OBJECTIVE:To investigate the independent influence of alterations in fat mass, body fat distribution and hormone release on pubertal increases in fasting serum insulin concentrations and on insulin resistance assessed by the homeostasis model (HOMA). DESIGN AND SUBJECTS: Cross-sectional investigation of pre-(n ¼ 11, n ¼ 8), mid-(n ¼ 10, n ¼ 11), and late-pubertal (n ¼ 10, n ¼ 11) boys and girls with normal body weight and growth velocity. MEASUREMENTS: Body composition (by a four-compartment model), abdominal fat distribution and mid-thigh interfascicular plus intermuscle (extramyocellular) fat (by magnetic resonance imaging), total body subcutaneous fat (by skinfolds), mean nocturnal growth hormone (GH) release and 06:00 h samples of serum insulin, sex steroids, leptin and insulin-like growth factor-I (IGF-I). RESULTS: Pubertal insulin resistance was suggested by greater (P < 0.001) fasting serum insulin concentrations in the latepubertal than pre-and mid-pubertal groups while serum glucose concentrations were unchanged and greater (P < 0.001) HOMA values in late-pubertal than pre-and mid-pubertal youth. From univariate correlation fat mass was most related to HOMA (r ¼ 0.59, P < 0.001). Two hierarchical regression models were developed to predict HOMA. In one approach, subject differences in sex, pubertal maturation, height and weight were held constant by adding these variables as a block in the first step of the model (r 2 ¼ 0.36). Sequential addition of fat mass (FM) increased r 2 (r 2 (inc)remental ¼ 0.08, r 2 ¼ 0.44, P < 0.05) as did the subsequent addition of a block of fat distribution variables (extramyocellular fat, abdominal visceral fat, and sum of skinfolds; r 2 inc ¼ 0.11, r 2 ¼ 0.55, P < 0.05). Sequential addition of a block of hormone variables (serum IGF-I and log (10) leptin concentrations; r 2 inc ¼ 0.04, P > 0.05) did not reliably improve r 2 beyond the physical characteristic and adiposity variables. In a second model, differences in sex and pubertal maturation were again held constant (r 2 ¼ 0.25), but body size differences were accounted for using percentage fat data. Sequential addition of percentage body fat (r 2 (inc)remental ¼ 0.11, r 2 ¼ 0.36, P < 0.05), then a block of fat distribution variables (percentage extramyocellular fat, percentage abdominal visceral fat, and percentage abdominal subcutaneous fat; r 2 inc ¼ 0.08, r 2 ¼ 0.44, P ¼ 0.058), and then a block of serum IGF-I and log (10) leptin concentrations (r 2 inc ¼ 0.07, r 2 ¼ 0.51, P < 0.05) increased r 2 . Mean nocturnal GH release was not related to HOMA (r ¼ 7 0.04, P ¼ 0.75) and therefore was not included in the hierarchical regression models. CONCLUSION: Increases in insulin resistance at puberty were most related to FM. Accumulation of fat in the abdominal visceral, subcutaneous and muscular compartments may increase insulin resistance at puberty beyond that due to total body fat. Serum concentrations of leptin and IGF-I may further modulate HOMA beyond the effects of adiposity and fat distribution. However, the results...

A friends' weight and the social context are unshared environmental factors associated with variability in adiposity among biologically-related weight-discordant siblings.

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