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Keywords:

  • disease;
  • hydration assessment;
  • obesity;
  • type 2 diabetes;
  • urolithiasis

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CHALLENGES TO RESEARCH
  5. EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

The purpose of this article is to review the effects of chronic mild dehydration and fluid consumption on specific health outcomes including obesity. The electronic databases PubMed and Google Scholar were searched for relevant literature published from the time of their inception to 2011, with results restricted to studies performed on human subjects and reports in the English language. Key words included the following: dehydration, hypohydration, water intake, fluid intake, disease, and the names of specific disease states. Strength of evidence categories were described for 1) medical conditions associated with chronic dehydration or low daily water intake, and 2) randomized-controlled trials regarding the effects of increased water consumption on caloric intake, weight gain, and satiety. This process determined that urolithiasis is the only disorder that has been consistently associated (i.e., 11 of 13 publications) with chronic low daily water intake. Regarding obesity and type 2 diabetes, evidence suggests that increased water intake may reduce caloric intake for some individuals. Recommendations for future investigations include measuring total fluid intake (water + beverages + water in solid food), conducting randomized-controlled experiments, identifying novel hydration biomarkers, and delineating hydration categories.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CHALLENGES TO RESEARCH
  5. EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

The inaugural ILSI North America Conference on Hydration and Health Promotion was convened in Washington, D.C. during November 2006. This conference emphasized the importance of total water intake in the promotion of health and well-being. In part, because of the recognition that little research had focused on the hydration needs of citizens with sedentary lifestyles and occupations,1,2 ILSI North America then organized the ILSI North America Second International Conference on Hydration & Health, which was held during November 2011 in Miami, Florida. One important aspect of these conference proceedings was the hydration of average citizens with sedentary lifestyles and occupations, who experience mild dehydration of −1% to −2% of body mass. Similarly, other conferences have emphasized the effects of chronic mild dehydration on health, physiological function, cognition, and performance. Published reports of both scientists and clinicians3–7 serve to document that few studies have evaluated mild dehydration. Today, this matter continues to be of interest to scientists.8 Therefore, the present article was prepared to 1) consider the effects of chronic mild dehydration and fluid consumption on specific health outcomes and diseases, and 2) describe the greatest challenges to the design of experiments and the interpretation of the relevant scientific literature.

Table 1 presents the available evidence for diseases and medical conditions that may be associated with chronic dehydration and underconsumption of water.7,9–17 During development of this article, the electronic databases PubMed and Google Scholar were searched for the period from their inception to 2011, with results limited to studies with human subjects and reports in the English language. Key words used to perform the literature searches included the following: dehydration, hypohydration, water intake, fluid intake, disease, and the names of specific disease states. Strength of evidence categories are described in the table's footnote. As the table shows, few studies have evaluated each of the diseases listed, in part because investigations that explore the effects of chronic low daily fluid intake (i.e., across months and years) on disease progression are difficult to design, conduct, maintain, and interpret. This lack of evidence, coupled with a variety of outcome variables (column 3), suggests that low daily water consumption may someday be associated with specific diseases. To date, however, urolithiasis is the only disorder that has been consistently associated (i.e., 11 of 13 publications) with chronic low daily water intake. With an eye toward obesity and type 2 diabetes, Table 2 (discussed below) summarizes the findings of eight intervention studies that evaluated the influence of increased water consumption on weight gain, caloric intake, and satiety.18–25

Table 1.  Evidence regarding diseases and medical conditions that may be associated with chronic, daily dehydration or underconsumption of water.
Disease (no. of studies)FindingsOutcome variablesReferencesa
  • a

     Six categories of evidence: Ia, meta-analysis of randomized, controlled trials; Ib, at least one randomized, controlled trial; IIa, at least one controlled trial without randomization; IIb, at least one other quasi-experimental study; III, descriptive studies such as comparative, correlational and case-control studies; IV, expert committee reports, opinions, or clinical experience of respected authorities.

Colon cancer (n = 3)Three studies (category III)a reported a positive influence of fluid intake on reduced cancer risk. Insufficient number of studiesWater intake (glasses/day); total fluid (solid food + water + beverages)Shannon et al. (1996)9; Institute of Medicine (2004)10
Bladder cancer (n = 8)The relationship between fluid intake and bladder cancer incidence has not been substantiated (category III)aTotal fluid (solid food + water + beverages); only beverages (mL/day); only beverages (mL/day)Institute of Medicine (2004)10; Michaud et al. (1999)11; Radosavljevic et al. (2003)12
Chronic kidney disease (n = 1)Participants who consumed the most fluid (3.18 L/day) had a significantly lower risk of CKD (odds ratio 0.5, P < 0.003) than those who consumed the least fluid (1.79 L/day) (category III).a Insufficient number of studiesTotal fluid (solid food + water + beverages)Strippoli et al. (2011)13
Urolithiasis (n = 13)11 of 13 publications identified a significant association between favorable hydration status and lower stone recurrence rate (category IIb)aEvaluated renal function not fluid intake, review articleManz and Wentz (2003)7
Urinary tract infection (n = 8)The influence of fluid volume on susceptibility to UTI is not substantiated by existing literature (category III).a Several expert panels recommend a high fluid intake for patients with UTI (category IV)aEvaluated renal function not fluid intake, review article; methods not stated, review paperManz and Wentz (2003)7; Beetz (2003)14
Fatal coronary heart disease (n = 1)One case control study (n = 246 fatal CHD events versus 8,280 males and 12,017 female controls, aged 38–100 y) reported that increased (>1.20 L/day) versus little water (≤0.48 L/day) intake had a relative risk of 0.33, when corrected for CHD risk factors plus intake of other beverages (category III)a. Insufficient number of studiesWater intake (glasses/day)Chan et al. (2002)15
Venous thromboembolism (n = 1)One study observed that patients with elevated serum osmolality exhibited an increased risk of VT (category III).a Insufficient number of studiesMeasured serum osmolality, not fluid intakeKelly et al. (2004)16
Exercise-induced bronchospasm (n = 1)One study reported that mild dehydration is a risk factor for EIB (category IIb).a Insufficient number of studiesMeasured hydration biomarkers, not fluid intakeKalhoff (2003)17
Cerebral infarct, dental diseases, gallstones, mitral valve prolapse, glaucoma, osteoporosis (<3 each)Insufficient number of studies and relationship unsubstantiatedTotal fluid (solid food + water + beverages); evaluated renal function not fluid intake, review articleInstitute of Medicine (2004)10; Manz and Wentz (2003)7
Table 2.  Controlled, randomized trials investigating the effects of increased water consumption on factors related to caloric intake, weight gain, and satiety.
Experimental approachFindingsReferencesa
  • a

     As controlled, randomized investigations, all studies in Table 2 exemplify the evidence category 1b (Table 1 footnote).

Consuming water with a mealWater (0, 237, or 474 mL) was consumed by men 30 min before lunch, 60 min before lunch, or with lunch. Energy intake of this meal was not altered by either water preload.Rolls et al. (1990)18
Consuming foods with a high water content more effectively reduced subsequent energy intake of lean women than did drinking water with food.Rolls et al. (1999)19
Women were served breakfast, with or without 2 glasses of water. The former decreased hunger and increased satiety during breakfast. This effect did not extend beyond breakfast. Food intake was not measured.Lappalainen et al. (1993)20
Laboratory-based meal study demonstrated that water consumed with a meal reduced ratings of hunger and increased rating of satiety.DellaValle et al. (2005)21
Drinking water before mealsBoth normal-weight and overweight/obese middle-aged and older adults ingested less energy during an ad libitum meal, when given a 500 mL water preload, 30 min prior to the meal (versus a no-preload meal condition).Van Walleghen et al. (2007)22; Davy et al. (2008)23
Young adults do not exhibit reduced energy intake following water preload. Coupled with the above findings, this suggests that age-related differences may exist.Rolls et al. (1990)18; Van Walleghen et al. (2007)22
Effect of water consumption on weight lossIn middle-aged and older adults, consumption of 500 mL of water prior to hypocaloric meals led to greater 12-week weight loss than a hypocaloric diet alone.Dennis et al. (2010)24
Increasing daily intake of waterIncreasing self-reported daily water consumption by ≥1 L in overweight women was associated with increased weight loss of ∼2 kg, during a 12-month dietary intervention, compared with women who consumed <1 L water daily.Stookey et al. (2008)25

CHALLENGES TO RESEARCH

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CHALLENGES TO RESEARCH
  5. EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

The following four subsections describe obstacles to conducting effective research in this field. These subtopics (i.e., evaluating drinking behavior; hydration indices; accurate measurement of water intake; the etiology of obesity) describe the difficulties that investigators experience when designing new studies and interpreting existing data. Until these challenges are overcome (i.e., within the design of experiments), it is unlikely that our understanding of the effects of chronic underconsumption of water on health outcomes will advance in meaningful ways.

Evaluating drinking behavior across years

Considering the diseases and medical conditions described in Table 1, it is obvious that some require days or weeks to develop, whereas others require years or decades (i.e., some forms of cancer). Controlled intervention trials spanning years or decades simply do not exist and are difficult to design, for the following reasons: 1) the number of subjects required for adequate statistical power is large, and research is costly; 2) lengthy intervention studies suffer from participant noncompliance and attrition, since it is difficult for any person to maintain a constant hydration state across years of his of her life; 3) over 40 different types of cancer exist, with the mechanisms and etiology of each type being subtly unique, and the influences of water consumption possibly different in each; 4) multiple personal characteristics, dietary habits, or lifestyle behaviors may concurrently encourage disease development, with their intercorrelation making interpretation of a single factor (e.g., daily water intake) difficult; 5) without careful experimental control of these factors, the probability of a type II statistical error increases (e.g., investigators declare there is no effect, when one actually exists) – this is scientifically important because studies that contain no significant differences (i.e., negative findings) are more difficult to publish.

Hydration indices: no gold standard exists

The physiological regulation of total body water volume and fluid concentrations is complex and dynamic.26 For example, the complex renal regulation of extracellular volume involves not only arginine vasopressin and aldosterone, but also atrial natriuretic peptide and urodilatin. Additionally, sweat gland, metabolic, and thirst responses are involved to varying degrees, depending on one's prevailing activities and dietary contents. The change of total body water throughout any 24-h period is best represented by a sinusoidal wave that oscillates around an average. Thus, all hydration indices (i.e., body mass change, plasma osmolality, urine osmolality, urine specific gravity, urine color, 24-h urine volume, salivary osmolality, rating of thirst, stable isotope dilution, bioelectrical impedance spectroscopy) are best viewed as singular measures of a dynamic fluid matrix, containing interconnected fluid compartments.

These facts have prompted several authorities to conclude that no single hydration biomarker is accurate and valid in all situations or all life activities.7,26–28 Recent evidence also suggests that biomarkers may accurately assess hydration changes acutely (within-day), but not chronically (across days) (unpublished observations of E. Perrier et al., 2011). Therein lies a major challenge to linking under-consumption of water to health outcomes. That is, the complex and dynamic nature of human fluid-electrolyte regulation makes it difficult to assess hydration state with great precision.26 And, if hydration state is not precisely identified, the relationship between chronic under-consumption of water and disease states cannot be adequately assessed.

Difficulties accurately measuring water intake

Measurements of water intake are essential to all hydration studies. Yet, even though the influence of water has been studied for decades, 1) no scientific or clinical consensus exists regarding precise values for the daily water requirements of men and women,10,29,30 and 2) surprisingly little data exist regarding the water turnover of healthy, free-living, sedentary adults who are not under environmental or physiological stress.31 Also, the published data describing human water requirements contain recognized limitations that have been described previously.32 The foregoing facts indicate that accurate and precise measurements of adult water consumption patterns are necessary but rare.

It is reasonable to ask why identifying chronic low- or underconsumption of water is a difficult task. One answer lies in the methods and instruments that assess human dietary intake, including water. Food diaries and recalls are resource-intensive, time-consuming, burdensome for participants, provide only recent intake data (i.e., not habitual intake patterns), and are not always feasible in large-scale epidemiological studies.33 Further, food diaries often do not describe 24-h human water turnover accurately because 1) the water content of solid food is not measured accurately,10,30 and 2) under-reporting errors are common,34 no matter how carefully researchers plan, control, and execute procedures. Indeed, different methods of measuring dietary intake may explain, in part, the differences between European Food Safety Authority30 recommendations for daily adequate intake of water (men, 2.5 L/day; women, 2.0 L/day) versus those of the Institute of Medicine (men, 3.7 L/day; women, 2.7 L/day).10 Other factors that may explain these international differences in AI recommendations, include: inclusion or exclusion of exercise and labor in the consideration of human water needs,10,29 as well as international and cultural differences in the selection of food items (i.e., salty foods, large soup intake).7

Elusive etiology of obesity

Many countries have taken steps to plan for the rising healthcare costs associated with an increasing incidence of obesity and type 2 diabetes. For example, healthcare costs for diabetes treatment in the United States exceeded $116 billion in 2007, which was 11% of all health care expenditures.35 Also, annual healthcare costs attributable to obesity and overweight in the United States are projected to be $861 to $957 billion by the year 2030; this may account for $1 of every $6 spent on health.36 Such trends, now experienced in many countries, have focused government and research attention on causes. However, many diseases have multifactorial origins.37 For example, the risk factors for obesity and type 2 diabetes are numerous (i.e., increased use of technology at home and at work, reduced physical activity, increased caloric intake, altered selection of food items, changes in the dietary carbohydrate : fat ratio) and interrelated. It also is likely that one causative factor predominates for some individuals, but not for others.

Water consumption may provide an approach to counteracting this growing international healthcare problem. Some researchers have hypothesized that consumption of water, before or during meals, helps overweight individuals manage their body weight by increasing satiety or altering food intake.21,38Table 2 summarizes the findings of eight intervention studies that evaluated the influence of increased water consumption. Energy intake, weight gain, sensation of hunger, and sensation of satiety were the primary outcome variables (evidence categories Ib, IIa, and IIb; see table footnote). The findings of these eight studies indicate the following: 1) they support a complex and dynamic network of behavioral and physiological factors that likely are influenced by the timing of intake, volume of fluid consumed, mode of fluid presentation (i.e., separated from or mixed with solid food), and chronological age; 2) they suggest that personal food and fluid preferences influence the relationship between water consumption and weight gain; 3) water consumption may be effective for some individuals but not others; and 4) they demonstrate that additional research is required to interpret the subtle interactions between water consumption, satiety, energy intake, and weight gain.

Interestingly, water intake may increase energy expenditure directly, independent of hunger and satiety. One research team reported a 30% increase of resting metabolic rate when obese men (n = 7) and women (n = 7) consumed 500 mL of water39; this increase (i.e., approximately 100 kJ) peaked after 30–40 min. Similarly, a 24% increase of metabolic rate was observed when a different sample of obese men (n = 8) and women (n = 8) consumed 500 mL of water40; however, no change of metabolic rate was measured when these subjects consumed only 50 mL of water. Investigators concluded that this inexpensive intervention might be useful to help obese individuals attain increased energy expenditure.39

The work of Keller et al.5 suggests that high levels of water intake may directly influence substrate utilization. They examined the effects of hypo-osmotic cell swelling, similar to a state of high fluid intake, by administering desmopressin (a synthetic analogue of AVP) plus intake of 2.4 L water in 12 h. The protein, glucose, and lipid metabolism responses of 10 healthy young men were observed. During the hypo-osmotic experiment, plasma osmolality dropped from 286 to 265 mOsm/kg, and resulted in protein sparing associated with increased whole body lipolysis, ketogenesis, and lipid oxidation; insulin sensitivity of glucose metabolism was also impaired. This acute metabolic state resembled fasting. The authors concluded that a chronic, increased fluid intake may reduce body fat stores.

The above studies demonstrate that science cannot adequately explain the ways in which water consumption, before or during meals, may help overweight individuals manage their body weight.

EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CHALLENGES TO RESEARCH
  5. EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Scientists today are discovering new ways that mild dehydration alters physiology and behavior. Contributing to previous studies41–43 regarding the effects of moderate and substantial dehydration (2.6–3.2% of body mass loss) on cognitive performance, our research team published two investigations that demonstrated cognitive impairment and mood changes in women and men subsequent to mild dehydration of only 1.39% and 1.59% of body mass, respectively.44,45 Prior to these publications, none had reported that dehydration of such a small magnitude, without concurrent hyperthermia, would alter cognitive function.

It appears there is also much to learn about chronic dehydration and fluid underconsumption. The relative lack of data in this field of study (Table 1), and the aforementioned challenges to the study of long-term health outcomes, prompted the following recommendations. These ideas are provided to encourage future research regarding the effects of short-term and chronic dehydration on disease and health outcomes. They are organized into two categories: fluid intake and hydration assessment. Future experiments (i.e., preferably controlled, randomized intervention trials) should investigate these areas.

Fluid intake and disease.  Such investigations would include the following: 1) total fluid consumption from all sources (solid foods + water + beverages)30; 2) consumption patterns for water, beverages, and water in solid food that are consumed in various countries, providing data to substantiate Adequate Intake recommendations for water intake; 3) techniques, slogans, memory cues, and devices that encourage humans to consume the daily AI of water, as recommended by international health organizations10,30; 4) a brief, self-administered, valid, and reliable beverage intake assessment tool to enhance measurements of water and beverage consumption33; 5) differences between high-volume drinkers and low-volume drinkers – this group comparison technique offers promise for observations of humans during ordinary daily activities (E. Perrier et al., unpublished observations, 2011); and 6) chronic underconsumption of water in older adults, other than dehydration encephalopathy, which has been associated with cognitive decline.46

Hydration assessment and disease.  Such investigations would include the following: 1) which hydration markers are most useful to identify chronic mild dehydration; 2) the definition of “optimal hydration,” which may be different for different physiological and cognitive functions and for different body organs; 3) serum hormone concentrations (e.g., arginine vasopressin, aldosterone, cortisol) as indicators of chronic underconsumption of water, or as components of the mechanisms that underlie disease; 4) changes in international patterns of beverage consumption, and their relationship to obesity and type 2 diabetes; 5) numerous concurrent hydration indices, within the context of an experimental intervention or a group comparison (E. Perrier et al., unpublished observations, 2011), to identify novel hydration biomarkers that are relevant to disease; 6) hydration categories (e.g., euhydration, extremely hyperhydrated, extremely dehydrated) that are defined via statistical techniques, as useful guidance for the lay public47,48; 7) the dynamics of water turnover and water balance of obese individuals versus individuals with a normal body mass index.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CHALLENGES TO RESEARCH
  5. EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

Considering the factors that influence weight gain, caloric intake and satiety, it is widely appreciated (Table 2) that a complex and dynamic network of behavioral, metabolic, and physiological factors exists. This suggests that caloric intake is influenced by the timing of intake, volume of fluid consumed, mode of fluid presentation, chronological age, as well as personal food and fluid preferences. Thus, because water consumption may be effective for some individuals but not for others, this simple intervention deserves future research attention.

Finally, recommendations for future research focus on the effects of dehydration and fluid consumption on disease and health outcomes. These recommendations are organized into two categories: fluid intake, and hydration assessment. Urolithiasis is the only disorder that consistently has been associated with chronic low daily water intake. However, investigations that explore the effects of chronic low daily fluid intake (i.e., across months and years) on disease progression are difficult to design, conduct, and interpret. Future investigations should measure the effects of total fluid intake (i.e., including water, beverages, and solid foods) on health outcomes, especially obesity and type 2 diabetes.

Acknowledgments

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CHALLENGES TO RESEARCH
  5. EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES

The concepts and opinions expressed by the author do not represent those of the University of Connecticut.

Declaration of interest.  The author is a consultant to ILSI North America and to Danone Research. He serves on the Danone Research Scientific Advisory Board and receives funding to conduct research from Danone Research, France.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. CHALLENGES TO RESEARCH
  5. EXPERIMENTAL APPROACHES FOR FUTURE INVESTIGATIONS
  6. CONCLUSION
  7. Acknowledgments
  8. REFERENCES