This observation led to the obvious hypothesis that weight loss might lower a person's risk of developing type-2 diabetes. Three large randomized clinical trials have tested the hypothesis that dietary change in persons at high risk of type-2 diabetes can reduce the incidence of diabetes during a treatment period of several years. The first two employed diet and exercise interventions compared with each other  or combined . The third, the US Diabetes Prevention Program, used a structured program of diet and exercise modification designed to produce weight loss compared with a program of diet and exercise advice only [4, 12]. It also included two pharmacologic treatment arms that will not be discussed here. Lifestyle modification lowered the incidence rate of diabetes in all three of these clinical trials [4, 10, 11]. The diet intervention in the Diabetes Prevention Program started with a reduction of fat to less than 25% of caloric intake [4, 12]. This was followed, when necessary, by total caloric restriction. Emphasis was put on self-monitoring of diet, exercise, and weight, and case managers were used to help each individual make the appropriate changes. The goal was gradual weight loss of 0.5-1 kg/week with a long-term goal of loss of 7% of body weight. This intervention resulted in a 58% reduction of the incidence rate of diabetes during a treatment and follow-up period averaging 2.8 years/participant . The weight-loss intervention also resulted in a significant fall in serum triglycerides compared with the advice-only group . Total and low-density lipoprotein cholesterol were also lowered, but not to an extent significantly different from the advice-only group.
The Diabetes Prevention Program followed only one diet and exercise approach to weight loss. Although it was successful compared with the advice-only group, this study did not evaluate different dietary approaches. Thus it is not known whether the weight loss induced in the Diabetes Prevention Program was due to the fat reduction, or whether other approaches to limiting caloric intake and increasing energy expenditure would have been more or less successful. It is likely that weight loss per se has more important effects on glycemia and lipids than does the type of diet used to achieve weight loss.
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