Phenotypic characteristics associated with insulin resistance in metabolically obese but normal-weight young women

Metabolically obese, normal-weight (MONW) individuals are a hypothesized subgroup of the general population. These normal-weight individuals potentially display a cluster of obesity-related features, although this has not been systematically tested in young women. We hypothesized that MONW young women would display higher levels of total and visceral fat and lower levels of physical activity than normal women. In a cohort of 71 healthy nonobese women (21-35 years old), we identified MONW women based on cut points for insulin sensitivity (normal = glucose disposal >8 mg x min(-1) x kg(-1) of fat-free mass [FFM], n = 58; impaired = glucose disposal <8 ml x min(-1) x kg(-1) of FFM, n = 13). Thereafter, we measured body composition (dual energy X-ray absorptiometry) and body fat distribution (computed tomography), cardiorespiratory fitness (VO2max on a treadmill), physical activity energy expenditure (doubly labeled water and indirect calorimetry), glucose tolerance (oral glucose tolerance test), serum lipid profile, and dietary intake. We found a higher body fat percentage (32 +/- 6 vs. 27 +/- 6%, P = 0.01) and higher subcutaneous (213 +/- 61 vs. 160 +/- 78 cm2, P = 0.03) and visceral (44 +/- 16 vs. 35 +/- 14 cm2, P < 0.05) abdominal adiposity in the MONW group versus the normal group. The MONW group showed a lower physical activity energy expenditure (2.66 +/- 0.92 vs. 4.39 +/- 1.50 MJ/day, P = 0.01), but no difference in cardiorespiratory fitness was noted between groups. In conclusion, despite a normal body weight, a subset of young, apparently healthy women displayed a cluster of risky phenotypic characteristics that, if left untreated, may eventually predispose them to type 2 diabetes and cardiovascular disease.