Courtney L. Jennings

Determinants of Insulin-resistant Phenotypes in Normal-weight and Obese Black African Women

Objective: Subsets of metabolically “healthy obese” and “at‐risk” normal‐weight individuals have been previously identified. The aim of this study was to explore the determinants of these phenotypes in black South African (SA) women.

The reliability of the FitroDyne as a measure of muscle power.

The FitroDyne is a device that attaches to conventional resistance-training equipment to measure speed of movement, from which muscle power is calculated. The aim of this study was to quantify the repeatability of the measurement of muscle power with the FitroDyne during squat jump and biceps curl exercises. Thirty male subjects completed 3 trials, each consisting of 6 squat jumps and 6 biceps curls of increasing loads. Upper body and lower body maximum power was predicted from the force-velocity curves derived from the range of weights used for each trial. Maximum power measurements of a squat jump (range, 911–1,673 W) and biceps curl (range, 45–110 W) had intraclass correlation coefficients (ICC) of R = 0.97 (95% CI, 0.95–0.98) and R = 0.97 (95% CI, 0.95–0.98), respectively. The limits of agreement for the squat jump and biceps curl trials were 217 ± 96 W and 0.11 ± 13.90 W, respectively. It may be concluded that muscle power can be measured with a high degree of reliability with the FitroDyne. The limits of agreement need to be considered when data are interpreted.

Maintenance of plasma volume and serum sodium concentration despite body weight loss in ironman triathletes.

Objective:To examine the relationship between body weight, plasma volume, and serum sodium concentration ([Na+]) during prolonged endurance exercise. Design:Observational field study. Settings:2000 South African Ironman Triathlon. Participants:181 male triathletes competing in an Ironman triathlon. Main Outcome Measures:Body weight, plasma volume, and serum ([Na+]) change from pre- to postrace. Results:Significant body weight loss occurred (−4.9 ± 1.7%; P < 0.0001), while both plasma volume (1.0 ± 11.2%; P = 0.4: NS) and serum [Na+] (0.6 ± 2.4%; P < 0.001) increased from pre- to postrace. Blood volume (−0.6 ± 6.6%) and red cell volume (−2.6 ± 5.5%; P < 0.001) decreased in conjunction with the body weight loss. There was a strong correlation between blood and plasma volume change, both as a percentage, and absolute change in fluid volume (r = 0.9; P < 0.001). Body weight change was positively correlated with plasma volume change (r = −0.4; P < 0.001), but inversely correlated with serum [Na+] change (r = −0.4; P < 0.001). Plasma volume change was not significantly correlated with serum [Na+] change (r = 0.0; NS). Serum [Na+] change was inversely correlated with both percentage of red cell volume change (r = −0.2; P < 0.05) and percentage body weight change (r = −0.4; P < 0.001). Conclusion:Plasma volume and serum [Na+] were maintained in male Ironman triathletes, despite significant (5%) body weight loss during the course of the race. Body weight was not an accurate “absolute” surrogate of fluid balance homeostasis during prolonged endurance exercise. Clinicians should be warned against viewing these three regulatory parameters as interchangeable during an Ironman triathlon.

The association of interleukin-18 genotype and serum levels with metabolic risk factors for cardiovascular disease.

OBJECTIVE Circulating levels of interleukin (IL)-18 are associated with the metabolic syndrome and risk for the development of cardiovascular disease (CVD). This study investigated the association between the circulating IL-18 levels and the -137 G/C polymorphism within the IL-18 gene with metabolic risk factors for CVD in normal-weight and obese black South African women. METHODS Blood pressure (BP), body composition (dual-energy X-ray absorptiometer), visceral adiposity (computerized tomography), as well as fasting glucose, insulin, lipid profile, IL-18 levels, and IL-18 genotype were measured in 104 normal-weight (body mass index (BMI) < or = 25 kg/m2) and 124 obese (BMI > or = 30 kg/m2) black South African women. RESULTS Subjects with a GC genotype (23%) had a greater mean arterial pressure (MAP, 90.6+/-11.1 vs 85.5+/-10.3 mmHg, P<0.001) than the subjects with the GG genotype. Serum IL-18 levels were not associated with IL-18 genotype (P=0.985); however, they significantly correlated with percentage of body fat (r=0.25, P<0.001), visceral adiposity (r=0.32, P<0.001), MAP (r=0.22, P=0.001), HOMA-IR (r=0.33, P<0.001), fasting insulin (r=0.25, P<0.001), triglyceride (r=0.16, P<0.05), and high-density lipoprotein-cholesterol (r=-0.14, P<0.05) levels, after adjusting for age and body fatness. CONCLUSIONS We show for the first time that the GC genotype of the IL-18 -137 G/C polymorphism and the circulating IL-18 levels are independently associated with raised BP. Moreover, fasting IL-18 levels are associated with the other metabolic risk factors for CVD in normal-weight and obese black South African women.

Tumor Necrosis Factor-α Gene -308 G/A Polymorphism Modulates the Relationship between Dietary Fat Intake, Serum Lipids, and Obesity Risk in Black South African Women

The prevalence of obesity and related disease risk is high in black South African (SA) women, possibly influenced by the dietary transition associated with urbanization. This study explored interactions between dietary fat intake and the tumor necrosis factor-alpha (TNFA) -308 G/A polymorphism on obesity, insulin resistance, and serum lipid concentrations in urbanized black SA women. Normal-weight (n = 105) and obese (n = 118) women underwent measurements of body composition, fat distribution, fasting serum lipids, glucose and insulin concentrations, and dietary intake. Participants were genotyped for the functional TNFA -308 G/A polymorphism. The genotype or allele frequency of the TNFA -308 G/A polymorphism did not differ between the BMI groups. However, when dietary fat intake was 30% of total energy intake [percentage energy (%E)], the odds of being obese with the TNFA GA+AA genotype was only 12% of that with GG, but increasing intake of dietary fat (%E) was associated with a significantly faster rate of increase in obesity risk in women with the TNFA GA+AA genotype compared with those with the GG genotype (P = 0.036). There were significant diet-gene interactions between alpha-linolenic acid (%E) and the total cholesterol:HDL-cholesterol ratio (P = 0.036), and PUFA (%E) and LDL cholesterol levels (P = 0.026), with participants with the A allele being more responsive to changes in relative fat intake. The TNFA -308 G/A polymorphism modified the relationship between dietary fat intake, obesity risk, and serum lipid concentrations in black SA women.

The atypical presentation of the metabolic syndrome components in black African women: the relationship with insulin resistance and the influence of regional adipose tissue distribution

The appropriateness of the metabolic syndrome criteria as an indicator of cardiovascular disease risk has been challenged in black Africans. Hence, the aims of this study were (1) to examine the level of agreement between the International Diabetes Federation (IDF) and the National Cholesterol Education Program Adult Treatment Panel III (ATP III) metabolic syndrome criteria, which differ in their emphasis on central obesity; (2) to investigate the degree to which these criteria predict insulin resistance, as estimated by the homeostasis model assessment of insulin resistance (HOMA-IR); and (3) to investigate the extent to which a diagnosis of the metabolic syndrome and insulin resistance may be explained by body fat and its distribution. In 103 normal-weight (body mass index <or=25 kg/m(2), mean: 22.0 +/- 1.8 kg/m(2)) and 119 obese (body mass index >or=30 kg/m(2), mean: 33.9 +/- 5.5 kg/m(2)) urbanized black South African women (27 +/- 7 years old), body composition (dual-energy x-ray absorptiometry), fat distribution (waist and computed tomography), blood pressure, fasting glucose, HOMA-IR, and lipid profiles were measured. Insulin resistance was defined as the upper tertile of HOMA-IR. The overall proportion of individuals who met the IDF and ATP III metabolic syndrome criteria were 13% and 10%, respectively. Agreement was high between the IDF and ATP III metabolic syndrome criteria (kappa = 0.87); however, neither criteria predicted HOMA-IR (kappa = 0.16, 95% confidence interval: 0.05-0.27 and 0.14, 95% confidence interval: 0.05-0.27, respectively). Visceral adipose tissue was the largest contributor to diagnosis of the metabolic syndrome, and waist alone (>80 cm or >88 cm) had an improved specificity (21% or 18% higher, respectively) and positive predictive value (64% or 57% higher, respectively) for identifying insulin resistance compared with the metabolic syndrome criteria. Waist circumference was a better predictor of HOMA-IR than the IDF or ATP III metabolic syndrome criteria in young black African women without known disease. The measurement of waist circumference, as an indicator of disease risk, should therefore be encouraged in the public health setting.

Dual-energy X-ray Absorptiometry and Anthropometric Estimates of Visceral Fat in Black and White South African Women

Visceral adipose tissue (VAT) is associated with increased risk for cardiovascular disease, and therefore, accurate methods to estimate VAT have been investigated. Computerized tomography (CT) is the gold standard measure of VAT, but its use is limited. We therefore compared waist measures and two dual‐energy X‐ray absorptiometry (DXA) methods (Ley and Lunar) that quantify abdominal regions of interest (ROIs) to CT‐derived VAT in 166 black and 143 white South African women. Anthropometry, DXA ROI, and VAT (CT at L4–L5) were measured. Black women were younger (P < 0.001), shorter (P < 0.001), and had higher body fat (P < 0.05) than white women. There were no ethnic differences in waist (89.7 ± 18.2 cm vs. 90.1 ± 15.6 cm), waist:height ratio (WHtR, 0.56 ± 0.12 vs. 0.54 ± 0.09), or DXA ROI (Ley: 2.2 ± 1.5 vs. 2.1 ± 1.4; Lunar: 2.3 ± 1.4 vs. 2.3 ± 1.5), but black women had less VAT, after adjusting for age, height, weight, and fat mass (76 ± 34 cm2 vs. 98 ± 35 cm2; P < 0.001). Ley ROI and Lunar ROI were correlated in black (r = 0.983) and white (r = 0.988) women. VAT correlated with DXA ROI (Ley: r = 0.729 and r = 0.838, P < 0.01; Lunar: r = 0.739 and r = 0.847, P < 0.01) in black and white women, but with increasing ROI android fatness, black women had less VAT. Similarly, VAT was associated with waist (r = 0.732 and r = 0.836, P < 0.01) and WHtR (r = 0.721 and r = 0.824, P < 0.01) in black and white women. In conclusion, although DXA‐derived ROIs correlate well with VAT as measured by CT, they are no better than waist or WHtR. Neither DXA nor anthropometric measures are able to accurately distinguish between high and low levels of VAT between population groups.

Diagnostic Ability of Obesity Measures to Identify Metabolic Risk Factors in South African Women

BACKGROUND Currently, guidelines for obesity thresholds relating to metabolic risk in South African women have not been established. Therefore, the aim of the study was to investigate the level and diagnostic ability of obesity measures [waist circumference (WC), waist-to-height ratio (WHtR), and visceral adipose tissue (VAT) area] to identify black and white South African women with elevated blood pressure, dyslipidemia, and insulin resistance. METHODS Blood pressure, fasting insulin, glucose, and lipids were measured in 241 black and 188 white South African women. Receiver operator characteristic (ROC) curve analyses were performed to determine the diagnostic ability of WC, WHtR, and computer tomography (CT)-derived VAT to identify subjects above metabolic risk thresholds. The Youden index was used to calculate obesity thresholds for metabolic risk variables. RESULTS WC, WHtR, and VAT were significant determinants of all metabolic risk variables (P<0.05), and differences in the ROC area under the curve (AUC) between obesity measures were small (≈0.08) for all metabolic risk variables, in both ethnic groups. However, the ROC AUC vales for all obesity measures were greater in white compared to black women (P<0.01). WC and VAT thresholds were lower in black women compared to white women, whereas WHtR thresholds varied less between ethnicities. CONCLUSIONS Due to the cost, access, and radiation exposure, CT-derived VAT is not recommended above the use of simple anthropometric measures (WC and WHtR) for the determination of metabolic risk. Furthermore, thresholds of WHtR, due to low variability between ethnicities, may be more useful than WC for ethnic comparisons of risk.

Comparison of body fatness measurements by near-infrared reactance and dual-energy X-ray absorptiometry in normal-weight and obese black and white women.

The aim of the present study was to compare body fat percent (BF %) using single-site near-IR reactance (NIR) and dual-energy X-ray absorptiometry (DXA) in a cohort of normal-weight (BMI < 25 kg/m2) black (n 102) and white (n 71); and obese (BMI > or = 30 kg/m2) black (n 117) and white (n 41) South African women (18-45 years). NIR-derived BF % was significantly correlated with DXA-derived BF % in all groups: normal-weight black (r 0.55, 95 % CI: 0.40, 0.67, P < 0.001) and white (r 0.69, 95 % CI: 0.53, 0.79, P < 0.001) women; obese black (r 0.59, 95 % CI: 0.46, 0.70, P < 0.001) and white (r 0.56, 95 % CI: 0.30, 0.74, P < 0.001) women. NIR under-predicted BF% compared to DXA in black women (normal-weight, - 4.36 (sd 4.13) % and obese, - 3.41 (sd 3.72) %), while smaller mean differences were observed in white women (normal-weight, - 0.29 (sd 4.19) % and obese, - 0.81 (sd 3.09) %), irrespective of normal-weight or obese status (P < 0.001). In obese subjects, NIR-derived BF % did not measure values greater than approximately 45 %, while the maximum DXA-derived measure was 58 %. In conclusion, although there was a significant relationship between NIR- and DXA-derived BF %, NIR under-predicted BF % in normal-weight and obese black South African women compared to DXA, but to a greater extent in subjects with very high levels of adiposity (>45 %). The results of single-site NIR as a measure of BF % should therefore be interpreted with caution, particularly in women of African descent and in those with very high levels of adiposity.

Association Between the 4 bp Proinsulin Gene Insertion Polymorphism (IVS-69) and Body Composition in Black South African Women

The objective of the study was to examine the association between a functional 4 bp proinsulin gene insertion polymorphism (IVS‐69), fasting insulin concentrations, and body composition in black South African women. Body composition, body fat distribution, fasting glucose and insulin concentrations, and IVS‐69 genotype were measured in 115 normal‐weight (BMI <25 kg/m2) and 138 obese (BMI ≥30 kg/m2) premenopausal women. The frequency of the insertion allele was significantly higher in the class 2 obese (BMI ≥35kg/m2) compared with the normal‐weight group (P = 0.029). Obese subjects with the insertion allele had greater fat mass (42.3 ± 0.9 vs. 38.9 ± 0.9 kg, P = 0.034) and fat‐free soft tissue mass (47.4 ± 0.6 vs. 45.1 ± 0.6 kg, P = 0.014), and more abdominal subcutaneous adipose tissue (SAT, 595 ± 17 vs. 531 ± 17 cm2, P = 0.025) but not visceral fat (P = 0.739), than obese homozygotes for the wild‐type allele. Only SAT was greater in normal‐weight subjects with the insertion allele (P = 0.048). There were no differences in fasting insulin or glucose levels between subjects with the insertion allele or homozygotes for the wild‐type allele in the normal‐weight or obese groups. In conclusion, the 4 bp proinsulin gene insertion allele is associated with extreme obesity, reflected by greater fat‐free soft tissue mass and fat mass, particularly SAT, in obese black South African women.