Does Protein Source Matter for Muscle Health?

1. Animal protein resulted in significantly greater muscle mass overall.

Thirty randomized trials involving 1,538 participants were included in the muscle-mass meta-analysis.

Compared with plant protein, animal protein resulted in significantly greater muscle mass after the intervention (SMD −0.20; P = .02).

The magnitude of the difference was modest. Results also varied considerably between studies.

2. In adults aged 60 and older, there was no significant difference in muscle mass.

Age changed the result.

Among younger adults, animal protein resulted in significantly greater muscle mass than plant protein (21 RCTs; SMD −0.28; P = .01).

Among adults aged 60 and older, no significant difference was found between plant and animal protein (9 RCTs; SMD −0.05; P = .74).

This finding is particularly relevant to healthy aging. However, considerably fewer trials were available in older adults.

3. Soy protein and milk protein produced similar results.

The type of plant protein mattered.

Across 17 randomized trials, there was no significant difference in muscle mass between soy and milk protein (SMD −0.02; P = .80).

Animal protein resulted in significantly greater muscle mass than the non-soy plant proteins studied, including rice, chia, oat and potato protein (5 RCTs; SMD −0.58; P = .02).

However, only five trials contributed to this comparison. The result should therefore not be generalized to all non-soy plant proteins.

4. The findings for muscle strength were more complex.

Across the overall analyses, there was no consistent difference between plant and animal protein for muscle strength.

However, among older adults, animal protein resulted in significantly greater improvement in lower-body strength (SMD −0.38; P = .03).

No significant difference was found for upper-body strength.

The strength analyses included fewer participants and trials than the muscle-mass analysis.

5. Protein source did not significantly affect physical performance.

Five randomized trials assessed physical performance using measures such as walking and functional tests.

There was no significant difference between plant and animal protein (SMD 0.12; P = .47).

Loss of muscle mass and strength can progressively affect mobility, physical function and independence with age.

These findings show that the question is not simply whether plant or animal protein is better.

The specific protein source matters.

Soy protein produced muscle-mass outcomes comparable with milk protein. Evidence for other plant proteins was more limited and showed different results.

For people aged 60 and older, current randomized evidence did not demonstrate an overall advantage of animal protein for muscle mass. However, animal protein was associated with greater lower-body strength in this age group.

Resistance exercise also remains central to muscle health. In this review, improvements in muscle outcomes were consistently observed in trials that combined protein interventions with resistance training.

This review has several strengths.

It included 43 randomized controlled trials and required plant and animal interventions to provide comparable amounts of protein. Thirty trials and 1,538 participants contributed to the main muscle-mass analysis.

However, the studies differed considerably in participant characteristics, protein sources, doses, duration and exercise interventions.

Only 15 of 43 trials were assessed as having an overall low risk of bias. Twenty-seven had some concerns and one had a high risk of bias.

Evidence specifically relevant to older adults was also more limited. Only ten trials had a mean participant age of at least 60 years, and very few involved people with clinically significant low muscle mass, frailty or sarcopenia.

Importantly, none of the included trials directly assessed sarcopenia as an outcome.

The study therefore provides stronger evidence about changes in muscle mass than about prevention of sarcopenia or preservation of long-term independence.