In a longitudinal cohort of 1,022 adults undergoing metabolic bariatric surgery (MBS), fat-free mass (FFM) loss varied meaningfully by sex and menopausal status. At 24 months post-surgery, the proportion of weight lost as FFM was lowest in premenopausal women (24.7%), intermediate in men (27.1%), and highest in postmenopausal women (28.7%). Despite these substantial muscle losses across all groups, maximal oxygen consumption (VO2 max) improved significantly in all three subgroups. Critically, postoperative FFM changes were not independently associated with CRF gains (β: −0.004; 95% CI: −0.008, 0.001).
The dissociation between FFM loss and cardiorespiratory fitness improvement is the most clinically important signal here. Conventional wisdom assumes preserving muscle is essential to fitness outcomes after weight loss, but this cohort suggests that the cardiovascular unloading effect of losing large amounts of fat may dominate over any FFM-related detriment — at least up to two years. Postmenopausal women emerge as a higher-risk subgroup, losing proportionally more lean mass, which has known downstream implications for bone health, insulin sensitivity, and functional independence as they age. The study cannot establish whether targeted resistance training or protein supplementation in these groups would shift the FFM/weight-loss ratio favorably, nor does it follow participants beyond six years to capture longer-term sarcopenic consequences. As an observational cohort, causality is limited. Still, the n=1,022 sample with repeated measures and stratified analysis makes this a substantive, practically relevant contribution to surgical obesity care.