Self-selected vegan diet reduces skeletal muscle mass in healthy older adults

The vegan group experienced a reduction in protein intake of 27.7 g/day in week 1, declining to 19.3 g/day by week 12—all significantly lower than baseline (P < 0.0001). In contrast, the omnivorous group's protein intake did not change. Over 12 weeks, the vegan group lost 0.61 liters of thigh muscle volume and 1.15 kg of appendicular lean mass, whereas the omnivorous group experienced no meaningful change (−0.01 L and +0.05 kg, respectively; P < 0.0001 for both comparisons). MPS rates also fell in the vegan group to 1.09 %/day versus 1.27 %/day in the omnivorous group (P = 0.0073). Importantly, resistance exercise substantially mitigated these losses: the VEG-RE group lost only 0.35 L of thigh muscle and 0.62 kg of lean mass, with MPS rates of 1.24 %/day—intermediate between vegan and omnivorous groups and statistically better than vegan alone. This study included only healthy, motivated older adults in a controlled setting, so results may not generalize to those with existing frailty or poor dietary adherence.

The protein deficit in the vegan group was substantial and persistent: even by week 12, when participants had adapted their diet, they consumed 19 g/day less protein than their baseline—equivalent to roughly one serving of tofu or legumes. This suggests that 'self-selected' vegan eating without deliberate protein optimization is insufficient for older adults, whose baseline protein needs are already higher than younger populations (1.0–1.2 g/kg/day is often recommended). The resistance exercise effect was striking: VEG-RE participants, who suffered the same protein deficit as VEG, retained approximately 43% more thigh muscle volume than VEG alone (−0.35 L vs −0.61 L). This points to a mechanistic interplay—mechanical loading from resistance training may partly compensate for lower circulating amino acid availability by increasing muscle sensitivity to the limited protein signal. The magnitude of muscle loss in VEG (1.15 kg over 12 weeks, or ~2% of typical appendicular lean mass) is clinically relevant in older adults, where even modest sarcopenia accelerates frailty risk. Prior observational studies have shown similar patterns, but this RCT is among the first to measure MPS directly and test exercise modification within a vegan context.