Older Adults on Self-Chosen Vegan Diet Lost Leg Muscle in 12 Weeks

This is a randomized controlled trial, the design best suited to testing cause and effect. It enrolled 72 healthy adults aged 65 or older. They were randomly assigned to one of three groups: a self-selected vegan diet, their usual omnivorous diet, or a self-selected vegan diet plus resistance exercise.

At the start and after 12 weeks, the researchers measured thigh muscle volume and fat within the thigh muscle using MRI. They measured lean mass in the arms and legs with a DXA scan, and muscle strength with a Biodex machine. They also measured muscle protein synthesis, the rate at which the body builds new muscle protein, over the first 10 days using a deuterium oxide (heavy water) method.

Protein intake in the vegan-only group fell by 27.7 grams a day in week 1, 22.0 grams in week 6 and 19.3 grams in week 12, compared with the start of the study. The authors conclude that a self-selected vegan diet reduced protein intake, skeletal muscle mass and muscle protein synthesis in these older adults. They also conclude that resistance exercise can mitigate the harm to muscle mass.

Note what the authors say exercise did: it mitigated the effect.

What does this mean for someone considering a plant-based diet later in life? The trial tested vegan diets that people chose for themselves, and protein intake dropped. The trial did not test a vegan diet built to match the protein of an omnivorous one, so it cannot say whether such a diet would protect muscle, and it did not test any advice on planning protein.

The findings also come with limits. The participants were healthy, and the trial lasted 12 weeks. It does not tell us what happens over years, or in frailer people.

The result adds a caution to our October 17 report on a low-fat vegan diet and liver fat in type 2 diabetes. That was a different study, a small single-arm trial in a different group of adults, and it looked at weight and liver fat rather than muscle. Benefits in one area do not settle what a diet does to muscle in older people.

Next, watch for the full paper, which should show the size of the muscle and strength changes, and for longer trials that test vegan diets planned to match protein intake, with and without resistance training.

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