Metabolic Syndrome, Inflammation, Heart Rate Variability, and Fitness in Obese African American Youth

Purpose. The purposes of this dissertation were to (1) explore the relationships among fitness, heart rate variability (HRV), inflammation and components of the Metabolic Syndrome in obese African American youth participating in a supervised weight management program, (2) examine the effect of change in severity of obesity on these variables, and (3) identify predictors of weight loss. Method. This study consisted of a post-hoc analysis of existing pre and post-intervention data obtained from 50 African-American youth (70% female, aged 7-18 years). Anthropometric assessment of height, weight, waist circumference, systolic blood pressure, body mass index, body mass index percentile ranges, and relative body mass index were conducted. Laboratory measures included high-density lipoprotein cholesterol, triglycerides, fibrinogen, and C-reactive protein levels, and fasting and 2-hr postprandial insulin and glucose levels obtained during a mixed meal tolerance test for determination of impaired glucose tolerance and insulin resistance. Aerobic fitness was assessed by cardiopulmonary maximal treadmill exercise using the Branching Protocol (VO2max). Heart rate variability was measured with a 24-hour Holter monitor and measures of circadian fluctuation (SDNN), parasympathetic function (HF), and sympathetic/ parasympathetic balance (LF/HF ratio) were calculated. Measures were repeated at 6 months. Data were analyzed using descriptive statistics, Pearson’s correlation coefficient, non-parametric t-tests and multiple linear regression. Metabolic Syndrome was defined based on World Health Organization definition with age, gender, and race-specific cut-points. Results. Baseline measures revealed the following means: age 12.9 ± 2.9, weight 92.2 kg ± 28.4, relative body mass index 195.5 ± 42.4, HF 5.9 [ln(ms2)] ± 0.9, SDNN 128.1(ms) ± 38.2, LF/HF ratio 1.1 ± 0.1, VO2max 18.8 ± 4.5 mg/kg/min, C-reactive protein 0.75 mg/L ± 1.34, fibrinogen 372.64 mg/dl ± 71.12. The prevalence of the Metabolic Syndrome was 46% based on 39 subjects with complete data sets. The presence of the individual Metabolic Syndrome components, for the entire sample, was as follows: systolic blood pressure 44%, high density lipoprotein 20%, triglycerides 40%, impaired glucose tolerance 18%, insulin resistance 82%, and waist circumference 73%. Inflammation was common (elevated C-reactive protein 45%, elevated fibrinogen 42%). VO2max was positively correlated with HF (r = 0.41, p = 0.004) and SDNN (r = 0.39, p = 0.007) and negatively with fibrinogen (r =-0.53, p = 0.0002) and relative body mass index (r = -0.47, p = 0.0007). Relative body mass index was positively associated with C-reactive protein (r = 0.38, p = 0.01) and fibrinogen (r = 0.51, p = 0.003). Those with impaired glucose tolerance did not differ in HRV measures, fitness, or inflammation. Insulin resistance was associated with HF (p = 0.004) and fibrinogen (p = 0.02). Waist circumference was associated with LF/HF ratio (p = 0.02). Lower high density lipoprotein was associated with higher fibrinogen (p = 0.05). There were no significant differences in HRV, VO2max, or inflammation in youth with or without Metabolic Syndrome. Change in relative body mass index was associated with improved SDNN (r = 0.36, p = 0.04) and there was a trend toward better VO2max (r = -0.30, p = 0.07) and HF (r = 0.33, p = 0.07). Male gender and higher baseline fitness accounted for 28% of the variance in RBMI change. Conclusions. Metabolic Syndrome is common in obese youth, even in younger children. Insulin resistance, even in the absence of the Metabolic Syndrome, negatively affects HRV. Mild weight loss is associated with improvements in SDNN, a risk factor for sudden death. Strategies to improve weight loss and fitness in severely obese African American youth are needed