A self-selected vegan diet reduces skeletal muscle mass in healthy older adults: a randomized controlled trial

BACKGROUND: Adopting a plant-based diet may compromise protein intake and impair skeletal muscle mass in older adults. OBJECTIVES: To compare the impact of a 12-wk self-selected vegan diet with that of an omnivorous diet and a self-selected vegan diet with resistance exercise (RE) on skeletal muscle mass, strength, and muscle protein synthesis (MPS) rates in older adults. METHODS: This randomized controlled trial included 3 parallel study arms in which 72 healthy older adults aged ≥65 y were randomly assigned to follow 1) a self-selected vegan diet (VEG), 2) their habitual omnivorous diet (OMNI), or 3) a self-selected vegan diet with RE (VEG-RE). At baseline and after 12 wk, thigh muscle volume, thigh muscle fat infiltration, and body fat distribution were assessed using magnetic resonance imaging, appendicular lean mass with dual x-ray absorptiometry, and muscle strength via Biodex. MPS rates were assessed during the first 10 d using a deuterium oxide protocol. Other outcome measures were body mass, dietary intake, and physical activity. Linear mixed models were applied to test differences between VEG and the other groups. Results are presented as estimated marginal mean or change in estimated marginal mean [95% confidence interval (CI)]. RESULTS: VEG reduced protein intake with ‒27.7 g/d (95% CI: ‒37.3, ‒18.2 g/d), ‒22.0 g/d (95% CI: ‒31.7, ‒12.4 g/d), and ‒19.3 g/d (95% CI: ‒29.2, ‒9.37 g/d) in weeks 1, 6, and 12 (all P < 0.0001 compared with baseline), which was similar to the decrease in VEG-RE (P > 0.05), but different from OMNI (P < 0.0001), in which protein intake did not change. Thigh muscle volume and appendicular lean mass significantly reduced in VEG [‒0.61 L (95% CI: ‒0.72, ‒0.50 L); ‒1.15 kg (95% CI: ‒1.46, ‒0.85 kg)] compared to OMNI [‒0.01 L (95% CI: ‒0.11, 0.09 L); +0.05 kg (95% CI: ‒0.23, 0.32 kg); both P < 0.0001] and VEG-RE [‒0.35 L (95% CI: ‒0.47, ‒0.24 L), P = 0.0038; ‒0.62 kg (95% CI: ‒0.93, ‒0.32 kg), P = 0.0033]. MPS was lower in VEG [1.09 %/d (95% CI: 1.01, 1.17 %/d)] compared with OMNI [1.27 %/d (95% CI: 1.18, 1.35 %/d); P = 0.0073] and VEG-RE [1.24 %/d (95% CI: 1.16, 1.33 %/d); P = 0.0246]. CONCLUSIONS: A self-selected vegan diet reduces protein intake, skeletal muscle mass, and MPS in older adults. The negative impact of a vegan diet on skeletal muscle mass can be mitigated by RE. This trial was registered at clinicaltrials.gov as NCT05809466. (https://clinicaltrials.gov/study/NCT05809466).