism in rats does not lower mitochondrial ADP/O and H+/O ratios. Biochem J 250, 477484.
  • 85
    Harper ME & Brand MD (1994) Hyperthyroidism stimulates mitochondrial proton leak and ATP turnover in rat hepatocytes but does not change the overall kinetics of substrate oxidation reactions. Can J Physiol Pharmacol 72, 899908.
  • 86
    Harper ME & Brand MD (1993) The quantitative contributions of mitochondrial proton leak and ATP turnover reactions to the changed respiration rates of hepatocytes from rats of different thyroid status. J Biol Chem 268, 1485014860.
  • 87
    Nobes CD, Brown GC, Olive PN & Brand MD (1990) Non-ohmic proton conductance of the mitochondrial inner membrane in hepatocytes. J Biol Chem 265, 1290312909.
  • 88
    Mitchell CS, Savage DB, Dufour S, Schoenmakers N, Murgatroyd P, Befroy D, Halsall D, Northcott S, Raymond-Barker P, Curran S et al. (2010) Resistance to thyroid hormone is associated with raised energy expenditure, muscle mitochondrial uncoupling, and hyperphagia. J Clin Invest 120, 13451354.
  • 89
    Hagopian K, Harper ME, Ram JJ, Humble SJ, Weindruch R & Ramsey JJ (2005) Long-term calorie restriction reduces proton leak and hydrogen peroxide production in liver mitochondria. Am J Physiol Endocrinol Metab 288, E674E684.
  • 90
    Bevilacqua L, Ramsey JJ, Hagopian K, Weindruch R & Harper ME (2005) Long-term caloric restriction increases UCP3 content but decreases proton leak and reactive oxygen species production in rat skeletal muscle mitochondria. Am J Physiol Endocrinol Metab 289, E429E438.
  • 91
    Bevilacqua L, Ramsey JJ, Hagopian K, Weindruch R & Harper ME (2004) Effects of short- and medium-term calorie restriction on muscle mitochondrial proton leak and reactive oxygen species production. Am J Physiol Endocrinol Metab 286, E852E861.
  • 92
    Asami DK, McDonald RB, Hagopian K, Horwitz BA, Warman D, Hsiao A, Warden C & Ramsey JJ (2008) Effect of aging, caloric restriction, and uncoupling protein 3 (UCP3) on mitochondrial proton leak in mice. Exp Gerontol 43, 10691076.
  • 93
    Raffaella C, Francesca B, Italia F, Marina P, Giovanna L & Susanna I (2008) Alterations in hepatic mitochondrial compartment in a model of obesity and insulin resistance. Obesity (Silver Spring) 16, 958964.
  • 94
    Ciapaite J, Bakker SJ, Van Eikenhorst G, Wagner MJ, Teerlink T, Schalkwijk CG, Fodor M, Ouwens DM, Diamant M, Heine RJ et al. (2007) Functioning of oxidative phosphorylation in liver mitochondria of high-fat diet fed rats. Biochim Biophys Acta 1772, 307316.
  • 95
    Almind K, Manieri M, Sivitz WI, Cinti S & Kahn CR (2007) Ectopic brown adipose tissue in muscle provides a mechanism for differences in risk of metabolic syndrome in mice. Proc Natl Acad Sci USA 104, 23662371.
  • 96
    Rabol R, Hojberg PM, Almdal T, Boushel R, Haugaard SB, Madsbad S & Dela F (2009) Improved glycaemic control decreases inner mitochondrial membrane leak in type 2 diabetes. Diabetes Obes Metab 11, 355360.
  • 97
    Boudina S, Sena S, Theobald H, Sheng X, Wright JJ, Hu XX, Aziz S, Johnson JI, Bugger H, Zaha VG et al. (2007) Mitochondrial energetics in the heart in obesity-related diabetes: direct evidence for increased uncoupled respiration and activation of uncoupling proteins. Diabetes 56, 24572466.
  • 98
    Zoll J, Sanchez H, N'Guessan B, Ribera F, Lampert E, Bigard X, Serrurier B, Fortin D, Geny B, Veksler V et al. (2002) Physical activity changes the regulation of mitochondrial respiration in human skeletal muscle. J Physiol 543, 191200.
  • 99
    Conley KE, Amara CE, Bajpeyi S, Costford SR, Murray K, Jubrias SA, Arakaki L, Marcinek DJ & Smith SR (2013) Higher mitochondrial respiration and uncoupling with reduced electron transport chain content in vivo in muscle of sedentary versus active subjects. J Clin Endocrinol Metab 98, 129136.
  • 100
    Befroy DE, Petersen KF, Dufour S, Mason GF, Rothman DL & Shulman GI (2008) Increased substrate oxidation and mitochondrial uncoupling in skeletal muscle of endurance-trained individuals. Proc Natl Acad Sci USA 105, 1670116706.
  • 101
    Das SK, Gilhooly CH, Golden JK, Pittas AG, Fuss PJ, Dallal GE, McCrory MA, Saltzman E & Roberts SB (2007) Long term effects of energy-restricted diets differing in glycemic load on metabolic adaptation and body composition. Open Nutr J 85, 10231030.
  • 102
    Doucet E, St-Pierre S, Almeras N, Despres JP, Bouchard C & Tremblay A (2001) Evidence for the existence of adaptive thermogenesis during weight loss. Br J Nutr 85, 715723.
  • 103
    Johannsen DL, Knuth ND, Huizenga R, Rood JC, Ravussin E & Hall KD (2012) Metabolic slowing with massive weight loss despite preservation of fat-free mass. J Clin Endocrinol Metab 97, 24892496.
  • 104
    Doucet E, Imbeault P, St-Pierre S, Almeras N, Mauriege P, Despres JP, Bouchard C & Tremblay A (2003) Greater than predicted decrease in energy expenditure during exercise after body weight loss in obese men. Clin Sci (Lond) 105, 8995.
  • 105
    Harris JA & Benedict FG (1918) A biometric study of human basal metabolism. Proc Natl Acad Sci USA 4, 370373.
  • 106
    Frankenfield DC (2013) Bias and accuracy of resting metabolic rate equations in non-obese and obese adults. Clin Nutr doi:10.1016/j.clnu.2013.03.022.
  • 107
    Westerterp KR, Meijer GA, Schoffelen P & Janssen EM (1994) Body mass, body composition and sleeping metabolic rate before, during and after endurance training. Eur J Appl Physiol Occup Physiol 69, 203208.
  • 108
    Schwartz A, Kuk JL, Lamothe G & Doucet E (2012) Greater than predicted decrease in resting energy expenditure and weight loss: results from a systematic review. Obesity (Silver Spring) 20, 23072310.
  • 109
    Rosenbaum M & Leibel RL (2010) Adaptive thermogenesis in humans. Int J Obes (Lond) 34 (Suppl 1), S47S55.
  • 110
    Thomas DM, Bouchard C, Church T, Slentz C, Kraus WE, Redman LM, Martin CK, Silva AM, Vossen M, Westerterp K et al. (2012) Why do individuals not lose more weight from an exercise intervention at a defined dose? An energy balance analysis Obes Rev 13, 835847.
  • 111
    Chaput JP, Drapeau V, Hetherington M, Lemieux S, Provencher V & Tremblay A (2007) Psychobiological effects observed in obese men experiencing body weight loss plateau. Depress Anxiety 24, 518521.
  • 112
    Kissileff HR, Thornton JC, Torres MI, Pavlovich K, Mayer LS, Kalari V, Leibel RL & Rosenbaum M (2012) Leptin reverses declines in satiation in weight-reduced obese humans. Am J Clin Nutr 95, 309317.
  • 113
    Rosenbaum M, Kissileff HR, Mayer LE, Hirsch J & Leibel RL (2010) Energy intake in weight-reduced humans. Brain Res 1350, 95102.
  • 114
    Rosenbaum M, Sy M, Pavlovich K, Leibel RL & Hirsch J (2008) Leptin reverses weight loss-induced changes in regional neural activity responses to visual food stimuli. J Clin Invest 118, 25832591.
  • 115
    Hinkle W, Cordell M, Leibel R, Rosenbaum M & Hirsch J (2013) Effects of reduced weight maintenance and leptin repletion on functional connectivity of the hypothalamus in obese humans. PLoS One 8, e59114.
  • 116
    Rosenbaum M, Hirsch J, Murphy E & Leibel RL (2000) Effects of changes in body weight on carbohydrate metabolism, catecholamine excretion, and thyroid function. Am J Clin Nutr 71, 14211432.
  • 117
    Doucet E, St Pierre S, Almeras N, Mauriege P, Richard D & Tremblay A (2000) Changes in energy expenditure and substrate oxidation resulting from weight loss in obese men and women: is there an important contribution of leptin? J Clin Endocrinol Metab 85, 15501556.
  • 118
    Rosenbaum M, Goldsmith R, Bloomfield D, Magnano A, Weimer L, Heymsfield S, Gallagher D, Mayer L, Murphy E & Leibel RL (2005) Low-dose leptin reverses skeletal muscle, autonomic, and neuroendocrine adaptations to maintenance of reduced weight. J Clin Invest 115, 35793586.
  • 119
    Rabol R, Svendsen PF, Skovbro M, Boushel R, Haugaard SB, Schjerling P, Schrauwen P, Hesselink MK, Nilas L, Madsbad S et al. (2009) Reduced skeletal muscle mitochondrial respiration and improved glucose metabolism in nondiabetic obese women during a very low calorie dietary intervention leading to rapid weight loss. Metabolism 58, 11451152.
  • 120
    Dent R, Mcpherson R & Harper ME (1999) Variability in weight loss in highly compliant women on a controlled dietary regimen. Obes Res 7 (Suppl), 1.
  • 121
    Gerrits MF, Ghosh S, Kavaslar N, Hill B, Tour A, Seifert EL, Beauchamp B, Gorman S, Stuart J, Dent R et al. (2010) Distinct skeletal muscle fiber characteristics and gene expression in diet-sensitive versus diet-resistant obesity. J Lipid Res 51, 23942404.
  • 122
    Mogensen M, Sahlin K, Fernstrom M, Glintborg D, Vind BF, Beck-Nielsen H & Hojlund K (2007) Mitochondrial respiration is decreased in skeletal muscle of patients with type 2 diabetes. Diabetes 56, 15921599.
  • 123
    Sun G, Ukkola O, Rankinen T, Joanisse DR & Bouchard C (2002) Skeletal muscle characteristics predict body fat gain in response to overfeeding in never-obese young men. Metabolism 51, 451456.
  • 124
    Ghosh S, Dent R, Harper ME, Stuart J & McPherson R (2011) Blood gene expression reveal pathway differences between diet-sensitive and resistant obese subjects prior to caloric restriction. Obesity (Silver Spring) 19, 457463.
  • 125
    Bonen A, Parolin ML, Steinberg GR, Calles-Escandon J, Tandon NN, Glatz JF, Luiken JJ, Heigenhauser GJ & Dyck DJ (2004) Triacylglycerol accumulation in human obesity and type 2 diabetes is associated with increased rates of skeletal muscle fatty acid transport and increased sarcolemmal FAT/CD36. FASEB J 18, 11441146.
  • 126
    Goodpaster BH, He J, Watkins S & Kelley DE (2001) Skeletal muscle lipid content and insulin resistance: evidence for a paradox in endurance-trained athletes. J Clin Endocrinol Metab 86, 57555761.
  • 127
    Furukawa S, Fujita T, Shimabukuro M, Iwaki M, Yamada Y, Nakajima Y, Nakayama O, Makishima M, Matsuda M & Shimomura I (2004) Increased oxidative stress in obesity and its impact on metabolic syndrome. J Clin Invest 114, 17521761.
  • 128
    Keaney JF Jr, Larson MG, Vasan RS, Wilson PW, Lipinska I, Corey D, Massaro JM, Sutherland P, Vita JA, Benjamin EJ et al. (2003) Obesity and systemic oxidative stress: clinical correlates of oxidative stress in the Framingham Study. Arterioscler Thromb Vasc Biol 23, 434439.
  • 129
    Chavez JA, Knotts TA, Wang LP, Li G, Dobrowsky RT, Florant GL & Summers SA (2003) A role for ceramide, but not diacylglycerol, in the antagonism of insulin signal transduction by saturated fatty acids. J Biol Chem 278, 1029710303.
  • 130
    Itani SI, Ruderman NB, Schmieder F & Boden G (2002) Lipid-induced insulin resistance in human muscle is associated with changes in diacylglycerol, protein kinase C, and IkappaB-alpha. Diabetes 51, 20052011.
  • 131
    Dresner A, Laurent D, Marcucci M, Griffin ME, Dufour S, Cline GW, Slezak LA, Andersen DK, Hundal RS, Rothman DL et al. (1999) Effects of free fatty acids on glucose transport and IRS-1-associated phosphatidylinositol 3-kinase activity. J Clin Invest 103, 253259.
  • 132
    Schmitz-Peiffer C, Craig DL & Biden TJ (1999) Ceramide generation is sufficient to account for the inhibition of the insulin-stimulated PKB pathway in C2C12 skeletal muscle cells pretreated with palmitate. J Biol Chem 274, 2420224210.
  • 133
    Kelley DE, He J, Menshikova EV & Ritov VB (2002) Dysfunction of mitochondria in human skeletal muscle in type 2 diabetes. Diabetes 51, 29442950.
  • 134
    Kim JY, Hickner RC, Cortright RL, Dohm GL & Houmard JA (2000) Lipid oxidation is reduced in obese human skeletal muscle. Am J Physiol Endocrinol Metab 279, E1039E1044.
  • 135
    Simoneau JA, Veerkamp JH, Turcotte LP & Kelley DE (1999) Markers of capacity to utilize fatty acids in human skeletal muscle: relation to insulin resistance and obesity and effects of weight loss. FASEB J 13, 20512060.
  • 136
    Steinberg GR, Parolin ML, Heigenhauser GJ & Dyck DJ (2002) Leptin increases FA oxidation in lean but not obese human skeletal muscle: evidence of peripheral leptin resistance. Am J Physiol Endocrinol Metab 283, E187E192.
  • 137
    Holloway GP, Thrush AB, Heigenhauser GJ, Tandon NN, Dyck DJ, Bonen A & Spriet LL (2007) Skeletal muscle mitochondrial FAT/CD36 content and palmitate oxidation are not decreased in obese women. Am J Physiol Endocrinol Metab 292, E1782E1789.
  • 138
    Holloway GP, Benton CR, Mullen KL, Yoshida Y, Snook LA, Han XX, Glatz JF, Luiken JJ, Lally J, Dyck DJ et al. (2009) In obese rat muscle transport of palmitate is increased and is channeled to triacylglycerol storage despite an increase in mitochondrial palmitate oxidation. Am J Physiol Endocrinol Metab 296, E738E747.
  • 139
    Hulver MW, Berggren JR, Cortright RN, Dudek RW, Thompson RP, Pories WJ, MacDonald KG, Cline GW, Shulman GI, Dohm GL et al. (2003) Skeletal muscle lipid metabolism with obesity. Am J Physiol Endocrinol Metab 284, E741E747.
  • 140
    Boushel R, Gnaiger E, Schjerling P, Skovbro M, Kraunsoe R & Dela F (2007) Patients with type 2 diabetes have normal mitochondrial function in skeletal muscle. Diabetologia 50, 790796.
  • 141
    Ritov VB, Menshikova EV, He J, Ferrell RE, Goodpaster BH & Kelley DE (2005) Deficiency of subsarcolemmal mitochondria in obesity and type 2 diabetes. Diabetes 54, 814.
  • 142
    Patti ME, Butte AJ, Crunkhorn S, Cusi K, Berria R, Kashyap S, Miyazaki Y, Kohane I, Costello M, Saccone R et al. (2003) Coordinated reduction of genes of oxidative metabolism in humans with insulin resistance and diabetes: potential role of PGC1 and NRF1. Proc Natl Acad Sci USA 100, 84668471.
  • 143
    Mootha VK, Lindgren CM, Eriksson KF, Subramanian A, Sihag S, Lehar J, Puigserver P, Carlsson E, Ridderstrale M, Laurila E et al. (2003) PGC-1alpha-responsive genes involved in oxidative phosphorylation are coordinately downregulated in human diabetes. Nat Genet 34, 267273.
  • 144
    Bell JA, Reed MA, Consitt LA, Martin OJ, Haynie KR, Hulver MW, Muoio DM & Dohm GL (2010) Lipid partitioning, incomplete fatty acid oxidation, and insulin signal transduction in primary human muscle cells: effects of severe obesity, fatty acid incubation, and fatty acid translocase/CD36 overexpression. J Clin Endocrinol Metab 95, 34003410.
  • 145
    Thyfault JP, Cree MG, Zheng D, Zwetsloot JJ, Tapscott EB, Koves TR, Ilkayeva O, Wolfe RR, Muoio DM & Dohm GL (2007) Contraction of insulin-resistant muscle normalizes insulin action in association with increased mitochondrial activity and fatty acid catabolism. Am J Physiol Cell Physiol 292, C729C739.
  • 146
    Koves TR, Ussher JR, Noland RC, Slentz D, Mosedale M, Ilkayeva O, Bain J, Stevens R, Dyck JR, Newgard CB et al. (2008) Mitochondrial overload and incomplete fatty acid oxidation contribute to skeletal muscle insulin resistance. Cell Metab 7, 4556.
  • 147
    Cogswell AM, Stevens RJ & Hood DA (1993) Properties of skeletal muscle mitochondria isolated from subsarcolemmal and intermyofibrillar regions. Am J Physiol 264, C383C389.
  • 148
    Hood DA (2001) Invited Review: contractile activity-induced mitochondrial biogenesis in skeletal muscle. J Appl Physiol 90, 11371157.
  • 149
    Benton CR, Nickerson JG, Lally J, Han XX, Holloway GP, Glatz JF, Luiken JJ, Graham TE, Heikkila JJ & Bonen A (2008) Modest PGC-1alpha overexpression in muscle in vivo is sufficient to increase insulin sensitivity and palmitate oxidation in subsarcolemmal, not intermyofibrillar, mitochondria. J Biol Chem 283, 42284240.
  • 150
    Chomentowski P, Coen PM, Radikova Z, Goodpaster BH & Toledo FG (2011) Skeletal muscle mitochondria in insulin resistance: differences in intermyofibrillar versus subsarcolemmal subpopulations and relationship to metabolic flexibility. J Clin Endocrinol Metab 96, 494503.
  • 151
    Holloway GP, Gurd BJ, Snook LA, Lally J & Bonen A (2010) Compensatory increases in nuclear PGC1alpha protein are primarily associated with subsarcolemmal mitochondrial adaptations in ZDF rats. Diabetes 59, 819828.
  • 152
    Dyck DJ, Peters SJ, Glatz J, Gorski J, Keizer H, Kiens B, Liu S, Richter EA, Spriet LL, van der Vusse GJ et al. (1997) Functional differences in lipid metabolism in resting skeletal muscle of various fiber types. Am J Physiol 272, E340E351.
  • 153
    Anderson EJ, Yamazaki H & Neufer PD (2007) Induction of endogenous uncoupling protein 3 suppresses mitochondrial oxidant emission during fatty acid-supported respiration. J Biol Chem 282, 3125731266.
  • 154
    Vincent AM, Edwards JL, McLean LL, Hong Y, Cerri F, Lopez I, Quattrini A & Feldman EL (2010) Mitochondrial biogenesis and fission in axons in cell culture and animal models of diabetic neuropathy. Acta Neuropathol 120, 477489.
  • 155
    Edwards JL, Quattrini A, Lentz SI, Figueroa-Romero C, Cerri F, Backus C, Hong Y & Feldman EL (2010) Diabetes regulates mitochondrial biogenesis and fission in mouse neurons. Diabetologia 53, 160169.
  • 156
    Jheng HF, Tsai PJ, Guo SM, Kuo LH, Chang CS, Su IJ, Chang CR & Tsai YS (2012) Mitochondrial fission contributes to mitochondrial dysfunction and insulin resistance in skeletal muscle. Mol Cell Biol 32, 309319.
  • 157
    Bach D, Pich S, Soriano FX, Vega N, Baumgartner B, Oriola J, Daugaard JR, Lloberas J, Camps M, Zierath JR et al. (2003) Mitofusin-2 determines mitochondrial network architecture and mitochondrial metabolism. A novel regulatory mechanism altered in obesity. J Biol Chem 278, 1719017197.
  • 158
    Joseph AM, Joanisse DR, Baillot RG & Hood DA (2012) Mitochondrial dysregulation in the pathogenesis of diabetes: potential for mitochondrial biogenesis-mediated interventions. Exp Diabetes Res 2012, 642038.
  • 159
    Toledo FG, Watkins S & Kelley DE (2006) Changes induced by physical activity and weight loss in the morphology of intermyofibrillar mitochondria in obese men and women. J Clin Endocrinol Metab 91, 32243227.
  • 160
    Menshikova EV, Ritov VB, Ferrell RE, Azuma K, Goodpaster BH & Kelley DE (2007) Characteristics of skeletal muscle mitochondrial biogenesis induced by moderate-intensity exercise and weight loss in obesity. J Appl Physiol 103, 2127.
  • 161
    Menshikova EV, Ritov VB, Toledo FG, Ferrell RE, Goodpaster BH & Kelley DE (2005) Effects of weight loss and physical activity on skeletal muscle mitochondrial function in obesity. Am J Physiol Endocrinol Metab 288, E818E825.
  • 162
    Goodpaster BH, Katsiaras A & Kelley DE (2003) Enhanced fat oxidation through physical activity is associated with improvements in insulin sensitivity in obesity. Diabetes 52, 21912197.
  • 163
    Toledo FG, Menshikova EV, Ritov VB, Azuma K, Radikova Z, DeLany J & Kelley DE (2007) Effects of physical activity and weight loss on skeletal muscle mitochondria and relationship with glucose control in type 2 diabetes. Diabetes 56, 21422147.
  • 164
    Wright DC, Han DH, Garcia-Roves PM, Geiger PC, Jones TE & Holloszy JO (2007) Exercise-induced mitochondrial biogenesis begins before the increase in muscle PGC-1alpha expression. J Biol Chem 282, 194199.
  • 165
    Holloszy JO (1967) Biochemical adaptations in muscle. Effects of exercise on mitochondrial oxygen uptake and respiratory enzyme activity in skeletal muscle. J Biol Chem 242, 22782282.
  • 166
    Talanian JL, Galloway SD, Heigenhauser GJ, Bonen A & Spriet LL (2007) Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. J Appl Physiol 102, 14391447.
  • 167
    Bruce CR, Thrush AB, Mertz VA, Bezaire V, Chabowski A, Heigenhauser GJ & Dyck DJ (2006) Endurance training in obese humans improves glucose tolerance and mitochondrial fatty acid oxidation and alters muscle lipid content. Am J Physiol Endocrinol Metab 291, E99E107.
  • 168
    Amati F, Dube JJ, Shay C & Goodpaster BH (2008) Separate and combined effects of exercise training and weight loss on exercise efficiency and substrate oxidation. J Appl Physiol 105, 825831.
  • 169
    Toledo FG, Menshikova EV, Azuma K, Radikova Z, Kelley CA, Ritov VB & Kelley DE (2008) Mitochondrial capacity in skeletal muscle is not stimulated by weight loss despite increases in insulin action and decreases in intramyocellular lipid content. Diabetes 57, 987994.
  • 170
    Bond DS, Phelan S, Leahey TM, Hill JO & Wing RR (2009) Weight-loss maintenance in successful weight losers: surgical vs non-surgical methods. Int J Obes (Lond) 33, 173180.
  • 171
    Catenacci VA, Ogden LG, Stuht J, Phelan S, Wing RR, Hill JO & Wyatt HR (2008) Physical activity patterns in the National Weight Control Registry. Obesity (Silver Spring) 16, 153161.
  • 172
    Goodpaster BH (2013) Mitochondrial deficiency is associated with insulin resistance. Diabetes 62, 10321035.
  • 173
    Holloszy JO (2013) ‘Deficiency’ of mitochondria in muscle does not cause insulin resistance. Diabetes 62, 10361040.