A randomised controlled trial put healthy older adults on a self-selected vegan diet, a vegan diet with resistance exercise, or an omnivorous diet for twelve weeks, and measured thigh muscle volume by imaging, appendicular lean mass, and muscle protein synthesis directly.
Protein intake on the vegan diet fell by about 28 g a day at week one and was still roughly 19 g a day lower at week twelve. This was self-selected, not prescribed — participants chose their own foods, which is the point and the limitation both.
The body composition results were clear. Thigh muscle volume fell 0.61 L and appendicular lean mass 1.15 kg on the vegan diet, against effectively no change on the omnivorous diet. Muscle protein synthesis was lower too, at 1.09%/day against 1.27%/day. Adding resistance exercise cut the losses roughly in half — 0.35 L and 0.62 kg — but did not prevent them.
This is not an argument against plant-based eating, and it should not be presented as one. It is an argument that an older adult moving to a vegan diet without deliberate attention to protein will lose muscle, measurably, within a season. The patients who ask about this are often the ones with the least reserve to spare.
- When an older patient adopts a vegan diet, set an explicit protein target rather than assuming intake will hold.
- Add resistance exercise at the same time: it halved the loss here but did not stop it.
- Recheck weight and function at three months rather than at the annual review.
- Ask specifically about protein sources — the shortfall came from self-selection, not from the diet being unable to supply protein.
- Be careful how this is framed: the finding is about protein intake and muscle, not a verdict on plant-based diets.
The statistics, in plain English
A loss of 1.15 kg of appendicular lean mass with a confidence interval of 1.46 to 0.85 kg is precise enough to trust the direction and the rough size — the interval nowhere approaches zero. The comparison that carries the weight is against the omnivorous group, which changed by +0.05 kg with an interval crossing zero, meaning no detectable change. Muscle protein synthesis measured directly, rather than inferred from weight, is what makes the mechanism credible. The trial was twelve weeks in healthy older adults, so it says nothing about what happens over years, or in someone already sarcopenic — where the same loss would matter considerably more.
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