Protein for Sarcopenia: Why Soy Holds Up to Whey

For older adults with sarcopenia, soy protein appears to build muscle mass about as well as animal protein. The better question is whether the protein plan includes resistance exercise—and whether a non-soy plant powder can deliver amino acids in a usable form.

A 2025 systematic review and meta-analysis of 43 randomized controlled trials found no meaningful difference in muscle-mass outcomes between plant and animal protein in adults aged 60 or older (standardized mean difference [SMD] −0.05; p=.74). In 17 soy-versus-animal comparisons, the difference was even closer to zero (SMD −0.02; p=.80). Those are null results, not a narrow win for whey. Reid-McCann et al. required groups to be matched within 5 g of protein, which matters because serving more protein to one group can make a source comparison meaningless.

That conclusion has an important boundary: the same review found non-soy plant proteins—including rice, chia, oat, and potato—performed worse than animal protein for muscle mass (SMD −0.58). The pooled estimate combines several very different proteins, so it cannot tell you the exact penalty for any one powder. It does tell plant-based older adults that "plant protein" is too broad a category for practical decisions.

Protein adds muscle mass. Exercise drives function.

The most diagnosis-specific evidence comes from a 2023 meta-analysis of six randomized trials in adults aged 60 or older with Asian Working Group for Sarcopenia-defined sarcopenia. Protein supplementation improved appendicular skeletal muscle mass, the lean tissue in the arms and legs most relevant to sarcopenia: SMD 0.41 (95% CI, 0.24 to 0.58; p<0.001; n=536). Heterogeneity was very low (I²=1%), so the mass result was consistent across the included trials. Kwon et al. provide good evidence that protein can improve this body-composition endpoint.

It did not reliably improve the outcomes people feel in daily life. Handgrip strength showed SMD 0.37, but the 95% CI ran from −0.32 to 1.07 (p=0.29); Short Physical Performance Battery scores showed SMD 0.35, with a 95% CI from −0.47 to 1.18 (p=0.40). Heterogeneity reached 94% for both outcomes. Those pooled estimates do not establish a typical functional benefit, because the trials differed too much to yield one useful average.

Five of Kwon's six trials paired protein with resistance or multicomponent exercise. That makes it hard to separate protein's contribution from training's contribution. A later systematic review by Yoshimura et al., designed to compare protein alone with protein plus exercise in sarcopenia and physical frailty trials, found that exercise was the critical ingredient for functional improvement. Protein can support adaptation; it does not substitute for loading your muscles.

This pattern also appears in a broader population of older adults. Kirwan et al. found that protein interventions augmented resistance exercise effects on appendicular lean mass and handgrip strength. The word "augmented" earns its place here: the evidence supports protein as part of a training program, not as a powder-only route to stronger legs or safer stair climbing.

Why soy and whey tie, while other plant proteins may not

Whey has long been the reference protein for muscle research because it is rich in leucine, an essential amino acid that helps trigger muscle protein synthesis. But a familiar reference protein is not automatically a superior one. In the dose-matched older-adult trials reviewed by Reid-McCann and colleagues, soy matched animal protein for muscle mass.

The non-soy gap may reflect more than an amino-acid label. Reid-McCann et al. cite evidence that even a plant meal matched to an omnivorous meal by amino-acid profile can produce less post-meal muscle protein synthesis. Their explanation centers on digestion and the food matrix: amino acids trapped in different protein structures may not appear in circulation with the same speed or amount. That is a plausible mechanism, not proof that every rice or oat isolate fails in the same way.

Leucine remains a credible target, but the practical case outruns the long-term trial evidence. Nasimi et al. reported positive lean-mass effects with whey intakes of 1.2 to 1.59 g/kg/day alongside resistance exercise. Chang et al. also found support for whey, leucine, and vitamin D combinations in sarcopenia research. A 2025 double-blind trial of leucine-enriched branched-chain amino acids in older patients with chronic kidney disease found slower sarcopenia progression, offering direct human support for leucine as an active variable. That trial involved a high-risk kidney-disease population with altered protein metabolism, however, so it cannot validate a leucine-fortified rice-protein protocol for otherwise community-dwelling adults with sarcopenia.

What dose can the evidence support?

Professional guidance commonly places total daily protein intake for older adults around 1.0 to 1.2 g/kg/day, while trials in the reviewed literature used supplements ranging from 10 to 40 g/day. The more operational 1.2 to 1.59 g/kg/day range comes from evidence in programs that combined whey with resistance exercise. It is not a dose-response curve proving that every person with sarcopenia needs that amount.

No dose-escalation meta-analysis has established the minimum intake that reverses appendicular muscle loss in adults with diagnosed AWGS or European Working Group on Sarcopenia in Older People sarcopenia. Daily intake also depends on dietary baseline, body size, appetite, kidney disease, and medical treatment. People with chronic kidney disease or other conditions requiring protein restriction should set targets with their clinician or renal dietitian rather than importing a gym protocol.

Vitamin D and multinutrient formulas: useful, but harder to unpack

Several reviews report benefits from protein combined with vitamin D, leucine, or other nutrients. In a meta-analysis of eight trials totaling 776 patients with sarcopenia, vitamin D at 100 to 1,600 IU/day plus 10 to 44 g/day of protein improved handgrip strength (SMD 0.38; 95% CI, 0.18 to 0.47; p=0.04). Gkekas et al. support the idea that combined interventions can help, but they do not show how much of the effect came from protein, vitamin D, or the combination in each participant.

That distinction matters when shopping for a crowded tub of powder. A multinutrient formula may work as a package, yet the evidence does not establish that every added ingredient earns its price or that vitamin D helps equally in people who already have adequate vitamin D status.

What the evidence supports now

• Use protein to support muscle mass: The appendicular muscle-mass result in diagnosed sarcopenia is statistically significant and consistent across the six-trial analysis.
• Make resistance exercise part of the plan: Current evidence does not show that protein alone reliably improves grip strength or physical performance.
• Choose soy or whey based on preference and tolerance: In older adults, the best dose-matched source comparison found no muscle-mass advantage for animal protein over soy.
• Treat non-soy powders cautiously: The pooled non-soy result favored animal protein. Leucine-fortified blends are a reasonable hypothesis, not a proven replacement for whey or soy in sarcopenic adults.

The useful hierarchy is simple: adequate total protein, progressive resistance exercise, then source selection. Soy has evidence comparable to whey for muscle mass in older adults; non-soy plant proteins still need better long-term trials.

The remaining uncertainty is clinical, not cosmetic. Researchers still need longer trials that test whether added muscle mass preserves independence, and direct trials of leucine-fortified non-soy blends in adults aged 60 or older with diagnosed sarcopenia. Until then, choosing soy over whey is evidence-based. Treating any plant powder as interchangeable is not.