A 12-week randomized controlled trial of 72 healthy adults aged ≥65 found that transitioning to a self-selected vegan diet slashed daily protein intake by ~20–28 g compared to baseline, triggering significant skeletal muscle losses: thigh muscle volume dropped 0.61 L and appendicular lean mass fell 1.15 kg versus near-zero change in omnivores. Muscle protein synthesis rates were measurably suppressed in vegans (1.09%/d) versus omnivores (1.27%/d). Critically, adding resistance exercise to the vegan diet halved but did not eliminate muscle loss (−0.35 L thigh volume; −0.62 kg lean mass).
This trial is notable precisely because it is randomized and directly measures muscle protein synthesis via deuterium oxide alongside MRI-confirmed volumetric muscle changes — a methodological combination rarely seen together. The finding lands in a contentious space: prior observational data suggested plant-based diets were roughly adequate for muscle maintenance, but those studies rarely tracked older adults who transition abruptly without dietary counseling. The "self-selected" design is both a strength and a limitation — it mirrors real-world behavior but likely reflects poor substitution planning rather than an optimized plant-protein diet.
For adults over 65, where sarcopenia already accelerates at 1–2% annually, losing 1.15 kg of lean mass in just three months is clinically meaningful and could accelerate frailty trajectories. The practical message is unambiguous: unsupported vegan transitions in older adults carry real anabolic risk. Resistance training mitigates but does not neutralize this risk. Protein-optimized plant diets with leucine-rich sources remain an untested but plausible alternative. This study is paradigm-shifting for plant-based dietary guidance in aging populations.
