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Cardiovascular Fitness and Exercise as Determinants of Insulin Resistance in Postpubertal Adolescent Females

Departments of Pediatrics (J.Z.K.-V., A.R., K.S.) and Internal Medicine (C.C.L., J.B.H.), University of Michigan, Ann Arbor, Michigan 48019

Address all correspondence and requests for reprints to: Josephine Z. Kasa-Vubu, M.D, M.S., Department of Pediatrics, University of Michigan Medical Center, Ann Arbor, Michigan 48019-0718. E-mail: jzkv{at}umich.edu.

In obese adolescents, body mass index (BMI) is a poor predictor of insulin resistance, and the potential role of diminished physical activity has not been quantitated. We measured possible determinants of sensitivity to insulin in 53 adolescent females with a BMI between the 10th and the 95th percentile. We hypothesized that across weight and fitness spectra, relative fat mass, rather than BMI, and cardiovascular fitness would be predictors of insulin resistance.

We measured body composition by total-body dual x-ray absorptiometry. Self-reported weekly frequency of aerobic exercise for 1 h (RDE) was recorded, and maximal oxygen consumption (VO₂max) was measured. Insulin sensitivity was estimated by homeostasis model assessment index (HOMA_IR) derived from fasting glucose and insulin concentrations.

BMI was not related to HOMA_IR (P = 0.20), RDE showed a marginal relationship (P = 0.049), whereas percent body fat and VO₂max were significantly related to HOMA_IR (P = 0.01 and 0.0008, respectively). In a multiple regression model, VO₂max was a more critical determinant of insulin resistance than percent body fat (P = 0.03 vs. P = 0.67) or RDE (P = 0.01 vs. 0.51).

For prevention strategies in youth, physical inactivity may represent a greater metabolic risk than obesity alone.

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