Comparing BMI with skinfolds to estimate age at adiposity rebound and its associations with cardio-metabolic risk markers in adolescence.

BACKGROUND: Body mass index (BMI) reaches a nadir in mid-childhood, known as the adiposity rebound (AR). Earlier AR is associated with a higher risk of cardio-vascular diseases in later life. Skinfolds, which are a more direct measure of adiposity, may give better insight into the relationship between childhood adiposity and later obesity and cardio-metabolic risk. OBJECTIVE: We aimed to assess whether AR corresponds to a rebound in skinfolds, and compare associations of BMI-derived AR and skinfold-derived AR with cardio-metabolic risk markers in adolescence. METHODS: We used penalised splines with random coefficients to estimate BMI and skinfold trajectories of 604 children from the Mysore Parthenon Birth Cohort. Age at AR was identified using differentiation of the BMI and skinfold growth curves between 2 and 10 years of age. At 13.5 years, we measured blood pressure, and glucose, insulin and lipid concentrations. RESULTS: BMI and skinfolds had different growth patterns. Boys reached BMI-derived AR earlier than skinfold-derived AR (estimated difference: 0.41 years; 95% CI:[0.23, 0.56]), whereas the opposite was observed in girls (estimated difference: -0.71 years; 95% CI:[-0.90, -0.54]). At 13.5 years, children with earlier BMI-derived AR had higher BMI (-0.58 SD per SD increase of AR; 95%CI:[-0.65, -0.52]), fat mass (-0.44; 95%CI:[-0.50, -0.37]), insulin resistance (HOMA-IR: -0.20; 95%CI:[-0.28, -0.12]) and systolic blood pressure (-0.20; 95%CI:[-0.28, -0.11]), and lower HDL-cholesterol (0.12; 95%CI:[0.04, 0.21]). The associations were independent of BMI at time of rebound, but were fully explained by fat mass at 13.5 years. Similar associations were found for skinfold-derived AR. CONCLUSION: BMI-derived adiposity rebound predicts later cardio-metabolic risk markers similarly to that derived from skinfolds, a direct measure of adiposity.

Adiposity and Cortisol Response to Stress in Indian Adolescents.

OBJECTIVE: We examined associations of different adiposity measures with cortisol responses during the Trier Social Stress Test for children (TSST-C). DESIGN: Descriptive study. SETTING: Holdsworth Memorial Hospital, Mysore, India. PARTICIPANTS: Adolescents aged 13.5y from a birth cohort were recruited (N=269, 133 boys). METHODS: The stressor (TSST-C) was 5-minutes each of public speaking and mental arithmetic tasks in front of two unfamiliar 'judges'. Salivary cortisol concentrations were measured at baseline and at regular intervals after TSST-C. Weight, height, sub scapular and triceps skinfold thickness, and waist and hip circumference were measured, and percentage body fat was estimated (fat%; bioimpedance). Body mass index (BMI) and Waist-to-hip ratio (WHR) were calculated. All variables were converted into within-cohort SD scores before analysis. Stress-induced change in cortisol concentrations from baseline (cortisol response) was examined in relation to adiposity. RESULTS: Stress increased cortisol concentrations significantly from baseline (mean (SD): 5.5 (6.4) ng/mL; P<0.001). Higher WHR was associated with lower cortisol response at 20 and 30-minutes after stress (~0.13 SD decrease in cortisol response per SD higher WHR, P<0.05). Higher fat% was also associated with lower cortisol response only in girls 20-minutes post-stress (0.23 SD lower response per SD higher fat%, P=0.004). Sum of skinfold thickness and BMI were not associated with cortisol responses. CONCLUSION: Abdominal adiposity is associated with reduced hypothalamic-pituitary-adrenal axis reactivity to stress in this adolescent population.