A Self-Planned Vegan Diet Cost Older Adults Real Muscle in 12 Weeks

If you coach clients past 60, or you're one yourself, you've probably had the "should I go vegan?" conversation at least once this year. Plant-based eating has been trending for health, ethics and environmental reasons across every age group. A study just published in the American Journal of Clinical Nutrition gives the clearest answer yet to what actually happens to muscle when an older adult switches to a self-planned vegan diet without professional guidance, and the results are a useful reality check for anyone assuming "vegan" and "muscle-safe" are the same thing.

What the study actually did

Researchers from Wageningen University and Maastricht University Medical Centre, led by Jacintha Domić, ran a 12-week randomized controlled trial (known as the Vold-study) on 72 healthy adults aged 65 and older. Participants were split into three groups: one kept eating their normal omnivorous diet (OMNI), one switched to a self-selected vegan diet with no dietitian oversight (VEG), and one switched to the same self-selected vegan diet but added structured resistance exercise twice a week (VEG-RE).

The word "self-selected" is the important design choice here. The researchers didn't hand participants a carefully balanced, protein-matched vegan meal plan. They told them to eat vegan and figure it out themselves, which is exactly what most real-world clients do when they decide to try plant-based eating on their own.

The measurement side of the study was unusually rigorous for a diet trial. Thigh muscle volume and fat infiltration were tracked with MRI scans, appendicular lean mass (the muscle on your arms and legs, which is what actually predicts falls and frailty in older adults) was measured with a DXA scan, and, most impressively, the researchers directly measured how fast participants' muscles were building new protein using a deuterium oxide ("heavy water") tracer protocol over the first 10 days. That's a gold-standard method for catching a real biological signal, not just a scale-weight change that could be water or gut contents.

What they found

The vegan-only group's protein intake fell off a cliff almost immediately: down about 27.7 grams per day in week one, still down 19.3 grams per day by week 12, compared with no change at all in the omnivorous group (p < 0.0001 at every checkpoint). Adding resistance training didn't fix that: the VEG-RE group's protein intake dropped by a similar amount.

That protein shortfall showed up directly in the muscle data. Over 12 weeks, thigh muscle volume dropped by 0.61 liters in the vegan-only group, compared with a negligible 0.01 liter change in the omnivore group, a statistically significant and clinically meaningful difference (p < 0.0001). Appendicular lean mass told the same story: down 1.15 kg in the vegan-only group versus essentially flat (+0.05 kg) in the omnivore group. Muscle protein synthesis rates, the tracer measurement, were lower in the vegan-only group (1.09%/day) than in the omnivore group (1.27%/day, p = 0.0073).

Here's the more encouraging half of the result: adding resistance training roughly halved the damage. The VEG-RE group still lost muscle volume and lean mass, but significantly less than the vegan-only group (thigh volume -0.35 L vs. -0.61 L; lean mass -0.62 kg vs. -1.15 kg), and their muscle protein synthesis rate (1.24%/day) was statistically no different from the omnivore group's. Training didn't fully cancel out the effect of a self-planned, lower-protein diet, but it did most of the heavy lifting in damage control.

Why this matters if you train, or you coach people who do

This isn't a study about veganism being inherently bad for muscle. The researchers are careful about that distinction, and it lines up with a growing body of contradictory evidence. A separate crossover trial in active older adults found that a well-balanced vegan diet, matched for total protein and leucine content, produced essentially the same muscle protein synthesis response as an omnivorous diet. The difference between that study and this one isn't the presence or absence of animal products, it's whether someone deliberately replaced the protein they lost.

That's the actionable insight for coaches and self-coached lifters alike: the risk isn't "plant-based," it's "unplanned." When people cut out meat, dairy and eggs without consciously building in replacements, protein intake drops by default, because most Western diets get a large share of daily protein from animal sources almost automatically. Nobody has to intend to under-eat protein on a vegan diet. It just happens unless you actively counter it.

A related study published around the same time reinforces the practical stakes: physically active vegans habitually eating around 0.98 g/kg of protein (versus 2.14 g/kg in omnivore comparisons) recovered from a standardized muscle-damage protocol just as well on objective markers like creatine kinase and jump performance, but reported meaningfully worse delayed-onset muscle soreness at 24 and 72 hours. In plain terms, under-fueled protein intake on a plant-based diet might not tank your numbers on paper, but it can make training feel a lot worse day to day, which matters enormously for adherence.

For coaches, this is a client-conversation trigger, not a reason to talk anyone out of plant-based eating. Anyone mentioning they're "trying vegan" or "cutting back on meat," especially older or perimenopausal/postmenopausal clients already fighting an uphill sarcopenia battle, deserves a five-minute protein-replacement conversation before week one, not a body-composition surprise at week twelve.

What to actually do this week

If you or a client is switching toward plant-based eating, don't wing the protein side of it. Before removing meat, dairy or eggs from a meal, name the specific replacement going into that same meal: tofu, tempeh, seitan, edamame, lentils, chickpeas, or a plant protein powder. Aim for a daily total in the range most sarcopenia and resistance-training research supports, roughly 1.6-2.2 g of protein per kg of bodyweight, spread across three to four meals rather than back-loaded into dinner. Prioritize the leucine-rich plant sources (soy products and pea-protein isolates test closest to animal protein on essential amino acid profile) since leucine threshold is what actually triggers the muscle-building signal.

Keep resistance training in the picture regardless of what's on the plate. The data above shows training doesn't erase a protein shortfall, but it meaningfully blunts it, and for older adults specifically, preserving strength and lean mass is one of the highest-leverage things a coach can protect.

Finally, don't rely on "feeling fine" as your feedback loop. Track total protein for the first two to three weeks of any diet change, and recheck weight and a basic strength benchmark (grip strength, a leg-press or squat number, whatever's already in your program) at the four-week mark. Protein deficits are invisible until they show up on a scan or a strength test, and by then you've already lost ground.

Sources and references

• Domić, J., Grootswagers, P., Pinckaers, P.J.M., Melchers, M., van Loon, L.J.C., & de Groot, L.C.P.G.M. (2026). A self-selected vegan diet reduces skeletal muscle mass in healthy older adults: a randomized controlled trial. American Journal of Clinical Nutrition. PMID: 42680254. DOI: 10.1016/j.ajcnut.2026.101416. Registered at ClinicalTrials.gov NCT05809466
• Mendes-Netto, R.S., et al. (2026). Similar Muscle Recovery but Greater Soreness in Active Vegans on Low-Protein Diets Compared to High-Protein Omnivores Following Exercise-Induced Damage. Nutrition Bulletin. PMID: 42584017. DOI: 10.1111/nbu.70064
• Hernandez-Martinez, J., et al. (2026). Combined effects of strength training, whey protein, and HMB on sarcopenia in older people: A systematic review and meta-analysis. Medicine (Baltimore). PMID: 42700006. DOI: 10.1097/MD.0000000000050477
• Domić, J., et al. (2025). The effect of a vegan diet with or without resistance exercise on thigh muscle volume in older adults. Research protocol of the Vold-study. BMC Geriatrics. DOI: 10.1186/s12877-025-06708-9
• For contrast, a well-balanced (protein-matched) vegan/omnivore crossover trial in active older adults found no muscle protein synthesis penalty: "A Well-Balanced Vegan Diet Does not Compromise Daily Mixed Muscle Protein Synthesis Rates when Compared with an Omnivorous Diet in Active Older Adults." PMID: 39732437

This article summarizes peer-reviewed research for general educational purposes. It is not a substitute for individualized medical, nutrition, or coaching advice. If you or a client is planning a major diet change, especially an older adult managing existing health conditions, please consult a qualified professional.