Association of waist circumference and body mass index with fasting blood insulin in severely obese children: a cross-sectional study.

We tested whether body mass index (BMI) and waist circumference (WC) are associated with fasting insulin in severely obese children. A number of 391 (204 female and 187 male) obese children were consecutively enrolled in the study at a Paediatric outpatient clinic. They were aged 10+/-3 yr (mean+/-SD; range: 3-17 yr) and had a relative weight for age of 160+/-27% (mean+/-SD). BMI and WC explained respectively 9 and 13% of the variance of log-transformed (lt) insulin (p<0.0001 for both). After correction for age, however, BMI lost its association with lt-insulin (p=NS) and WC explained only 3% (p<0.001) of lt-insulin variance. Sex and pubertal status did not influence the relationship between WC, BMI and lt-insulin (p=NS, ANCOVA). We conclude that in severely obese children: 1) WC is a marginally better predictor of fasting blood insulin than BMI, 2) age has a substantial influence on the relationship between BMI, WC and insulin and, 3) the contribution of BMI and WC to insulin is of doubtful clinical relevance because it leaves a substantial portion (> or = 87%) of lt-insulin variance unexplained.