OBJECTIVE: This study aimed to evaluate the impact of timing of exercise initiation on weight loss within a behavioral weight loss program. METHODS: Adults with overweight or obesity (N = 170; age 18 to 55 years; BMI 25 to 42 kg/m2; 83.5% women) were enrolled in an 18-month behavioral weight loss program consisting of a reduced-calorie diet, exercise, and group-based support. The standard group (STD) received a supervised exercise program (progressing to 300 min/wk of moderate-intensity aerobic exercise) during months 0 to 6. The sequential group (SEQ) was asked to refrain from changing exercise during months 0 to 6 and received the supervised exercise program during months 7 to 12. On completion of supervised exercise, both groups were instructed to continue 300 min/wk of moderate-intensity exercise for the study duration. RESULTS: At 6 months, STD exhibited greater reductions in body weight (-8.7 +/- 0.7 kg) versus SEQ (-6.9 +/- 0.6 kg; p = 0.047). Between 6 to 18 months, STD regained more weight (2.5 +/- 0.8, 0.0 +/- 0.8 kg; p = 0.02). At 18 months, there were no between-group differences in changes in weight (STD -6.9 +/- 1.2, SEQ -7.9 +/- 1.2 kg), fat mass, lean mass, physical activity, or attrition. CONCLUSIONS: Both immediate and delayed exercise initiation within a behavioral weight loss program resulted in clinically meaningful weight loss at 18 months. Thus, timing of exercise initiation can be personalized based on patient preference.

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