A self-selected vegan diet reduces protein intake, skeletal muscle mass, and muscle protein synthesis in older adults, but the negative impact on skeletal muscle mass can be mitigated by resistance exercise.
Key Findings
Results
A self-selected vegan diet significantly reduced protein intake in older adults across all measured timepoints compared to an omnivorous diet.
Protein intake decreased by -27.7 g/d (95% CI: -37.3, -18.2 g/d) at week 1, -22.0 g/d (95% CI: -31.7, -12.4 g/d) at week 6, and -19.3 g/d (95% CI: -29.2, -9.37 g/d) at week 12 in the VEG group (all P < 0.0001 compared with baseline).
Protein intake reduction in VEG was similar to that in VEG-RE (P > 0.05), indicating resistance exercise did not alter dietary protein intake.
Protein intake did not change in the OMNI group, making the difference between VEG and OMNI statistically significant (P < 0.0001).
The trial ran for 12 weeks with 72 healthy older adults aged ≥65 years randomized across three parallel arms.
Results
Thigh muscle volume was significantly reduced in the vegan diet group compared to both the omnivorous and vegan diet plus resistance exercise groups.
VEG showed a reduction in thigh muscle volume of -0.61 L (95% CI: -0.72, -0.50 L), compared to -0.01 L (95% CI: -0.11, 0.09 L) in OMNI (P < 0.0001).
VEG-RE showed an intermediate reduction of -0.35 L (95% CI: -0.47, -0.24 L), which was significantly less than VEG (P = 0.0038).
Thigh muscle volume was assessed using magnetic resonance imaging at baseline and after 12 weeks.
Even VEG-RE showed significant muscle volume loss compared to OMNI, but the loss was approximately half that seen in VEG alone.
Results
Appendicular lean mass was significantly reduced in the vegan diet group compared to both the omnivorous and vegan diet plus resistance exercise groups.
VEG showed a reduction in appendicular lean mass of -1.15 kg (95% CI: -1.46, -0.85 kg), compared to +0.05 kg (95% CI: -0.23, 0.32 kg) in OMNI (P < 0.0001).
VEG-RE showed a reduction of -0.62 kg (95% CI: -0.93, -0.32 kg), which was significantly less than VEG (P = 0.0033).
Appendicular lean mass was assessed using dual x-ray absorptiometry (DXA).
Resistance exercise approximately halved the lean mass loss associated with the vegan diet.
Results
Muscle protein synthesis rates were significantly lower in the vegan diet group compared to both the omnivorous and vegan diet plus resistance exercise groups.
MPS rates were 1.09 %/d (95% CI: 1.01, 1.17 %/d) in VEG, compared to 1.27 %/d (95% CI: 1.18, 1.35 %/d) in OMNI (P = 0.0073).
MPS rates in VEG-RE were 1.24 %/d (95% CI: 1.16, 1.33 %/d), significantly higher than VEG (P = 0.0246).
MPS was assessed during the first 10 days of the intervention using a deuterium oxide protocol.
Both OMNI and VEG-RE had statistically similar and higher MPS rates compared to VEG alone.
Results
Resistance exercise mitigated, but did not fully prevent, the negative impact of a vegan diet on skeletal muscle mass in older adults.
VEG-RE showed significantly less thigh muscle volume loss (-0.35 L) and appendicular lean mass loss (-0.62 kg) compared to VEG (-0.61 L and -1.15 kg, respectively).
Despite the mitigation by resistance exercise, VEG-RE still showed greater muscle mass losses compared to the OMNI group.
MPS rates in VEG-RE (1.24 %/d) were similar to OMNI (1.27 %/d) and both were significantly higher than VEG (1.09 %/d).
The protein intake reduction was similar between VEG and VEG-RE groups (P > 0.05), indicating exercise benefits occurred independent of protein intake differences.
Methods
This randomized controlled trial enrolled 72 healthy older adults aged ≥65 years across three parallel arms over 12 weeks.
Participants were randomly assigned to a self-selected vegan diet (VEG), habitual omnivorous diet (OMNI), or self-selected vegan diet with resistance exercise (VEG-RE).
Outcome measures included thigh muscle volume (MRI), thigh muscle fat infiltration (MRI), body fat distribution (MRI), appendicular lean mass (DXA), and muscle strength (Biodex).
Linear mixed models were applied to test differences between VEG and the other groups.
The trial was registered at clinicaltrials.gov as NCT05809466.
What This Means
This research suggests that when older adults (age 65 and above) switch to a self-selected vegan diet for 12 weeks, they consume significantly less protein and lose meaningful amounts of muscle mass compared to older adults who continue eating an omnivorous diet. The vegan group lost an average of about 1.15 kg of lean mass and 0.61 liters of thigh muscle volume, while the omnivorous group showed essentially no change. The vegan group also showed lower rates of muscle protein synthesis — the biological process by which muscles are built and maintained — which helps explain the muscle mass losses observed.
This research also suggests that adding resistance exercise (such as strength training) to a vegan diet can substantially reduce, though not entirely prevent, this muscle loss. Older adults who followed a vegan diet combined with resistance exercise lost roughly half as much muscle mass as those following a vegan diet alone, and their muscle protein synthesis rates were comparable to the omnivorous group. Notably, both the vegan-only and vegan-with-exercise groups reduced their protein intake by similar amounts, meaning the protective effect of exercise appeared to work through mechanisms beyond simply eating more protein.
These findings matter because muscle loss in older adults is linked to weakness, falls, and reduced independence. This research suggests that older adults considering a vegan diet should be aware of potential muscle-related risks, particularly if they are not meeting recommended protein intakes, and that incorporating regular resistance exercise may be an important strategy to help preserve muscle health when following a plant-based diet.
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Domić J, Grootswagers P, Pinckaers P, Melchers M, van Loon L, de Groot L. (2026). A self-selected vegan diet reduces skeletal muscle mass in healthy older adults: a randomized controlled trial.. The American journal of clinical nutrition. https://doi.org/10.1016/j.ajcnut.2026.101416