man in black long sleeve shirt sitting on a leg press exercise machine

Can Vegetarians Build as Much Muscle?

A 12-week trial in 108 adults 55+ found both vegetarian and omnivorous diets built muscle — but non-inferiority wasn't proven and vegetarian protein was lower.

Both vegetarians and omnivores aged 55 and older doing supervised resistance training built muscle and strength — but protein intake was not matched, so results can’t be taken as definitive.

In a 12-week randomized non-inferiority trial, 108 adults aged 55 and older doing supervised resistance training built muscle and strength on both a self-selected vegetarian and an omnivorous diet — but the vegetarian group ate less protein (1.24 versus 1.58 g/kg/d), and the trial could not prove the vegetarian diet was non-inferior for muscle mass, nor prove a significant difference.

What this means

In a 12-week randomized trial, 108 adults aged 55 and older doing supervised resistance training gained arm-and-leg muscle mass on both a lacto-ovo vegetarian and an omnivorous diet. The vegetarian group ate less protein — 1.24 versus 1.58 grams per kilogram of body weight per day — though both met the trial’s ≥1.2 g/kg/d target. But “non-inferiority not established” means the trial failed to prove the vegetarian diet matched the meat diet for muscle, not that it was worse: the between-group difference, −0.33 kg (about −0.7 lb), was not statistically significant. This covers 12 weeks in mostly healthy older adults, not years.

What the trial did

The IMPACT trial (NCT06172725) randomly assigned 108 community-dwelling adults aged 55 and older — average age 66, 58% women, average BMI 25.7 — to a lacto-ovo vegetarian or an omnivorous diet, while both groups completed the same supervised resistance-training program three times a week for 12 weeks. Both received dietary counseling and aimed for at least 1.2 g of protein per kilogram of body weight daily; the vegetarian arm targeted 60% or more of protein from plants, the omnivorous arm 60% or more from animals. Foods were self-selected, not supplied.

What the numbers show

Both groups gained appendicular (arm-and-leg) muscle mass by DXA: the vegetarian group by +0.38 kg (about 0.8 lb; 95% CI 0.12–0.64) and the omnivorous group by +0.66 kg (about 1.5 lb; 95% CI 0.40–0.93). Leg-press strength and the chair-stand test improved significantly in both groups, and the trial’s secondary analyses found no statistically significant differences between diets (a secondary-outcome comparison that cannot prove equivalence). Protein intake was lower on the vegetarian diet — 1.24 versus 1.58 g/kg/d, a between-group difference of −0.33 g/kg/d (95% CI −0.49 to −0.17) — even though both averages met the trial’s ≥1.2 g/kg/d target.

Why “not established” does not mean “worse”

The primary comparison was a non-inferiority test: it asked whether the vegetarian diet was no more than a preset margin worse. For muscle mass, the between-group difference was −0.33 kg (about −0.7 lb; 95% CI −0.69 to 0.03), which crossed the 0.25 kg margin, so the trial could not declare non-inferiority. But that same interval includes zero, so the trial also showed no statistically significant difference between the diets — proving neither equivalence nor inferiority. The vegetarian arm still met the protein target and gained muscle; and because the two diets differed in total protein, the comparison mixes diet pattern with protein level.

Where this sits

Prior evidence splits. An acute feeding study in 16 older adults found an omnivorous meal drove muscle-protein synthesis about 47% more than an isonitrogenous vegan meal, while multi-day studies found daily muscle-protein synthesis equal between vegan and omnivorous diets when protein was adequate. A separate 12-week, three-arm trial in 72 adults 65 and older found a self-selected vegan diet reduced appendicular lean mass and protein intake versus an omnivorous diet, with resistance exercise blunting but not eliminating the gap — longer, stricter plant-based diets can cost muscle when total protein is not protected, which is what IMPACT’s vegetarian protein target tried to avoid. IMPACT adds a trained, 12-week muscle-mass result on the “equivalent when targets are met” side — plus the observation that a vegetarian pattern made a high protein target harder to reach. Whether a protein-matched vegetarian diet costs older adults muscle over longer stretches stays open. The trial’s funding statement discloses a public–private partnership grant from the Dutch Top Consortia for Knowledge and Innovation (TKI, LWV21.57) in which the dairy company Royal FrieslandCampina participated as an industrial partner; the authors state they received no personal funding or financial remuneration from the partnership and that the researchers had full responsibility for study design, analysis, and the decision to publish, and they report no conflicts of interest. For a related question, see does-creatine-build-muscle-without-exercise.

Terms explained

  • Non-inferiority trial — a trial asking whether one approach is not unacceptably worse than another, rather than proving it is better. A preset margin defines “unacceptably worse.”
  • Appendicular muscle mass — the skeletal muscle of the arms and legs, measured here by DXA (dual-energy X-ray absorptiometry); a surrogate for muscle, distinct from whole-body lean mass.
  • Lacto-ovo vegetarian — a plant-based diet that still includes eggs and dairy.
  • g/kg/d — grams of protein per kilogram of body weight per day.

Sources

  1. Primary source: van Oppenraaij SL, Weijs PJM, Verlaan S. “Effects of a self-selected vegetarian compared with omnivorous diet on protein intake and muscle mass in older adults during resistance exercise: the IMPACT randomized controlled trial.” Am J Clin Nutr 2026, article 101533; published 2026-09, online 2026-09-22. DOI: 10.1016/j.ajcnut.2026.101533 · PMID: 42772454. Trial registration: NCT06172725. Peer-reviewed randomized controlled trial (non-inferiority, two-arm parallel). Open-access (CC-BY 4.0) publisher full text examined; funding statement and conflict-of-interest declaration read and cited in the body.
  2. Landscape — acute feeding trial: Pinckaers PJM, et al. “Higher Muscle Protein Synthesis Rates Following Ingestion of an Omnivorous Meal Compared with an Isocaloric and Isonitrogenous Vegan Meal in Healthy, Older Adults.” J Nutr 2024. DOI: 10.1016/j.tjnut.2023.11.004 · PMID: 37972895 (abstract-level access).
  3. Landscape — multi-day vegan diet: Monteyne AJ, et al. “A mycoprotein-based high-protein vegan diet supports equivalent daily myofibrillar protein synthesis rates compared with an isonitrogenous omnivorous diet in older adults.” Br J Nutr 2021. DOI: 10.1017/s0007114520004481 · PMID: 33172506 (abstract-level access).
  4. Landscape — whole-diet vegan trial: Domic J, et al. “A Well-Balanced Vegan Diet Does not Compromise Daily Mixed Muscle Protein Synthesis Rates when Compared with an Omnivorous Diet in Active Older Adults: A Randomized Controlled Cross-Over Trial.” J Nutr 2025. DOI: 10.1016/j.tjnut.2024.12.019 · PMID: 39732437 (abstract-level access).
  5. Landscape — parallel-arm vegan diet trial: Domić J, et al. “A self-selected vegan diet reduces skeletal muscle mass in healthy older adults: a randomized controlled trial.” Am J Clin Nutr 2026, article 101416; published 2026-09, online 2026-08-14. DOI: 10.1016/j.ajcnut.2026.101416 · PMID: 42680254 (abstract-level access).
  6. Press signal (framing only, not evidence): “New Trial Compared Muscle Gains in Vegetarians And Meat Eaters. One Difference Stood Out.” ScienceAlert, 2026-10-10. https://www.sciencealert.com/new-trial-compared-muscle-gains-in-vegetarians-and-meat-eaters-one-difference-stood-out

Related Articles