How we rate
“Strong” = several concordant meta-analyses or systematic reviews. “Moderate” = one synthesis, or several controlled trials. “Limited” = early trials only. The rating is about the quality of the studies, not about how useful the supplement is.
Soy protein
Soy protein: with resistance training, as much strength and muscle as whey in long trials, despite lower muscle protein synthesis right after the training session. And slightly less LDL cholesterol (3 to 4%).
Benefits
- Muscle and strength with resistance training: in a meta-analysis of 9 trials (266 people), soy protein yields the same gains in strength and lean mass as whey or other animal proteins.
- Vegan: in young men consuming 1.6 g of protein per kilo per day (soy isolate), 12 weeks of resistance training resulted in a gain of 1.2 kg of leg lean mass, just as much as in omnivores supplemented with whey.
- Beginners: over 6 weeks of resistance training, 1.2 g per kilo of soy protein or whey yielded greater gains in lean mass and strength than a placebo; the authors consider the effect minimal.
- Postmenopausal women: 25 g of soy protein in milk almost doubled the bench press gain over 16 weeks of resistance training (+12.5 kg versus +6.7 kg under placebo), with no extra muscle.
- Cholesterol: across 43 trials, 25 g per day for 6 weeks lowered LDL by 4.76 mg/dL, or 3 to 4%, and total cholesterol by 6.41 mg/dL.
- Lipids: in another meta-analysis of 43 trials (median of 30 g per day), LDL −4.2 to −5.5 %, HDL +3.2 % and fasting triglycerides −10.7 %.
Precautions
Testosterone: a meta-analysis finds no effect of soy protein or isoflavones on testosterone, free or total, in men. Female hormones: before menopause, soy or its isoflavones lower FSH and LH by about 20% and lengthen the cycle by 1.05 days on average; the clinical significance remains unknown. Limits: muscle, the most recent meta-analysis: in young adults, neither whey nor soy significantly increases lean mass, and only whey improves the bench press and squat. Muscle protein synthesis: in men averaging 71 years of age, soy isolate stimulates muscle protein synthesis less than whey, at both 20 g and 40 g; milk also performs better than soy in young men. Weight loss: over 23 weeks, about 56 g per day of soy protein changed neither weight nor fat mass compared to carbohydrates, unlike whey. Without resistance training: in postmenopausal women, 25 g per day for 16 weeks changed nothing regarding body composition; training was what built the muscle. Bones: over 12 months, isoflavone-rich soy protein changed neither hip nor spinal bone density in men.
Recommended dosage
Muscle: 20 to 40 g post-workout, or enough to reach 1.2 to 1.6 g of protein per kilo per day; cholesterol: 25 to 30 g per day.
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How to take it
- For muscle: with a weight training programme, enough to reach 1.2 to 1.6 g of protein per kilo per day; without training, no trial from this sheet shows any gain.
- Dose per serving: in elderly men, 20 g of soy after the training session did no better than nothing, 40 g did, but less than whey.
- Right after the training session: whey stimulates muscle synthesis slightly more (+31 % compared to soy in one trial), but with 20 g and carbohydrates, another trial finds no difference.
- Over time: in 6- to 16-week weight training trials, this gap is not found in muscle and strength gains.
- Cholesterol: 15 to 30 g per day, over 4 to 18 weeks in the trials; the effect is small, around 3 to 4 % less LDL.
- Form: soy protein isolate is the form tested in most of these trials.
Did you know?
Soy does not lower testosterone: a meta-analysis of 15 treated groups versus placebo finds no effect of soy protein or isoflavones on testosterone, free or total, in men.
What the studies say
This systematic review and meta-analysis, including 261 young adults, evaluated the impact of whey and soy protein supplements on lean mass and strength. The analysis found no significant effect of whey or soy protein supplementation on lean mass. Protein supplementation increased total essential amino acids in plasma, and whey protein specifically improved bench press and squat performance.
Davis BE, Young I, Giglotti JC, Yao J, Tou JC. — Journal of dietary supplements (2026) — View the source study
For information only: not a substitute for medical advice. Dietary supplements are not a replacement for a varied, balanced diet. Learn more