The Relation of Fatness to Insulin is Independent of Fitness in 9-but Not 15-yr-olds

To explore the relationship between varying aerobic fitness (fitness), fatness, and fasting insulin levels in healthy children. A population-based sample of 9-yr-old (9YO, 47 boys, 56 girls) and 15-yr-old (15YO, 53 boys, 51 girls) Icelandic children. Body fatness was evaluated via body mass index, waist circumference adjusted for height (waist adj), and sum of four skinfolds. Fitness was assessed with a graded maximal cycle ergometer test. Fasting insulin was measured using an ECLIA. Fasting insulin correlated to all fatness measures (9YO, r = 0.43-0.46, P < 0.001; 15YO, r = 0.30-0.37, P < 0.003) and fitness (9YO, r = -0.29, P = 0.003; 15YO, r = -0.32, P = 0.001). Adjustment for fitness did not affect the relations between fatness and fasting insulin in 9YO (r = 0.33-0.37, P < 0.001); however, only waist adj remained significantly related to fasting insulin (r = 0.24, P = 0.016) in 15YO. Children in the upper half of fitness and fatness split on the median did not differ in fasting insulin from children in the upper half of fitness but lower half of fatness. Fatness was related to fasting insulin in 9YO (r = 0.51-0.54, P = 0.001) and 15YO (r = 0.31-0.35, P = 0.011-0.028) in the lower half of fitness, but no association was observed in the upper half of fitness in either group. Fatness has a greater association with fasting insulin than fitness, especially among 9YO; however, fitness attenuates the adverse relation of fatness to fasting insulin in 15YO but does not change it in 9YO. In both age groups, being fitter and fatter does not result in greater fasting insulin than being fitter and leaner, and fatness is primarily associated with fasting insulin in lower-fit children.