BACKGROUND: The efficacy of ad libitum low-fat diets in reducing body weight and fat in overweight and obese adults remains controversial. METHODS: We examined the effect of a 12-week low-fat, high-complex carbohydrate diet alone (HI-CHO) and in combination with aerobic exercise training (HI-CHO+EX) on body weight and composition in 34 individuals with impaired glucose tolerance (20 women and 14 men; mean +/- SEM age 66 +/- 1 years). Participants were randomly assigned to a control diet (41% fat, 14% protein, 45% carbohydrates, and 7 g of fiber per 1,000 kcal), a HI-CHO diet (18% fat, 19% protein, 63% carbohydrates, and 26 g of fiber per 1,000 kcal), or a HI-CHO diet plus endurance exercise 4 d/wk, 45 min/d, at 80% peak oxygen consumption (HI-CHO+EX). Participants were provided 150% of estimated energy needs and were instructed to consume food ad libitum. Total food intake, body composition, resting metabolic rate, and substrate oxidation were measured. RESULTS: There was no significant difference in total food intake among the 3 groups and no change in energy intake over time. The HI-CHO+EX and HI-CHO groups lost more body weight (-4.8 +/- 0.9 kg (p = 0.003) and -3.2 +/- 1.2 kg (p = 0.02)) and a higher percentage of body fat (-3.5% +/- 0.7% (p = 0.01) and -2.2% +/- 1.2% (p = 0.049)) than controls (-0.1 +/- 0.6 kg and 0.2% +/- 0.6%). In addition, thigh fat area decreased in the HI-CHO (p = 0.003) and HI-CHO+EX (p < 0.001) groups compared with controls. High carbohydrate intake and weight loss did not result in a decreased resting metabolic rate or reduced fat oxidation. CONCLUSION: A high-carbohydrate diet consumed ad libitum, with no attempt at energy restriction or change in energy intake, results in losses of body weight and body fat in older men and women.

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