Strong evidencePhysical performance

Soy Protein

Soy is a complete plant protein that produces the same strength and lean-mass gains as whey in head-to-head training trials, and modestly lowers LDL cholesterol. The persistent worry that it lowers testosterone in men is not supported by controlled trials.

What the science says

Equals animal protein for muscle mass and strength Strong evidence
A meta-analysis of 9 resistance-training trials (266 participants, all at least 6 weeks) found bench press and squat strength and lean body mass increased significantly with supplementation, with no difference between soy and whey or other animal proteins (all between-group p > 0.38).
Modestly lowers LDL cholesterol Strong evidence
Pooling the 46 trials the FDA used to reconsider soy's heart health claim, a median 25 g/day for a median 6 weeks lowered LDL by 4.76 mg/dL and total cholesterol by 6.41 mg/dL - roughly a 3-4% LDL reduction. Real and consistent, but small: this is a diet-quality nudge, not a substitute for statin therapy.
Does not lower testosterone or feminize men Moderate evidence
Meta-analyses of clinical trials in men find no significant effect of soy protein or isoflavones on total testosterone, free testosterone or estradiol. The handful of gynecomastia case reports involve extreme intakes (around 360 mg isoflavones daily, an order of magnitude above typical supplement use).

Dosage & safety

Studied dose25 g/day of soy protein is the cholesterol-lowering dose used in trials and the FDA claim threshold. For training, 20-40 g per serving. Isoflavone content varies widely by processing: alcohol-washed isolates retain very little, while concentrates may supply 1-2 mg isoflavones per gram of protein.

SafetySoy is one of the major food allergens. Isoflavones can interfere with levothyroxine absorption, so separate doses by at least 4 hours and ensure adequate iodine intake. Trial and cohort data in breast cancer survivors are neutral to favorable, but anyone on endocrine therapy should discuss it with their oncologist. Soy protein concentrate commonly causes gas and bloating; isolates are better tolerated.

Scientific references

No Difference Between the Effects of Supplementing With Soy Protein Versus Animal Protein on Gains in Muscle Mass and Strength in Response to Resistance Exercise
Messina M, Lynch H, Dickinson JM, Reed KE — Int J Sport Nutr Exerc Metab, 2018
A Meta-Analysis of 46 Studies Identified by the FDA Demonstrates that Soy Protein Decreases Circulating LDL and Total Cholesterol Concentrations in Adults
Blanco Mejia S, Messina M, Li SS, et al. — J Nutr, 2019

Where to buy

Nutricost, Soy Protein Isolate, Unflavored, 2 lb (907 g)

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Related in Physical performance

Creatine (monohydrate) Strong evidence
Whey Protein Strong evidence
Caffeine Strong evidence
Beta-Alanine Moderate evidence
BCAA (Branched-Chain Amino Acids) Limited evidence
EAA (Essential Amino Acids) Moderate evidence