Insulin sensitivity and fitness - Diabetes

Nutrition Research Newsletter, Feb, 2004

The prevalence of overweight in youth has increased dramatically in recent years and has disproportionately affected many ethnic groups including Hispanics. The most recent data from the 1999 to 2000 National Health and Nutrition Examination Survey indicate that approximately 40% of 6- to 11-year-old Hispanic youth possess an age- and gender-specific BMI > 85% percentile, a level that is approximately double that found in whites. It is evident that the health risks, such as fat hyperinsulinemia, impaired glucose tolerance (IGT), and type 2 diabetes, that accompany a high degree of body fat manifest early in life. Although a host of hereditary and environmental variables are linked to these health risks, the relationships between metabolic risk factors for type 2 diabetes and lifestyle related variables, including cardiorespiratory fitness and physical activity (PA), have not been investigated previously in overweight Hispanic youth with a family history of type 2 diabetes. Therefore, a recent study evaluated the relationships between insulin sensitivity/secretion and fitness/activity in the population described above.

Fifty-five and forty Hispanic boys and girls, respectively, participated in this study. After an overnight fast, a frequently sampled intravenous glucose tolerance test was used to determine the insulin sensitivity index (SI), the acute insulin response, and the disposition index. Cardiorespiratory fitness (maximal oxygen uptake [V[O.sub.2max]]) was evaluated using a treadmill protocol, and PA was determined by an interviewer-administered questionnaire. Body composition was measured using DXA.

Regardless of how cardiorespiratory fitness was expressed--relative to body weight, or relative to lean tissue mass--boys possessed greater fitness levels than girls. Boys also had higher fasting glucose levels than girls, but both group means were within the normally accepted range. Unadjusted correlations indicated that V[O.sub.2max] was negatively related to SI and disposition index, and positively associated with fasting insulin. However, these relationships were no longer significant once gender, Tanner stage, fat mass and soft lean tissue mass were included. Body mass explained 53% of the variance in SI and that V[O.sub.2max] was not independently related to SI. Cardiorespiratory fitness was positively related to both fat mass and soft lean tissue mass. PA was not related to any measure of insulin sensitivity and secretion.

The findings demonstrated that cardiorespiratory fitness was not independently related to insulin sensitivity and secretion in overweight Hispanic children with a family history of type 2 diabetes once gender, Tanner stage and body composition were controlled for. The results also highlight the fact that conclusions may vary depending on how fitness data are expressed, which fitness indicators are included in the analyses, and how differences in body composition are considered in the analyses. However, it is evident that to reduce the risk of diabetes, improvements in body composition may be required, and increasing the time spent performing various physical activities may facilitate this.

Geoff D.C. Ball, Gabriel Q. Shaibi, Martha L. Cruz, et al. Insulin sensitivity cardiorespiratory fitness, and physical activity in overweight Hispanic youth. Obesity Research 12(1): 77-85 (January 2004) [Correspondence: Michael I. Goran, Departments of Preventive Medicine and Physiology and Biophysics, University of Southern California, Health Sciences Campus, 1540 Alcazar Street, CHP-208D, Los Angeles, CA 90089. E-mail: goran@usc.edu.]