Level 3: Lacking direct evidence
Combined exercise and diet interventions may prevent type 2 diabetes in patients with impaired glucose tolerance or metabolic syndrome, based on a Cochrane review of eight randomized trials lasting at least six months and involving 5,956 patients (Cochrane Database Syst Rev. 2008:CD003054). In this context, type 2 diabetes is a surrogate outcome. This review does not directly establish whether an intervention that prevents or delays a diagnosis of diabetes will delay onset of diabetic complications.
Trial duration ranged from one to six years, with follow-up from one to seven years. Compared with controls, a combined diet-and-exercise intervention was associated with reduced incidence of type 2 diabetes (NNT 10) and decreased BP (weighted mean difference -4 mm Hg systolic and -2 mm Hg diastolic). The intervention was also associated with favorable effects on weight, BMI, waist-to-hip ratio, and waist circumference. Most results were limited by statistical heterogeneity.
Two trials had a diet-only arm with 167 patients; one trial had an exercise-only arm with 178 patients. There were no significant differences comparing exercise only vs. standard recommendations in an analysis of two trials with 351 patients. There were also no significant differences comparing an exercise-only vs. diet-only intervention in an analysis of two trials with 345 patients. These results are supported by two previously published systematic reviews (BMJ. 2007;334:299 and Cochrane Database Syst Rev. 2005:CD005270).