November 4, 2026

Plant or Animal Protein for Muscle? What Trials Show About Lean Mass, Leucine, and Digestibility

By Dr. Zakira Naureen, Associate Professor, University of Nizwa
Nutrition Muscle Health Young Adults

In Simple Terms

  • If I eat mostly lentils, beans, and soy instead of meat, will I lose muscle?
  • Is whey or chicken really better for building muscle than plant protein, or is that a myth?
  • Does it matter more what kind of protein I eat, or whether I lift weights and eat enough of it?
  • What do we still not know for people who carry extra body fat and have low muscle?

Few nutrition questions create as much confusion as the choice between plant and animal protein. Fitness advice often claims that meat, eggs, and whey are the only proteins that truly build muscle. Plant based advocates say the difference is irrelevant. For a young adult trying to avoid sarcopenic obesity, the practical stakes are real. Many people in the Gulf region eat a mix of rice, bread, legumes, dairy, fish, and meat, and budget, culture, and religion all shape which proteins end up on the plate. This article looks at what the trials actually measured, where the two sides agree, and what remains uncertain.

The mechanism: why protein source could matter at all

Muscle is rebuilt from essential amino acids, and one of them, leucine, acts as a trigger that switches on the cell's protein building machinery. A 2019 critical review in Nutrients by Berrazaga and colleagues explains three reasons plant proteins might be at a disadvantage. First, most plant sources are less digestible, partly because of their structure, fiber, and compounds such as phytic acid and protease inhibitors, so fewer amino acids reach the blood. Second, plant proteins contain less leucine on average (about 7.1 percent of protein compared with 8.8 percent in animal sources). Third, specific essential amino acids run short in specific foods, such as lysine in cereals and sulfur containing amino acids in legumes.

The same review notes that these gaps are not fixed. Heat treatment and processing can improve digestibility, blending sources (for example cereals with legumes) fills in missing amino acids, and eating a larger portion narrows the difference. In short-term laboratory studies, muscle protein synthesis after soy or wheat protein was generally lower than after whey or casein, especially in older adults. That is the biological case for caution. The question is whether it shows up in actual muscle after weeks of training.

This connects directly to sarcopenic obesity. Excess body fat can blunt how well muscle responds to protein, a problem we describe in our post on insulin resistance and muscle protein synthesis. If muscle already responds weakly, a lower quality protein source could in theory matter more, not less.

Evidence 1: the pooled picture across randomized trials

The broadest view comes from Lim and colleagues (Nutrients, 2021), who screened more than 3,000 articles and pooled 16 randomized controlled trials comparing animal and plant protein for lean mass and strength. Overall, protein source did not change absolute lean mass or muscle strength, and whether people also did resistance training made no difference to that conclusion. However, animal protein did favor percent lean mass. In adults under 50, animal protein produced a modestly larger gain in lean mass (a weighted mean difference of 0.41 kg, 95 percent CI 0.08 to 0.74) and in percent lean mass (0.50 percent, CI 0.00 to 1.01). The authors concluded that animal protein tends to benefit lean mass more than plant protein, especially in younger adults.

Two things deserve emphasis. The advantage was small, under half a kilogram, and the confidence interval for percent lean mass touches zero. It is also a finding in adults under 50, which is the group our research focuses on, so it cannot be dismissed as only relevant to older people.

Evidence 2: soy against animal protein during resistance training

Soy is the best studied plant protein. A meta-analysis in the International Journal of Sport Nutrition and Exercise Metabolism (2018), pooling nine studies with 266 participants, found that soy supplementation produced gains in strength and lean body mass similar to those from whey or other animal proteins during resistance training. Both whey and soy increased bench press and squat strength with no difference between them. The limitation is the modest total sample size, and the supplementation studies mostly used protein powders in young, healthy, trained or training volunteers, not people with excess body fat and low muscle quality.

Evidence 3: a fully vegan diet in young adults

Perhaps the most relevant recent trial is by Monteyne and colleagues (Journal of Nutrition, 2023). In its first phase, 16 healthy young adults ate a high protein diet (1.8 g per kg of body weight per day) for 3 days, either omnivorous or exclusively non animal, with single leg resistance exercise. Daily muscle protein synthesis was about 12 percent higher in the exercised leg than the rested leg, and it did not differ between diets. In the second phase, 22 young adults followed a 10 week, five days per week progressive resistance program on about 2 g of protein per kg per day. Gains in lean mass, thigh muscle volume, muscle fiber size, and strength were similar in both groups.

The result is encouraging, but the design matters. The groups were small, the vegan diet was rich in mycoprotein (a fungal protein), and every participant had a normal body mass index. The finding applies to a well planned, high protein, non animal diet combined with demanding training. It should not be read as proof that any plant diet with low protein will perform the same.

What this means for sarcopenic obesity and young adults

Weighing these sources together, a few practical points follow:

  • Total protein and resistance training matter more than source. In both the 2021 meta-analysis and the vegan trial, training and adequate intake were the shared ingredients. Our posts on protein during a calorie deficit and strength training basics cover those in detail.
  • If you eat mostly plant protein, plan for it. Aim for a generous total amount, combine cereals with legumes, include soy, and consider larger portions at meals, which Berrazaga and colleagues suggest narrows the anabolic gap.
  • If you eat animal protein, a mixed approach is reasonable. Fish, eggs, dairy, and lean meat supply leucine efficiently and fit regional diets, and mixing plant and animal sources is also an option the review describes.
  • Do not assume that a plant based diet is automatically protective or harmful for muscle. The evidence supports neither extreme. Our complete guide for young adults sets out general nutrition steps that apply either way.

Limitations and open questions

Most trials enroll healthy, lean to normal weight volunteers and run for weeks, not years. Berrazaga and colleagues note that most evidence comes from young adults or animals and call for studies in people with sarcopenia, and the same gap applies to sarcopenic obesity. Studies differ in dose, protein type, training programs, and how lean mass was measured, which makes pooled results imprecise. Lean mass from body scans includes water and organs, not only functional muscle, so small differences of a few hundred grams may not be meaningful in daily life. Industry sponsorship appears in some trials. Observational studies linking plant protein intake to muscle loss are inconsistent. No trial that we are aware of has directly compared plant and animal protein in young Gulf adults with excess body fat and low muscle, and local diets, cooking methods, and food processing may change digestibility. That is the type of question that precision nutrition research is designed to answer.

This post is for general education and is not medical advice. Please speak with a qualified healthcare provider or registered dietitian before making major changes to your diet, especially if you have a medical condition, allergy, or take medication.

References

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