Macro partitioning
Incretin-based therapies (GLP-1 and GLP-1/GIP RAs) induce substantial weight loss that is predominantly derived from fat mass, resulting in the relative preservation or improvement of lean body mass percentage and muscle quality, despite absolute reductions in lean mass.
If you are using GLP-1 or GLP-1/GIP medications for weight loss, expect to lose mostly fat. You will likely lose some muscle mass in absolute terms, but your body composition will improve because you are losing fat faster than muscle. To maximize this benefit, prioritize protein intake (1.2-1.6 g/kg/day) and engage in resistance training to signal your body to keep muscle tissue.
Most clinical studies have focused on body composition measurements... Recent meta-analyses of incretin-based anti-obesity medications report diverse reductions in lean body mass... ranging from nonsignificant decreases (less than 20%) to significant reductions of up to 30%... These findings were consistent even in multidisciplinary settings... liraglutide 3 mg/day combined with a multidisciplinary intervention over 1 year decreased lean body mass by 2.7 kg, representing 23% of total weight loss. This lower lean body mass loss compared to fat mass loss increased lean body percentage on follow-up.
Why this rating
Based on multiple randomized controlled trials and DXA substudies (e.g., STEP-1, SURMOUNT-1), though absolute lean mass loss is acknowledged.
Source
Impact of Incretin-Based Therapy on Skeletal Muscle Health
Andrijana Koceva et al. · Medicina · 2025
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