A self-selected vegan diet may compromise protein intake, skeletal muscle mass, and muscle protein synthesis (MPS) in healthy, older adults, suggests a study. However, resistance exercise (RE) may mitigate the negative effects of this diet on skeletal muscle.
This study included three parallel study arms, wherein 72 healthy older adults aged ≥65 years were randomly assigned to follow a self-selected vegan diet, their habitual omnivorous diet, or a self-selected vegan diet with RE.
At baseline and after 12 weeks, the investigators assessed thigh muscle volume, thigh muscle fat infiltration, and body fat distribution using MRI, appendicular lean mass using dual x-ray absorptiometry, and muscle strength via Biodex. They also assessed MPS rates during the first 10 days using a deuterium oxide protocol and other outcome measures including body mass, dietary index, and physical activity.
Differences between the vegan diet and other groups were examined using linear mixed models, with results presented as estimated marginal mean or change in estimated marginal mean.
Muscle loss
Adherence to a vegan diet resulted in reduced protein intake with ‒27.7 g/day (95 percent confidence interval [CI], ‒37.3 to ‒18.2), ‒22.0 g/day (95 percent CI, ‒31.7 to ‒12.4), and ‒19.3 g/day (95 percent CI, ‒29.2 to ‒9.37) at weeks 1, 6, and 12, respectively (p<0.0001 for all vs baseline). This was similar to the decrease seen with vegan diet plus RE (p>0.05) and different from the omnivorous diet (p<0.0001), which saw no change in protein intake. [Am J Clin Nutr 2026;124:101416]
Vegan diet also significantly reduced thigh muscle volume and appendicular lean mass (‒0.61 L, 95 percent CI, ‒0.72 to ‒0.50; ‒1.15 kg, 95 percent CI, ‒1.46 to ‒0.85, respectively) compared with omnivorous diet (‒0.01 L, 95 percent CI, ‒0.11 to 0.09; 0.05 kg, 95 percent CI, ‒0.23 to 0.32; p<0.0001 for both) and vegan diet plus RE (‒0.35 L, 95 percent CI, ‒0.47 to ‒0.24; p=0.0038; ‒0.62 kg, 95 percent CI, ‒0.93 to ‒0.32; p=0.0033).
Similarly, MPS was lower with vegan diet (1.09 percent/day, 95 percent CI, 1.01‒1.17) than with omnivorous diet (1.27 percent/day, 95 percent CI, 1.18‒1.35; p=0.0073) and vegan diet plus RE (1.24 percent/day, 95 percent CI, 1.16‒1.33; p=0.0246).
“[S]everal other studies show that vegan diets result in a reduced dietary protein intake and muscle mass loss in other population groups as well,” the investigators said. [JAMA Netw Open 2020;3:e2025454; J Int Soc Sports Nutr 2024;21:2413961; Nutrients 2020;12:2917]
“Conversely, previous work with more controlled diets, including ours, showed no differences in MPS between a vegan or omnivorous diet in older adults,” they added. [Br J Nutr 2020;126:674-684; J Nutr 2024;155:1141-1150]
Plant-based diet
However, the benefits of plant-based diets must not be ignored. In the current study, both vegan diet and vegan diet plus RE resulted in lower visceral adipose tissue by approximately 9 percent.
Visceral adipose tissue has been shown to be associated with cardiometabolic risk factors, and its reduction may prove beneficial to health. [Circulation 2007;116:39-48]
Notably, both vegan diets reduced liver fat fraction from ∼7.5 percent to ∼5 percent, which is highly clinically relevant. [Hepatology 2018;67:328-357]
“Altogether, our findings emphasize the importance of establishing a balance between the health benefits of transitioning to more plant-based diets and the risks associated with completely excluding all animal-based food products,” the investigators said.