SEARCH

SEARCH BY CITATION

Keywords:

  • Adiposity;
  • isoprostane;
  • metabolic syndrome

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information
What is already known about this subject
  • African Americans are disproportionately affected by obesity and other metabolic risk factors in comparison to White Americans.
  • Increasing prevalence of obesity has been associated with concomitant increases in childhood hypertension, dyslipidaemia and type 2 diabetes.
  • Oxidative stress is associated with obesity in both adults and children.
What this study adds
  • Oxidative stress is positively associated with total body fat and truncal fat, but not with body mass index (BMI) or BMI z-score in healthy youth.
  • Oxidative stress is associated with diastolic blood pressure in African American but not in White American healthy youth.

Background

Oxidative stress is elevated in obese youth, but less is known regarding racial disparities in the relationship of oxidative stress with metabolic risk factors.

Objectives

To determine the relationship between oxidative stress and metabolic risk factors, adiposity, leptin, adiponectin and cardiovascular fitness (VO2PEAK) in healthy African American and White American youth.

Methods

A marker of oxidative stress (F2-isoprostane), validated markers of metabolic risk factors, fitness and body composition were measured in African American (n = 82) and White American (n = 76) youth (8–17 years old) recruited over a range of BMI percentiles (4th to 99th).

Results

F2-isoprostane concentration was positively correlated with percentage body fat (r = 0.198) and percentage truncal fat (r = 0.173), but was not different between African American and White American males and females (P = 0.208). African American youth had significantly higher mean systolic and diastolic blood pressure (P = 0.023 and P = 0.011, respectively), body weight, BMI percentile and Tanner stage. After adjusting for gender, age, BMI and Tanner stage, African American youth varied from White Americans in the association of F2-isoprostane with diastolic blood pressure (P = 0.047), but not with systolic blood pressure, triglycerides, VO2PEAK or homeostatic model assessment for insulin resistance (all P > 0.05).

Conclusions

Oxidative stress, as measured by urinary F2-isoprostane concentrations, was positively associated with percent body fat and truncal fat in youth. Oxidative stress levels were similar among African American and White American youth. Among markers of the metabolic syndrome, a significant difference between African American and White American youth was demonstrated only in the association of oxidative stress with diastolic blood pressure.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information

African Americans are disproportionately affected by obesity compared to White Americans, and these disparities begin in childhood [1]. The recent rapid increase in childhood obesity has been accompanied by concomitant increases in hypertension, dyslipidaemia and type 2 diabetes [2, 3]. It has been established that compared to White American adults, African Americans are at higher risk for cardiovascular disease and type 2 diabetes [4, 5]. Because of the increased metabolic risk for African American youth and consequent obesity-related pathophysiologies which track into adulthood, it is imperative to determine clinically relevant racial disparities in early markers of metabolic dysfunction.

Oxidative stress, defined as an imbalance between production of reactive oxygen species and antioxidant defences, is an early marker of metabolic dysfunction and has been implicated in atherosclerosis, microvascular complications of diabetes, beta-cell failure in type 2 diabetes and insulin resistance, making it a unifying mechanism of metabolic dysfunction [6, 7]. Previous studies have shown that oxidative stress is increased in obese adults [8, 9] and children [10-16]. In addition, oxidative stress was elevated in individuals who displayed metabolic risk factors associated with metabolic syndrome compared to individuals with no metabolic dysfunction [9-11, 17].

While these studies support the role of oxidative stress as an early marker of metabolic derangements, more data are needed regarding racial differences in the relationship of oxidative stress and individual metabolic risk factors. Despite an increased risk of developing obesity, diabetes type 2 and hypertension in African American compared to White American adults and children, certain cardiometabolic risk factors tend to be more favourable in African Americans (higher high-density lipoprotein [HDL], lower triglycerides) [18, 19]. No study, to the best of our knowledge, has evaluated and compared racial disparities in the relationship of oxidative stress with metabolic risk factors, adipokine concentration or cardiorespiratory fitness. Therefore, the objectives of the current study were to determine the relationship between F2-isoprostane, an established marker of oxidative stress, and metabolic risk factors, adiposity, adipokine concentrations and cardiorespiratory fitness in African American and White American youth.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information

Participants

A group of 158 healthy African American and White American youth (8 to 17 years old) were recruited over a range of body mass index (BMI) percentiles from the Nashville general population using flyers, e-mail distribution lists and personal contacts. Participants or their parents classified their own ethnicity according to investigator-defined options (African American, White American or other). All volunteers were healthy as determined by a physical exam performed by a board-certified paediatrician. Participants were not involved in a weight loss programme or in an intensive exercise programme in the 6 months before the study. Exclusion criteria included smoking or using tobacco products, diabetes, cardiovascular disease, significant recent weight change, chronic pulmonary conditions (asthma, sleep apnoea) or other health issues that would preclude participation in physical activities as assessed by a paediatrician. All applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed during this study, in accordance with the ethical principles of the Helsinki-II Declaration. All participants and their parents or legal guardians signed an informed consent or assent document approved by the university-affiliated Institutional Review Board.

Protocol

The details of the protocol were discussed, and all questions answered by the study staff before scheduling a study visit. Participants were asked to maintain normal daily routine but avoid any unusual patterns of physical activity, such as strenuous exercise and stress, on the day before the study. No dietary restrictions were stipulated before the study visit. Participants reported to the Clinical Research Center after an overnight fast for baseline measurements for a study on the role of physical activity in adolescent obesity.

Anthropometric and body composition measurements

The National Health and Nutrition Examination Survey protocols were followed for all anthropometrical measurements [20]. Stature (height) was measured within 0.5 cm using a calibrated wall-mounted stadiometer (Perspective Enterprises, Portage, MI, USA). Body weight was measured within 0.1 kg using a calibrated beam platform scale (Detecto-Medic, Detecto Scales, Inc, Northbrook, IL, USA) with participants wearing light clothing and no shoes. BMI, BMI percentiles and BMI z-scores were calculated from height, weight and age using Centers of Disease Control growth charts [21]. Total body fat mass and truncal fat mass were measured using dual-energy X-ray absorptiometry (DXA) (GE Medical Systems, Madison WI, USA, enCORE 2007 software version 11.40.004). For quality assurance and equilibration, a calibration block was scanned each morning and a spine phantom was scanned on a weekly basis. The coefficient of variation in for DXA measurements in our laboratory for youth is 0.7%.

Blood pressure

Systolic and diastolic blood pressure (SBP and DBP, respectively) were measured after 10 min of resting in a supine position using an automatic inflating blood pressure cuff (DINAMAP, GE Medical Systems, Milwaukee, WI, USA). SBP and DBP percentiles, corrected for age, sex and height, were calculated according to 2004 National High Blood Pressure Education Working Group guidelines [22].

Cardiorespiratory fitness

Peak oxygen uptake (VO2PEAK) was measured using a modified Bruce treadmill exercise test protocol [23]. Breath-by-breath oxygen consumption and carbon dioxide production were measured using a MedGraphics Ultima Series system (Medical Graphics Corp., St. Paul, MN, USA), and processed and analyzed with the BreezeSuite software Version 6.4.023 (St. Paul, MN, USA).

Urine collection and isoprostane analysis

Urine was collected and pooled from a 24-h period and stored at −80°C until analysis. The major urinary metabolite of 15-F2t-IsoP, 2,3-dinor-5,6-dihydro-15-F2t-IsoP (2,3-dinor-5,6-dihydro-8-iso-PGF2α) was used as a marker of oxidative stress. The metabolite (F2-isoprostane) was measured by gas chromatography/negative ion chemical ionization mass spectrometry, as previously reported in detail [24]. Precision of the assay is ± 4%, accuracy is 97%, and the lower limit of sensitivity is approximately 20 pg [24]. F2-isoprostane concentrations were normalized to urinary creatinine measured using Sirrus Clinical Chemistry analyzer (Stanbio Laboratory, Boerne, TX, USA).

Plasma collection and measurements

Fasting blood samples were collected and plasma was separated by centrifugation and stored at −80°C. Plasma triglycerides, total cholesterol, low-density lipoprotein (LDL) and HDL concentrations were measured using enzymatic kits from Cliniqa Corp. (San Marcos, CA, USA). Free fatty acids were measured using the NEFA-C kit by Wako (Nneuss, Germany) and by gas chromatography. Glucose was measured using the Vitros Chemistry analyzer (Ortho Clinical Diagnostics, Rochester, NY, USA). Insulin and leptin measurements were performed using radioimmunoassays (RIAs). Adiponectin analysis was done using a kit from Millipore (Billerica, MA, USA) and Luminex multiplexing technology. Homeostatic model assessment for insulin resistance (HOMA-IR) was calculated using fasting glucose and insulin measures (HOMA-IR = fasting glucose (mmol L−1) x fasting insulin (μU mL−1)/22.5) [25].

Statistical analysis

Descriptive statistics were calculated as the mean with standard deviation for continuous variables. For categorical variables, frequency and percentage were presented. Analysis of variance and t-test were used to compare the continuous variables between groups. Categorical variables were tested using Pearson chi-square test. Partial correlations between F2-isoprostane and metabolic and anthropometric parameters were adjusted for age, gender, race, Tanner stage and BMI (metabolic parameters only). Separate linear models, adjusted for age, gender, race, BMI and Tanner stage, were performed to assess the association between the F2-isoprostane data and the percent body fat, glucose, insulin, HOMA-IR, mean SBP and DBP, triglycerides, VO2PEAK and leptin. Interaction terms of the clinical factor and race were included in all models. All continuous variables were modelled as a linear trend due to the limited sample size. All tests were two-tailed, with a significance level of 5%. All statistical analyses were performed using open source R statistical software (version 2.13.0. Vienna, Austria).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information

Personal characteristics

Seventy-six White American (55% male) and 82 African American (48% male) youth between the ages of 8 and 17 years participated in our study. BMI percentile distribution of the entire study group was 41% normal (<85th percentile), 17% overweight (≥85th percentile to <95th percentile) and 42% obese (≥95th percentile). There were no significant differences in the mean height, weight, BMI percentile, BMI z-score or percent truncal fat between White American and African American males and females (Table 1). Significant differences were seen in Tanner stage (P = 0.020) and percent body fat (P = 0.021) between White American and African American males and females (Table 1).

Table 1. Comparison of baseline characteristics and anthropometric measurements
Age (years)FemaleMaleP-value
African American (n = 43)White American (n = 34)African American (n = 39)White American (n = 42)
13.6 ± 2.013.2 ± 2.413.4 ± 2.312.7 ± 2.40.415a
  1. Data are presented as mean and standard deviation or percentage and number.

  2. Differences between values measured by analysis of variance for continuous variables (a) or Pearson chi-square test for categorical variables (b).

  3. BMI, body mass index.

Tanner stage    0.020b
10% (0/43)6% (2/33)0% (0/37)5% (2/40) 
214% (6/43)18% (6/33)35% (13/37)48% (19/40) 
326% (11/43)30% (10/33)22% (8/37)10% (4/40) 
426% (11/43)21% (7/33)27% (10/37)25% (10/40) 
535% (15/43)24% (8/33)16% (6/37)12% (5/40) 
Height (cm)160.8 ± 7.1157.5 ± 7.3162.9 ± 13.3160.0 ± 11.90.178a
Weight (kg)67.3 ± 16.960.6 ± 19.068.7 ± 23.461.7 ± 20.00.200a
BMI percentile81.2 ± 23.773.2 ± 30.682.1 ± 22.477.1 ± 25.80.419a
BMI z-score1.2 ± 1.00.9 ± 1.21.3 ± 1.01.1 ± 1.00.383a
Body fat (% total mass)33.8 ± 10.434.9 ± 10.628.4 ± 12.428.2 ± 12.90.021a
Truncal fat (% total mass)14.6 ± 5.815.2 ± 7.112.4 ± 6.612.2 ± 7.00.117a

Oxidative stress

Urinary F2-isoprostane concentrations were used as a biomarker of oxidative stress. There was a significant correlation between F2-isoprostane concentrations and age (r = −0.232, P = 0.005). There was no significant difference in the mean F2-isoprostane concentrations between White American and African American groups (35.2 ± 18.3 and 32.0 ± 16.4 ng mg−1 creatinine, respectively; P = 0.265) (Table 2). There were also no significant differences in the mean F2-isoprostane concentrations of male and female participants (male = 33.0 ± 18.9 ng mg−1 creatinine vs. female = 34.2 ± 15.5 ng mg−1 creatinine, P = 0.660; Supporting Information Table S1).

Table 2. Comparison of baseline measures of metabolic risk in Black and White youth by race
 African AmericanWhite AmericanP-value
  1. Data are presented as mean and standard deviation.

  2. Differences between values in African Americans and White Americans as measured by t-test.

  3. DBP, diastolic blood pressure; FFA, free fatty acids; HDL, high-density lipoprotein; HOMA-IR, homeostasis model assessment-insulin resistance; LDL, low-density lipoprotein; SBP, systolic blood pressure; VO2PEAK, peak oxygen uptake.

Triglycerides (mg dL−1)64.5 ± 36.275.8 ± 38.10.071
FFA (mg dL−1)11.6 ± 6.910.2 ± 4.10.163
LDL (mg dL−1)83.3 ± 25.886.0 ± 30.30.560
HDL (mg dL−1)52.8 ± 14.248.4 ± 14.10.066
SBP (mmHg)115.4 ± 11.0111.2 ± 10.20.022
DBP (mmHg)66.2 ± 5.863.3 ± 5.40.003
Glucose (mg dL−1)99.9 ± 17.899.1 ± 16.50.774
Insulin (μU mL−1)31.6 ± 25.925.4 ± 15.30.097
HOMA-IR8.1 ± 7.96.3 ± 4.10.124
VO2PEAK (mL kg−1 min−1)30.5 ± 7.337.2 ± 11.1<0.001
Leptin (ng mL−1)17.6 ± 14.315.4 ± 14.80.375
Adiponectin (mcg mL−1)23.9 ± 21.323.4 ± 15.30.888
F2-isoprostane (ng mg−1 creatinine)32.0 ± 16.135.2 ± 18.30.265

Metabolic risk factors and oxidative stress

Lipid profile

The mean triglyceride concentration was lower in African American (64.5 ± 36.4 mg dL−1) compared to White American (75.8 ± 38.1 mg dL−1) youth, though not significantly (P = 0.071, Table 2). There was no significant difference between African American and White American youth in LDL, HDL or free fatty acid measures (Table 2). There were no significant correlations between triglyceride and F2- isoprostane measures (Table 3). When adjusted for gender, age, BMI and Tanner stage, there was no significant racial difference in the association between F2-isoprostane and triglyceride concentrations (P = 0.548) (Fig. 1a).

figure

Figure 1. (a-i) Linear regression of F2-ispotrostane as a function of risk factors for metabolic syndrome (x-axis), adjusted for age, gender, and Tanner stage in African American (black solid line, labelled ‘AA’) and White America (gray solid line, labelled ‘White’) youth. Dashed lines represent 95% confidence intervals for African American (black dashed lines) and White American (gray dashed lines) youth. Significant racial differences demonstrated only in the association of diastolic blood pressure and F2-isoprostane (P < 0.05).

HOMA-IR, homeostasis model assessment-insulin resistance; VO2PEAK, peak oxygen update.

Download figure to PowerPoint

Table 3. Partial correlation of metabolic risk factors with oxidative stress (F2-isoprostane), adjusted for age, gender, race, Tanner stage, and BMI
 Correlation (r)P-value
  1. DBP, diastolic blood pressure; FFA, free fatty acids; HDL, high-density lipoprotein; HOMA-IR, homeostasis model assessment-insulin resistance; SBP, systolic blood pressure; VO2PEAK, peak oxygen uptake.

Triglycerides (mg dL−1) (n = 137)0.0560.525
FFA (mg dL−1) (n = 124)−0.0910.338
HDL (mg dL−1) (n = 138)−0.0160.857
SBP (mmHg) (n = 132)−0.0340.708
DBP (mmHg) (n = 132)−0.0570.525
Glucose (mg dL−1) (n = 136)0.0150.891
Insulin(μU mL−1) (n = 129)−0.1030.252
HOMA-IR (n = 126)−0.1270.161
Leptin (n = 127)0.0020.985
Adiponectin (mcg mL−1) (n = 127)0.0130.889
VO2PEAK(mL kg−1 min−1) (n = 140)−0.1630.059
Blood pressure

Mean SBP was significantly higher in African American (115.4 ± 11.0 mmHg) compared to White American youth (111.2 ± 10.2 mmHg) (P = 0.022). Similarly, DBP was also significantly higher in African American (66.2 ± 5.8 mmHg) compared to White American youth (63.3 ± 5.4 mmHg) (P = 0.003) (Table 2). Neither SBP nor DBP was significantly correlated with F2-isoprostane concentrations (Table 3). However, when adjusted for gender, age, BMI and Tanner stage, there was a significant racial difference in the association between F2-isoprostane concentrations and mean DBP (P = 0.047) (Fig. 1b). No racial difference was seen in the association between F2-isoprostane concentrations and mean SBP (P = 0.884) (Fig. 1c).

Insulin sensitivity

There were no significant differences between White American and African American males and females in glucose, insulin or HOMA-IR measures (Table 2). All insulin and HOMA-IR values were included in analyses, including outliers. Additionally, there were no significant correlations between F2-isoprostane and glucose, insulin or HOMA-IR measures (Table 3). However, insulin significantly correlated with HOMA-IR (r = 0.976, P < 0.001) and leptin (r = 0.265, P = 0.002). When adjusted for gender, age, BMI and Tanner stage, no racial differences were seen in the association of F2-isoprostane concentrations with glucose (P = 0.754), insulin (P = 0.245) or HOMA-IR (P = 0.341) (Fig. 1d–f).

Measures of adiposity and oxidative stress

Percent body fat (r = 0.175, P = 0.041) and percent truncal fat (r = 0.173, P = 0.045) were significantly and positively correlated with F2-isoprostane concentrations. Correlations between BMI (r = 0.090, P = 0.288) and BMI z-score (r = 0.120, P = 0.153) with F2-isoprostane concentration were not significant (Table 4). When adjusted for gender, age and Tanner stage, no racial differences were seen in the association between F2-isoprostane concentrations and either percent truncal fat (P = 0.976) or percent body fat (P = 0.974) (Fig. 1g).

Table 4. Partial correlation of body composition measures with oxidative stress (F2-isoprostane), adjusted for age, gender, race, and Tanner stage
 Correlation (r)P-value
  1. *Significant at P < 0.05.

  2. BMI, body mass index; DBP, diastolic blood pressure; SBP, systolic blood pressure.

BMI (n = 147)0.0900.288
BMI z-score (n = 147)0.1200.153
Body fat (% total mass) (n = 141)0.1750.041*
Truncal fat (% total mass) (n = 140)0.1730.045*
Plasma leptin and adiponectin concentrations and oxidative stress

There were no significant differences in the mean plasma leptin (P = 0.375) or adiponectin (P = 0.888) concentrations between White American and African American groups (Table 2). Plasma leptin in females was higher than in males (P < 0.001, Supporting Information Table S1). Unadjusted leptin concentrations demonstrated a significant, positive relationship with F2-isoprostane measures (r = 0.231, P = 0.009). However, correlation between F2-isoprostane and leptin adjusted for age, race, gender, Tanner stage and BMI was non-significant (r = 0.002, P = 0.985) (Table 3). No racial differences were seen in the association between F2-isoprostane concentrations and leptin (P = 0.222) (Fig. 1h). There was no significant correlation between adiponectin and F2-isoprostane concentrations (r = 0.013, P = 0.889) When adjusted for gender, age, BMI and Tanner stage, no racial differences were seen in the association between F2-isoprostane concentrations and leptin (P = 0.222) (Fig. 1-h).

Cardiorespiratory fitness and oxidative stress

Average VO2PEAK was significantly lower in African Americans (30.5 ± 7.3 ml kg−1 min−1) compared to White Americans (37.2 ± 11.1 ml kg−1 min−1) (P < 0.001, Table 2) and in females (30.6 ± 8.6 ml kg−1 min−1) compared to males (37.0 ± 10.3 ml kg−1 min−1) (P < 0.001, Supporting Information Table S1). VO2PEAK was not significantly correlated with F2-isoprostane concentration (r = −0.163, P = 0.059) (Table 3). When adjusted for gender, age, BMI and Tanner stage, no racial differences were seen in the association between F2-isoprostane concentrations and VO2PEAK (P = 0.781) (Fig. 1i).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information

In this study, we explored racial differences in oxidative stress and potential associations between oxidative stress and specific metabolic risk factors in healthy African American and White American youth. First, we found that F2-isoprostane concentrations were significantly and positively associated with total body fat and truncal fat in both African American and White American youth. Second, we did not observe significant differences in F2-isoprostane concentrations between African American and White American males and females when controlled for body fat content. Third, F2-isoprostane concentrations were associated significantly and positively with DBP in the African American but not in the White American youth. And fourth, triglycerides, SBP, VO2PEAK, HOMA-IR and BMI content were not significantly associated with F2-isoprostane concentration.

Cardiovascular disease, the leading cause of death in the US, is highly prevalent in African Americans [5]. Plasma lipids are well-known risk factors for cardiovascular disease, but there are considerable racial differences in lipid profiles between African Americans and White Americans. Multiple previous reports in adults [26, 27] and children [28, 29] have demonstrated significantly lower plasma triglycerides in African American compared to White American youth. While this difference did not reach significant in our study, the racial disparity may explain, at least in part, the lower than expected prevalence of the metabolic syndrome in African Americans [19, 30]. Clinical consequences of the more favourable plasma lipid profile in African American compared to White American youth found in our study are unclear and cannot be overstated. For example, it has been postulated that African Americans have a different lipid profile threshold for cardiovascular disease than White Americans, explaining, at least in part, the well-documented ethnic disparity in cardiovascular disease prevalence in the US [31].

In addition to the impact of lipid profiles, oxidative stress has also been linked with important cardiovascular risk factors, in particular hypertension [32-34]. Although it has been shown that increased oxidative stress is associated with overt hypertension in obese children, there is minimal evidence of its association with early, pre-clinical abnormalities of blood pressure. The reason for the racial differences in our study is unclear, although the African American group had higher average body weight, BMI percentile and Tanner stage than in the White American group. Despite these differences, previous research has demonstrated that elevations in SBP and DBP seen in African American compared to White American children were not explained by body composition [35]. Also, correlation analyses pertaining to blood pressure measures used in our study were adjusted for several relevant factors (BMI, age and Tanner stage). While our small study size might have limited the detection of a racial difference in association of oxidative stress concentrations with other cardiometabolic risk factors, other recent studies with homogenous study populations suggest the potential for such racial differences. For example, Kelly et al. [11] found higher oxidative stress in children with metabolic syndrome. In a study of predominantly African American and Hispanic youth, Ostrow et al. [17] found a significant association between oxidative stress and mean 24-h SBP, but not with other markers of metabolic syndrome. These results are in agreement with the results for African American youth found in the present study. Although trends in racial differences in the association between oxidative stress and other cardiometabolic risk factors could be conceived from trends seen in our statistical models (Fig. 1), studies with larger, heterogenous study populations are needed to further clarify the relationships.

Despite the lack of a clear link between body composition and blood pressure in children and adolescents, the association of adiposity with oxidative stress has been well documented. Previous studies in both youth and adults have demonstrated that obese individuals had elevated oxidative stress levels when compared to normal weight persons, and this association was further augmented by the presence of other risk factors associated with metabolic syndrome [9-12]. In the present study, we evaluated several measures of obesity and found that both measures obtained through DXA (percent whole body and truncal fat) were significantly and positively associated with F2-isoprostane concentration. This finding suggests that overall adiposity is associated with oxidative stress more than general measures of obesity based solely upon height and weight (i.e., BMI) in youth.

A major present clinical concern is that obesity-related risk factors appearing early in childhood are tracking into adulthood. Previous studies in lean and obese children have shown significant associations of urinary isoprostanes with carotid intima-media thickness [36], with isoprostanes considered an independent risk factor for coronary heart disease [37]. We did not measure the intima-media thickness and thus, could not speculate about the relationship of adiposity-induced increased oxidative stress and cardiovascular risk in our study population.

The study has several strengths. The collection of urine for F2-isoprostane concentration, as well as all metabolic and cardiorespiratory measures, took place within the highly controlled environment of the indirect room calorimeter. Second, we used reference standard DXA measurements for estimations of adiposity. While waist circumference is often used as a proxy measure of truncal adiposity, DXA measures of truncal/abdominal fat mass are highly correlative with abdominal and visceral fat estimates obtained from computed tomography scans [38]. Third, we have a study sample with a wide range of age and BMI percentiles (from 4th to 99th percentiles for age and gender).

The study also has some limitations. First, it is a cross-sectional study limited to White American and African American youth, which does not enable us to determine whether there is a causative link among oxidative stress, adiposity and metabolic risk factors. Second, the study would have benefited from the measurement of water-soluble markers of early oxidation such as thiobarbituric acid reactive substances [39] or total antioxidant capacity [40, 41]. However, F2-isoprostane concentrations, which assess oxidation of lipids [42], have been shown to provide one of the most accurate assessments of oxidative stress status, and in turn, a more likely occurrence of endothelial dysfunction [43, 44] and increased risk of cardiovascular disease. Additionally, the F2-isoprostane data are reported per mg creatinine, which is linked to lean body mass, as opposed to fat mass, which is the key correlate of F2-isoprostane. Finally, we used HOMA-IR as a surrogate marker of insulin resistance in our population [45]. A reference standard hyperinsulinemic-euglycemic clamp method would have provided results that are more reliable, but the method is invasive, less practical and costly in large studies. Several previous studies have demonstrated acceptable correlation between HOMA-IR and the hyperinsulinemic-euglycemic clamp or IV glucose tolerance test [46, 47], but a recent international consensus statement concluded the correlation was actually quite low [48]. In our study, HOMA-IR results were on average higher than reported in other studies in healthy youth [49-51]. A plausible explanation is that with higher than expected average insulin and HOMA-IR values, and with large standard deviations, some study participants did not comply fully with the prescribed overnight fast before the study visit. However, the results also bring into question the reliability of insulin assays in children in general [48].

In summary, oxidative stress, as measured by urinary F2-isoprostane concentrations, was positively associated with percent body fat and leptin in youth. Oxidative stress levels were similar among African American and White American youth. Among markers of the metabolic syndrome, a significant difference between African American and White American youth was demonstrated only in the association of oxidative stress with DBP.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information

This work was supported by grants from the NIH (RO1HL082988) and the National Center for Research Resources, Grant UL1 RR024975-01, and is now at the National Center for Advancing Translational Sciences, Grant 2 UL1 TR000445-06. KRC was supported by a Vanderbilt Research Training in Diabetes and Endocrinology grant (NIH T32 DK07061-35) and JW by a NIDDK training grant (NIH T32 DK007673-17). Additional support came from an NIH MERIT Award (GM42056) awarded to LJR. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Health. Hormone, lipid and isoprostane assays were performed in the core laboratories of the Vanderbilt Diabetes Research and Training Center supported by NIH grant DK20593. We would like to thank Stephane Daphnis, Denise Dunlap, Cindy Dorminy, Natalie Meade, Elizabeth Provenzano and Daniel Short for their contributions to the acquisition of the data for this study.

KRC and JLK contributed to study conception, design and conduct of the experiments. LEW contributed to design and conduct of the experiments. LW contributed to data analysis and interpretation. JW contributed to data analysis and interpretation and writing of the article. SAA and LJR contributed to design and data analysis and interpretation. MSB contributed to the study conception, design, data analysis and interpretation, and revision of the article.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information
  • 1
    Ogden CL, Carroll MD, Curtin LR, McDowell MA, Tabak CJ, Flegal KM. Prevalence of overweight and obesity in the United States, 1999–2004. JAMA 2006; 295: 15491555.
  • 2
    Freedman DS, Dietz WH, Srinivasan SR, Berenson GS. The relation of overweight to cardiovascular risk factors among children and adolescents: the Bogalusa Heart Study. Pediatrics 1999; 103(6 Pt 1): 11751182.
  • 3
    Hannon TS, Rao G, Arslanian SA. Childhood obesity and type 2 diabetes mellitus. Pediatrics 2005; 116: 473480.
  • 4
    Clark LT, El-Atat F. Metabolic syndrome in African Americans: implications for preventing coronary heart disease. Clin Cardiol 2007; 30: 161164.
  • 5
    Cossrow N, Falkner B. Race/ethnic issues in obesity and obesity-related comorbidities. J Clin Endocrinol Metab 2004; 89: 25902594.
  • 6
    Brownlee M. The pathobiology of diabetic complications: a unifying mechanism. Diabetes 2005; 54: 16151625.
  • 7
    Robertson RP. Chronic oxidative stress as a central mechanism for glucose toxicity in pancreatic islet beta cells in diabetes. J Biol Chem 2004; 279: 4235142354.
  • 8
    Furukawa S, Fujita T, Shimabukuro M, et al. Increased oxidative stress in obesity and its impact on metabolic syndrome. J Clin Invest 2004; 114: 17521761.
  • 9
    Van Guilder GP, Hoetzer GL, Greiner JJ, Stauffer BL, Desouza CA. Influence of metabolic syndrome on biomarkers of oxidative stress and inflammation in obese adults. Obesity 2006; 14: 21272131.
  • 10
    Araki S, Dobashi K, Yamamoto Y, Asayama K, Kusuhara K. Increased plasma isoprostane is associated with visceral fat, high molecular weight adiponectin, and metabolic complications in obese children. Eur J Pediatr 2010; 169: 965970.
  • 11
    Kelly AS, Steinberger J, Kaiser DR, Olson TP, Bank AJ, Dengel DR. Oxidative stress and adverse adipokine profile characterize the metabolic syndrome in children. J Cardiometab Syndr 2006; 1: 248252.
  • 12
    Oliver SR, Rosa JS, Milne GL, et al. Increased oxidative stress and altered substrate metabolism in obese children. Int J Pediatr Obes 2010; 5: 436444.
  • 13
    Sinaiko AR, Steinberger J, Moran A, et al. Relation of body mass index and insulin resistance to cardiovascular risk factors, inflammatory factors, and oxidative stress during adolescence. Circulation 2005; 111: 19851991.
  • 14
    Stringer DM, Sellers EA, Burr LL, Taylor CG. Altered plasma adipokines and markers of oxidative stress suggest increased risk of cardiovascular disease in First Nation youth with obesity or type 2 diabetes mellitus. Pediatr Diabetes 2009; 10: 269277.
  • 15
    Ustundag B, Gungor S, Aygun AD, Turgut M, Yilmaz E. Oxidative status and serum leptin levels in obese prepubertal children. Cell Biochem Funct 2007; 25: 479483.
  • 16
    Codoñer-Franch P, Boix-García L, Simó-Jordá R, del Castillo-Villaescusa C, Maset-Maldonado J, Valls-Bellés V. Is obesity associated with oxidative stress in children? Int J Pediatr Obes 2010; 5: 5663.
  • 17
    Ostrow V, Wu S, Aguilar A, Bonner R, Jr, Suarez E, De Luca F. Association between oxidative stress and masked hypertension in a multi-ethnic population of obese children and adolescents. J Pediatr 2011; 158: 628633 e621.
  • 18
    Hoffman RP. Metabolic syndrome racial differences in adolescents. Curr Diabetes Rev 2009; 5: 259265.
  • 19
    Sumner AE. Ethnic differences in triglyceride levels and high-density lipoprotein lead to underdiagnosis of the metabolic syndrome in black children and adults. J Pediatr 2009; 155: S7 e7–e11.
  • 20
    CDC. National Health and Nutrition Examination: Anthropometry Procedures Manual. US Department of Health and Human Services, Centers for Disease Control and Prevention: Hyattsville, MD, 2009.
  • 21
    Centers for Disease Control and Prevention, National Center for Health Statistics. CDC growth charts: United States. [WWW document]. URL http://www.cdc.gov/growthcharts/ (accessed 30 May 2000).
  • 22
    National High Blood Pressure Education Program Working Group on High Blood Pressure in Children and Adolescents. The fourth report on the diagnosis, evaluation, and treatment of high blood pressure in children and adolescents. Pediatrics 2004; 114(2 Suppl. 4th Report): 555576.
  • 23
    Bruce RA. Exercise testing of patients with coronary heart disease. Principles and normal standards for evaluation. Ann Clin Res 1971; 3: 323332.
  • 24
    Morrow JD, Zackert WE, Yang JP, et al. Quantification of the major urinary metabolite of 15-F2t-Isoprostane (8-iso-PGF2α) by a stable isotope dilution mass spectrometric assay. Anal Biochem 1999; 269: 326331.
  • 25
    Matthews DR, Hosker JP, Rudenski AS, Naylor BA, Treacher DF, Turner RC. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 1985; 28: 412419.
  • 26
    Donahue R, Jacobs DR, Jr, Sidney S, Wagenkneckt L, Albers J, Hunley S. Distribution of lipoproteins and apolipoproteins in young adults: the CARDIA Study. Arteriosclerosis 1989; 9: 656664.
  • 27
    Sumner A, Cowie CC. Ethnic differences in the ability of triglyceride levels to identify insulin resistance. Atherosclerosis 2008; 196: 696703.
  • 28
    Srinivasan SR, Frerichs R, Webber L, Berenson GS. Serum lipoprotein profile in children from a biracial commnity: the Bogalusa Heart Study. Circulation 1976; 54: 309318.
  • 29
    Cook S, Weitzman M, Auinger P, Nguyen M, Dietz WH. Prevalence of a metabolic syndrome phenotype in adolescents: findings from the third national health and nutrition examination survery, 1988–1994. Arch Pediatr Adolesc Med 2003; 157: 821827.
  • 30
    Yu SSK, Castillo DC, Courville AB, Sumner AE. The triglyceride paradox in people of African descent. Metab Syndr Relat Disord 2012; 10: 7782.
  • 31
    Stein E, Kushner H, Gidding S, Falkner B. Plasma lipid concentrations in nondiabetic African American adults: associations with insulin resistance and the metabolic syndrome. Metabolism 2007; 56: 954960.
  • 32
    Rodrigo R, Prat H, Passalacqua W, Araya J, Guichard C, Bachler J. Relationship between oxidative stress and essential hypertension. Hypertens Res 2007; 30: 11591167.
  • 33
    Grossman E. Does increased oxidative stress cause hypertension? Diabetes Care 2008; 31(Suppl. 2): S185S189.
  • 34
    Ceriello A. Possible role of oxidative stress in the pathogenesis of hypertension. Diabetes Care 2008; 31(Suppl. 2): S181S184.
  • 35
    Cruz ML, Huang TT, Johnson MS, Gower BA, Goran MI. Insulin sensitivity and blood pressure in black and white children. Hypertension 2002; 40: 1822.
  • 36
    Giannini C, de Giorgis T, Scarinci A, et al. Increased carotid intima-media thickness in pre-pubertal children with constitutional leanness and severe obesity: the speculative role of insulin sensitivity, oxidant status, and chronic inflammation. Eur J Endocrinol 2009; 161: 7380.
  • 37
    Schwedhelm E, Bartling A, Lenzen H, et al. Urinary 8-iso-prostaglandin F2alpha as a risk marker in patients with coronary heart disease: a matched case-control study. Circulation 2004; 109: 843848.
  • 38
    Clasey JL, Bouchard C, Teates CD, et al. The use of anthropometric and dual-energy X-ray absorptiometry (DXA) measures to estimate total abdominal and abdominal visceral fat in men and women. Obes Res 1999; 7: 256264.
  • 39
    Yagi K. A simple fluorometric assay for lipoperoxide in blood plasma. Biochem Med 1976; 15: 212216.
  • 40
    Davalos A, Gomez-Cordoves C, Bartolome B. Extending applicability of the oxygen radical absorbance capacity (ORAC-fluorescein) assay. J Agric Food Chem 2004; 52: 4854.
  • 41
    Apak R, Guclu K, Ozyurek M, Bektasoglu B, Bener M. Cupric ion reducing antioxidant capacity assay for antioxidants in human serum and for hydroxyl radical scavengers. Methods Mol Biol 2010; 594: 215239.
  • 42
    Morrow JD, Hill KE, Burk RF, Nammour TM, Badr KF, Roberts LJ, II. A series of prostaglandin F2-like compounds are produced in vivo in humans by a non-cyclooxygenase, free radical-catalyzed mechanism. Proc Natl Acad Sci USA 1990; 87: 93839387.
  • 43
    Milne GL, Yin H, Hardy KD, Davies SS, Roberts LJ. Isoprostane generation and function. Chem Rec 2011; 111: 59735996.
  • 44
    Kadiiska MB, Gladen BC, Baird DD, et al. Biomarkers of oxidative stress study II: are oxidation products of lipids, proteins, and DNA markers of CCl4 poisoning? Free Radic Biol Med 2005; 38: 698710.
  • 45
    Shaibi GQ, Davis JN, Weigensberg MJ, Goran MI. Improving insulin resistance in obese youth: choose your measures wisely. Int J Pediatr Obes 2011; 6: e290e296.
  • 46
    Gungor N, Saad R, Janosky J, Arslanian S. Validation of surrogate estimates of insulin sensitivity and insulin secretion in children and adolescents. J Pediatr 2004; 144: 4755.
  • 47
    Keskin M, Kurtoglu S, Kendirci M, Atabek ME, Yazici C. Homeostasis model assessment is more reliable than the fasting glucose/insulin ratio and quantitative insulin sensitivity check index for assessing insulin resistance among obese children and adolescents. Pediatrics 2005; 115: e500e503.
  • 48
    Levy-Marchal C, Arslanian S, Cutfield W, et al. Insulin resistance in children: consensus, perspective, and future directions. J Clin Endocrinol Metab 2010; 95: 51895198.
  • 49
    d'Annunzio G, Vanelli M, Pistorio A, et al. Insulin resistance and secretion indexes in healthy Italian children and adolescents: a multicentre study. Acta Biomed 2009; 80: 2128.
  • 50
    Conwell LS, Trost SG, Brown WJ, Batch JA. Indexes of insulin resistance and secretion in obese children and adolescents: a validation study. Diabetes Care 2004; 27: 314319.
  • 51
    Adam TC, Hasson RE, Lane CJ, et al. Fasting indicators of insulin sensitivity: effects of ethnicity and pubertal status. Diabetes Care 2011; 34: 994999.

Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conflicts of Interest Statement
  8. Acknowledgements
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
ijpo135-sup-0001-si.docx15K

Table S1. Comparison of baseline measures of metabolic risk in Black and White youth by gender.

Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.