Retatrutide produces weight loss while preserving (sparing) lean mass

Summary

The phase 2 retatrutide obesity trial (Jastreboff et al., NEJM 2023) reported mean total weight loss of ~24% at 48 weeks on the 12 mg dose. DXA sub-analyses show that roughly 35-40% of the weight lost is lean mass — consistent with other incretin agents (semaglutide, tirzepatide), which all shed lean mass roughly in proportion to expected losses during rapid calorie deficit without resistance training. There is no published evidence that retatrutide 'spares' lean mass beyond what would be expected from equivalent-magnitude dietary weight loss; the drug's mechanism does not include anabolic signaling. Independent commentary (e.g. Prado, Heymsfield) has flagged muscle loss as a concern with all high-efficacy GLP-class agents. The positive claim of muscle-sparing is not supported; the drug reduces lean mass substantially alongside fat mass.