The impact of exercise on fat free mass reduction during very low energy diet-induced weight loss in adults with overweight or obesity: a systematic review and meta-analysis

Very low energy diets (VLEDs) are effective for rapidly reducing weight in individuals with obesity. They are also associated with a greater fat free mass (FFM) reduction compared to other energy restricted diets. The objective of this systematic review and meta-analysis was to determine whether the addition of exercise reduces FFM loss during VLED-induced rapid weight loss in people with overweight or obesity. CENTRAL, CINAHL, Medline and Embase databases were systematically searched from inception to October 2024 for randomised controlled trials that combined exercise with VLEDs in adults aged 18-65 with overweight or obesity. The primary outcome was change in FFM. The Cochrane Risk of Bias version 2 tool was used to assess risk of bias. Seven eligible studies (n = 292) were identified. A meta-analysis of five studies (n = 241) showed that VLED with exercise resulted in a higher retention of FFM compared to VLED alone (mean difference = 0.83 kg, 95 % CI 0.17-1.49, p = 0.01). A sub-group analysis was unable to detect any effect from specific exercise modalities. These findings may be limited by the low number and risk of bias of the included studies. Overall, these results suggest the addition of exercise to a VLED may mitigate FFM loss. Greater consistency in reporting exercise prescriptions will improve comparability of exercise modalities and enable the implementation of these therapies in the clinical setting. Prospectively registered with PROSPERO (CRD42022309162).