Fitness and weight cycling in relation to body fat and insulin sensitivity in normal-weight young women.

A direct link has been established between insulin resistance and obesity; however, the influence of body fat content on insulin sensitivity in normal-weight individuals has not been explored. The aim of this cross-sectional study was to examine differences in insulin sensitivity, glycemic control, and blood lipid levels in normal-weight, healthy participants differing in their body fat. Physical activity, cardiorespiratory fitness, weight cycling, nutrient intake, and eating behavior were also assessed. Two groups of normal-weight (body mass index <25) young, healthy women were recruited: a high-body-fat group with body fat >30% of body weight (n=15) and a low-body-fat group (fat < or =30%, n=17). Data were collected between November 2003 and March 2004. Participants were matched for age and body fat distribution (assessed by the waist-to-hip ratio). The high-body-fat group displayed lower values of insulin sensitivity compared to the low-body-fat group (1/homeostasis model assessment of insulin resistance 1.003+/-0.083 vs 1.385+/-0.158, respectively; P<0.05), whereas no differences were observed on blood lipid levels and glycemic control between groups. High-body-fat participants had lower cardiorespiratory fitness (25.2+/-1.2 vs 28.2+/-1.4 mL/kg/min in the low-body-fat group, P<0.05) and higher incidence of weight cycling of 1.0 to 2.5 kg loss per lifetime (18+/-3 kg vs 8+/-2 kg in the low-body-fat group, P<0.05). The latter variable was the only factor significantly associated with body fat. These data suggest that increased adiposity in normal weight, healthy women may adversely affect insulin sensitivity and that it may be related to differences in physical fitness and eating behavior.