Soy Protein vs Whey Protein: What Trials Show

Soy protein vs whey protein, compared on what the long-term trials found rather than on laboratory markers, plus three claims we have removed.

Both are complete proteins. Both work. The difference people argue about turns out to be much smaller than the marketing on either side implies, and the long-term trials say so plainly.

This page has been corrected in three places. The previous version concluded that whey wins for lean mass, repeated a claim that soy may reduce sperm counts, and said heavy whey use has been linked to organ strain. None of those survive contact with the evidence.

Key Takeaways

• Over 6+ weeks of training, soy and whey produced no difference in strength or lean mass (1).
• Soy does not lower testosterone or other male hormones (2).
• High protein intake did not harm kidney function in healthy adults (3).
• Whey scores higher on protein quality by DIAAS; the practical gap is small (4).
• Soy intake was associated with better breast cancer survival, not worse (5).

What They Are

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Whey is what separates from milk when casein is removed during cheesemaking. Concentrates retain more lactose and fat; isolates are further refined, with minimal lactose, which matters if dairy sugar upsets you (9).

Soy protein comes from soybeans that have been ground and had most of the fibre, carbohydrate and fat removed, leaving soy flour, concentrate or isolate. It is a complete protein containing all nine essential amino acids (6).

Protein content varies by brand and grade, so the label on the tub tells you more than any general figure. We have removed the specific per-100-gram numbers that used to appear here, because they did not match the products people actually buy.

The Muscle Question, Answered Properly

This is where the previous version of this page went wrong, and the reason is interesting.

Whey does stimulate muscle protein synthesis more strongly than soy in the hours after a dose. That is real, and it is largely because whey carries more leucine, the amino acid that triggers the response. If you measure the acute signal, whey wins.

But the acute signal is not the outcome anyone cares about. A meta-analysis identified nine studies in 266 participants comparing soy protein with animal protein supplementation alongside resistance training for at least six weeks. Five compared whey with soy; four compared soy with beef, milk or dairy protein. Supplementing training with either whey or soy produced significant increases in strength, and there was no difference between the groups for bench press, squat or lean body mass (1).

So the laboratory difference does not translate into a difference in what your body ends up looking like or lifting. Total protein intake and the training itself do the work.

On protein quality scores

The previous version used biological value (104 vs 74) and net protein utilisation (92 vs 61). Those are older measures. The current standard is the Digestible Indispensable Amino Acid Score, DIAAS, which is based on how much of each essential amino acid is actually absorbed in the small intestine rather than on total digestibility.

In a study comparing four animal and four plant proteins, whey protein isolate and concentrate had higher digestibility for most indispensable amino acids than soy protein isolate, soy flour, pea protein concentrate or wheat. The researchers also found that the older PDCAAS-style calculation overestimates the quality of plant proteins relative to DIAAS (4).

Whey scores higher. Both are complete proteins and both build muscle equally over time (1). Both statements are true.

Three Claims We Removed

Soy and male hormones

This one circulates endlessly, so here is the actual analysis. Researchers pooled 41 clinical studies in adult men consuming soy foods, soy protein or isoflavone extracts. Total testosterone was measured in 1,753 men and free testosterone in 752; estradiol was measured in 1,000 and sex hormone binding globulin in 967.

Regardless of the statistical model used, there was no significant effect on any of those hormones. Sub-analysis by isoflavone dose and by study duration also showed no effect (2).

Protein and kidneys

A systematic review examined 26 studies of healthy adults, comparing intakes at the recommended level with higher intakes. Most randomised trials found higher glomerular filtration rates with more protein, but all of those rates were still consistent with normal kidney function. Higher protein intake had little or no effect on blood markers of kidney function and no adverse effect on blood pressure (3).

The honest caveat, which the review states: studies in people with existing kidney disease were excluded, and most trials ran under six months. So this addresses the healthy-person myth, not people with diagnosed kidney disease, who should follow their own clinician’s advice.

The Soy and Breast Cancer Question

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This is the concern that stops many women using soy, and it is worth answering because the evidence runs the opposite way to the fear.

A systematic review and meta-analysis of 12 articles covering 37,275 women with breast cancer found that the highest versus lowest pre-diagnosis soy and isoflavone consumption was associated with better overall survival (HR 0.84). Both pre-diagnosis and post-diagnosis isoflavone intake were associated with a reduced risk of recurrence. The authors describe the evidence as limited, and the association with overall survival was clearest in post-menopausal patients (5).

NCCIH’s summary of soy is the right companion reading for anyone weighing this up (8). And as always: if you have or have had breast cancer, this is a conversation with your oncology team rather than a decision made from an article.

How to Actually Choose

The question is less interesting than it looks, which is good news.

• Vegan, or avoiding dairy? Soy. It is a complete protein and it builds muscle as well as whey does over a training block (1).
• Lactose intolerant? Whey isolate contains minimal lactose, or use soy (9).
• Soy allergy? Whey.
• No constraints? Pick on taste, texture and price. You will drink the one you like.
• Either way, the totals matter more. Protein is needed to build and repair muscle, bone, skin and connective tissue (6, 7), and getting enough across the day beats optimising the source.

Our protein shake guide covers blending your own, the bodybuilding diet guide covers the wider eating plan, and the balanced diet guide covers getting protein from food first.

Frequently Asked Questions

Is whey better than soy for building muscle?
Not in the trials that matter. Whey does stimulate muscle protein synthesis more strongly in the hours after a dose, because it carries more leucine. But a meta-analysis of studies lasting six weeks or more found no difference between soy and whey or other animal proteins for gains in strength or lean body mass.

Does soy protein lower testosterone in men?
No. A meta-analysis of 41 clinical studies, measuring total testosterone in 1,753 men and free testosterone in 752, found no significant effect of soy protein or isoflavone intake on any male reproductive hormone, regardless of dose or study duration. We have removed the sperm claim that was on this page.

Does protein powder damage your kidneys?
Not in people with healthy kidneys. A systematic review of 26 studies found that protein intake above the recommended amount raised glomerular filtration rate in most trials, but to rates still consistent with normal kidney function, with little or no effect on blood markers. If you already have kidney disease, that is a different situation and a question for your doctor.

Can women with breast cancer eat soy?
The evidence points the reassuring way. A meta-analysis covering 37,275 women with breast cancer found pre-diagnosis soy and isoflavone intake associated with better overall survival, and both pre- and post-diagnosis intake associated with reduced recurrence. Ask your oncology team about your own situation, but soy is not the hazard it is sometimes made out to be.

Which should I buy?
Whichever you will actually drink and tolerate. Soy if you are vegan or avoid dairy; whey if you like the texture and have no problem with milk. Total daily protein and resistance training decide the outcome far more than the label on the tub.

The Bottom Line

Whey stimulates muscle protein synthesis more strongly in the hours after a shake, and it scores higher on protein quality. Over six weeks or more of resistance training, that advantage disappears: pooled trials found no difference between soy and whey for strength or lean body mass. Soy does not lower testosterone, protein powder does not damage healthy kidneys, and soy intake is associated with better breast cancer survival rather than worse. Choose on diet, tolerance and taste, then spend your attention on hitting your daily protein target and showing up to train.

This article is for informational purposes only and is not a substitute for professional medical advice. If you have kidney disease, a soy or milk allergy, or a history of breast cancer, talk to your doctor or a registered dietitian before changing your protein intake.

Related Articles

9 Homemade Protein Shakes Made With Real Food

Bodybuilding Diet: Protein, Calories and Meal Plan Guide

Balanced Diet Guide With the 5 Food Groups

Plant-Based Diet: Benefits, Risks and How to Start

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