Weight loss in obese persons is associated with a decrease in insulin resistance and postprandial glucose and insulin levels. It has been suggested that additional benefit may be derived by combining weight loss and exercise. Weinstock and associates investigated the effects of diet and exercise on weight loss and insulin sensitivity in 45 obese women without diabetes.

The subjects were randomly assigned to one of three 48-week weight loss programs: diet alone; diet and aerobic training; and diet and strength training. All women participated in the same group behavior modification program and diet program. They consumed a diet of 925 calories per day for the first 16 weeks, followed by an increase to 1,500 calories per day for the remainder of the supervised diet and exercise program. Exercise consisted of three sessions per week for the first 28 weeks and two sessions per week for the next 20 weeks. Exercise was unsupervised during the remainder of the follow-up period.

Twenty-two subjects were also evaluated approximately one year after the study (week 96). To assess the effects of weight loss and exercise on insulin sensitivity, oral glucose tolerance tests were performed at baseline and at weeks 16, 24, 44 and 96.

Subjects in all three groups lost weight during the first 16 weeks. At week 16, the mean weight loss was 13.8 kg (30.4 lb), and this weight loss was maintained through weeks 24 and 44. In the 22 subjects who returned for a final visit at week 96, weight had increased from the 44th week to the 96th week, resulting in a mean net weight loss of 9.9 kg (21.8 lb) from baseline weight. At week 44, these subjects demonstrated a mean 15.2-kg (33.4-lb) weight loss. From weeks 44 to 96, during the unsupervised period, 14 of the 22 subjects (64 percent) gained more than 5 kg (11 lb). No significant differences were observed among the women in the three diet and exercise groups at week 96.

Assessment of glucose tolerance during the study period revealed that fasting glucose levels and glucose levels obtained after a 75-g glucose load did not differ among the groups throughout the study. The mean fasting insulin level and the mean insulin level in response to oral glucose decreased significantly from baseline to the 44th week, after weight loss had been achieved. The type of exercise program (i.e., aerobic or strength training) or the lack of an exercise program did not have a bearing on insulin levels.

In the 22 subjects followed for 96 weeks, fasting insulin levels and insulin levels after the glucose load rose from baseline in 19 of the subjects (86 percent) so that at week 96 their insulin levels were not significantly different from baseline levels. This was in contrast to weight, which remained significantly lower than baseline weight.

The authors conclude that their study corroborates the benefit of weight loss on hyper-insulinemia in obese persons. The addition of exercise, however, was not found to provide additional improvement, and a marked increase in insulin levels was noted with only a partial regaining of weight. Further studies are needed to investigate whether insulin sensitivity can be improved with more sustained weight loss or whether a defined basal metabolic index is required to improve insulin sensitivity.