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Physical activity (PA) reduces plasma insulin levels among people with and without type 2 diabetes. The mechanism by which PA decreases insulin resistance and hyperinsulinemia is not yet fully understood. Much of the evidence for this effect is from estimates of total daily PA and participation in vigorous PA. The purpose of a recently published study was to determine the association between moderate-intensity PA and fasting insulin levels and to examine possible differences in this association by race/ethnicity, central adiposity, and cardiorespiratory fitness level.

The study was a part of the Cross-Cultural Activity Participation Study, a five-year community study affiliated with the Women's Health Initiative community studies. The present analysis was conducted on 142 of the women enrolled in the study (mean age 54.8 years). Subjects consisted of 47 African Americans, 46 Native Americans, and 49 Caucasians. Surveys for physical activity, health history, and demographic data were completed in subjects' homes or worksites in New Mexico and South Carolina during five study visits. Cardiorespiratory fitness, body composition, and blood lipids were obtained during visits to the Exercise Testing Laboratories at the University of New Mexico and the University of South Carolina. PA records were used to obtain a detailed account of all PA performed during two consecutive four-day periods, scheduled one month apart. For each activity performed, subjects recorded the time they began an activity, the purpose and type of activity, and the perceived intensity of the activity in the PA record. Cardio-respiratory fitness was determined from the subject's time to exhaustion on a treadmill graded exercise test.

The results of the study found that 30 min/day of PA performed at a moderate intensity was associated with a 6.6% lower fasting insulin level among African American, Native American, and Caucasian women. The association between PA and fasting insulin levels was present after adjustment for potential confounding variables of race/ethnicity, age, educational attainment, and site. The median time spent in moderate/vigorous PA among women in the study was 98 min/day in the form of household, lawn and garden, child care, walking, and occupational activities. The association between PA and fasting insulin was also similar among centrally lean and obese women and fit and unfit women. An inverse association between PA and fasting insulin levels is clinically relevant, since fasting insulin levels are a strong predictor of type 2 diabetes.