Does a self-selected vegan diet result in muscle loss in older adults?

Does a self-selected vegan diet result in muscle loss in older adults?

Vegan diets are increasing in popularity, mostly for health, ethical, or sustainability reasons. However, plant-based food sources are often relatively low in protein. In addition, plant-based sources are typically of lower protein quality than animal-based sources, because of lower digestibility and suboptimal amino acid composition.

When protein intake is strictly matched, most studies find no meaningful differences in muscle protein synthesis or muscle mass when comparing vegan and omnivorous diets, suggesting that protein quality is not always impactful. However, what happens when people adopt a vegan diet freely (self-selected)?

This study investigated the impact of a 12-week self-selected vegan diet versus an omnivorous diet on muscle protein synthesis and muscle mass in older adults. Participants had monthly nutrition meetings where they received guidance to adopt the vegan diet.

Muscle protein synthesis was assessed over a 10-day period at the start of the intervention using labeled water (D2O). Muscle mass was determined with MRI. 

Key findings:

• The vegan diet resulted in a reduced protein intake (from 1.0 to 0.7 g/kg/day)
• The vegan diet had 14% lower muscle protein synthesis
• The vegan diet reduced muscle mass by 5%

Interpretation?

This study shows that a self-selected vegan diet lowers protein intake and muscle mass in older adults. These findings highlight the importance of maintaining total protein intake when switching to an all plant-based diet.

Strengths of this study include the measurement of muscle protein synthesis over 10 days using D2O and the assessment of muscle mass with MRI. In addition, external validity is high, as the diet was self-selected. A limitation is that the diets differed in many nutrients, making it difficult to determine whether and to what extent each nutrient contributed to the results.

Link to study:
Domić J et al., Does a self-selected vegan diet result in muscle loss in older adults? Am J Clin Nutr, 2026

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Q1: Does this mean you will always lose muscle mass on a vegan diet?
No. When protein intake is high and matched, studies generally find no clear disadvantage of a vegan or vegetarian diet. The key message of this study is that a vegan can reduce muscle mass if protein intake drops.

Q2: What is the practical implication?
Make sure protein intake does not drop when switching to a vegan diet. Participants in this study received guidance from a dietician, including a session focused on protein, and their intake still dropped by ~30%. So don’t assume protein intake will be fine: check it. Be especially careful when recommending a vegan diet to others, who may be less knowledgeable or motivated.

Q3: Is this study consistent with literature?
Mostly yes. Most studies on this topic matched protein intake to isolate the impact of protein quality. The few studies of self-selected vegan diets also tend to find lower protein intakes.

Q4: How large is this effect?
Muscle mass declined by ~5% in the vegan diet. After age 50, ~1% of muscle mass is lost per year. So this 3-month vegan diet caused roughly the equivalent of 5 years of age-related muscle loss.

Q5: What were some of the other findings?
The vegan diet increased fiber intake by 45% and reduced body weight (3kg), visceral fat (9%), and liver fat. Focusing only on muscle would oversimplify diet choice: the best diet may depend on which outcome matters most for the individual (e.g. risk of frailty vs cardiovascular disease).

Q6: Did energy intake play a role?
Probably. Reported energy intake did not change, but the vegan group lost 3 kg (of which 1.2 kg aLBM), suggesting a caloric deficit. A possible explanation is lower energy absorbed due to the higher fiber intake. An energy deficit makes muscle loss more likely.

Q7: What can the third group teach us?
The third group combined the vegan diet with resistance exercise. They still lost muscle, but less than the vegan group without exercise (3 and 5%, respectively). So, exercise partly offset, but did not prevent, the effect of lower protein intake and quality.

Does this study change how you think about vegan diets and muscle health? Have questions about the study or its interpretation? Drop them in the comments.