Improvements in glucose tolerance and insulin action induced by increasing energy expenditure or decreasing energy intake: a randomized controlled trial

Abstract

Background: Weight loss, through calorie restriction or increases in energy expenditure via exercise, improves glucose tolerance and insulin action. However, exercise-induced energy expenditure may further improve glucoregulation through mechanisms independent of weight loss.

Objective: The objective was to assess the hypothesis that weight loss through exercise-induced energy expenditure improves glucoregulation and circulating factors involved in insulin action to a greater extent than does similar weight loss through calorie restriction.

Design: Sedentary men and women aged 50-60 y with a body mass index (kg/m(2)) of 23.5-29.9 were randomly assigned to 1 of 2 weight-loss interventions [12 mo of exercise training (EX group; n = 18) or calorie restriction (CR group; n = 18)] or to a healthy lifestyle (HL) control group (n = 10). The insulin sensitivity index and areas under the curve for glucose and insulin were assessed with an oral-glucose-tolerance test. Adiponectin and tumor necrosis factor alpha concentrations were measured in fasting serum. Fat mass was measured by dual-energy X-ray absorptiometry.

Results: Yearlong energy deficits were not significantly different between the EX and CR groups, as evidenced by body weight and fat mass changes. The insulin sensitivity index increased and the glucose and insulin areas under the curve decreased in the EX and CR groups, remained unchanged in the HL group, and did not differ significantly between the EX and CR groups. Marginally significant increases in adiponectin and decreases in the ratio of tumor necrosis factor alpha to adiponectin occurred in the EX and CR groups but not in the HL group.

Conclusions: Weight loss induced by exercise training or calorie restriction improves glucose tolerance and insulin action in nonobese, healthy, middle-aged men and women. However, it does not appear that exercise training-induced weight loss results in greater improvements than those that result from calorie restriction alone.

Figure 1

Consort diagram indicating sample sizes at each stage and in each arm of the study.

Figure 2

Physical activity and energy intake (mean ± SE) before and during the intervention. Panel A is physical activity level determined by physical activity recall questionnaire, Panel B is energy intake determined by DLW/DXA, and panel C is energy intake determined by food diary. Baseline values were not different among groups for any of the outcomes according to ANOVA. Interactions between study group and time were significant for data in panel A (p=0.0497), panel B (p = 0.01) and panel C (p=0.003) by repeated measures mixed-model ANOVAs that included baseline values as a covariate. * P ≤ 0.05 versus baseline within group by repeated measures, mixed-model ANOVA and Tukey tests. Sample sizes are 18, 18, and 10 for the EX, CR, and HL groups, respectively.

Figure 3

Relationships between changes in the TNFa to adiponectin ratio and the changes in insulin sensitivity index in the EX (n=17; excludes 1 subject due to technical problems during data collection) and CR (n=18) groups. Raw data were used for presentation; however, the associations were assessed using Pearson correlations after parsing out the effects of baseline ISI. The correlation in the EX group was significantly greater than that in the CR group (p=0.03) as determined using Fisher’s z transformation.