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Guidelines on Overweight and Obesity: Electronic Textbook
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3.a. Lifestyle Trials

Evidence Statement: Weight loss produced by lifestyle modifications reduces blood glucose levels in overweight and obese persons without type 2 diabetes, and reduces blood glucose levels and HbA1c in some patients with type 2 diabetes. Evidence Category A.

Rationale: Nine RCTs examined lifestyle therapy (70, 362, 367, 369, 373, 403, 404, 412, 413). Five RCTs among normoglycemic overweight individuals examined the effects of behavior therapy or lifestyle change on weight loss. Changes in fasting glucose or fasting insulin are used as secondary outcome measures. Improvements in the levels of fasting insulin with weight loss ranged from 18 to 30 percent. While weight loss was maintained, improvements were observed for fasting glucose, fasting insulin, and glucose and insulin levels during oral glucose tolerance testing compared to controls (367, 373, 403, 412).

Three RCTs of lifestyle modifications were conducted among overweight diabetic patients (362, 404, 413). One RCT of diet and physical activity in African Americans aged 55 to 79 years with type 2 diabetes showed a 2.4 kg (5.3 lb) weight loss at 6 months compared to the usual-care control group. The intervention group also decreased HbA1c by 2.4 percentage units at 6 months (362). In another study, weight loss of 5 kg (11 lb) through diet resulted in a reduction of HbA1c by 2 percentage units at 6 months compared with the controls (413). At 1 year, the difference in weight loss between the two groups was reduced, and diabetic control was similar in each group. In the third study, weight loss through physical activity in 25 type 2 diabetes patients randomly assigned to physical activity (13 patients) or control (12 patients) groups, was associated with improved HbA1c (404). 

One RCT of 530 individuals in China with impaired glucose tolerance compared the effects of diet, physical activity, and diet plus physical activity on the incidence of diabetes. The data were analyzed for the group as a whole and for the lean and overweight groups separately. The overweight individuals in the diet and diet plus physical activity groups lowered their BMIs, while the lean individuals did not. After 6 years, the cumulative incidence rates of diabetes were significantly lower in all three intervention groups than they were in the controls (43.8 percent in the diet group, 41.1 percent in the physical activity group, 46 percent in the diet plus physical activity group, and 67.7 percent in the controls). This was true also if only the overweight persons were included in the analysis (70).

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