Protein extracted from defatted soybean, at 70% in concentrates and 90% or more in isolates. It is the only widely available plant protein with a complete essential amino acid profile and a protein quality score comparable to dairy.
Soy is the plant protein that does not require an apology. Its digestible indispensable amino acid score approaches that of dairy, and it contains enough leucine to raise muscle protein synthesis, though less per gram than whey. Trials comparing soy with whey during resistance training generally find whey slightly ahead on acute synthesis and roughly comparable on longer-term outcomes when total protein is adequate. Its regulatory story is a caution about how claims age: the FDA authorised a soy protein and heart disease health claim in 1999 and proposed to revoke it in 2017 after the accumulated evidence became inconsistent.
Protein percentage; isoflavone content varies with processing and is often unstated — a product without this marker may not resemble what was studied.
Also known as
Soy protein isolate, SPI, Soy protein concentrate
Regulation: The FDA authorised a health claim in 1999 linking 25 g/day of soy protein to reduced coronary heart disease risk, and in 2017 proposed to revoke it after later trials produced inconsistent results — the first time the agency proposed revoking an authorised health claim. EFSA also concluded a cause-and-effect relationship was not established for soy protein and cholesterol.
What the research used
The amounts, forms and durations published trials of Soy protein actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Resistance training and muscle protein synthesis
25–40 g
Moderate
Soy protein isolate per serving
Soy contains roughly 8% leucine against whey’s 10–11%, so a slightly larger serving is needed to cross the leucine threshold. At matched total daily protein, longer-term training outcomes are broadly comparable.
Studied for Blood lipid trials
25 g
Moderate
Soy protein daily replacing animal protein · 4 weeks to 6 months
The intake behind the original FDA claim. Meta-analyses now put the LDL reduction at around 3–4%, smaller than the early estimates, and part of it may come from what the soy displaces from the diet.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
Each timing below was actually compared against an alternative in a trial, which is unusual.
🔀Split through the day
Moderate
The same protein distribution logic applies as for dairy protein, with the leucine threshold slightly harder to reach per gram. Tested for protein generally rather than for soy specifically.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Moderate
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Meta-analyses find soy protein lowers LDL cholesterol modestly, on the order of 3–5%, which is considerably less than the effect that supported the original authorised health claim. The FDA proposed revoking that claim in 2017 on the basis of the inconsistency.
Moderate evidence. Several human studies point the same way, with real caveats.
Soy protein raises muscle protein synthesis and supports gains during resistance training, performing slightly below whey in acute comparisons and broadly comparably over longer training studies when total protein is matched.
Moderate evidence. Several human studies point the same way, with real caveats.
Soy isoflavones, rather than the protein itself, have been studied for hot flushes with modest and inconsistent results that appear to depend on whether an individual can produce the metabolite equol. Isolates vary widely in isoflavone content and many are largely stripped of them.
Limited evidence. Early human work, or mostly lab and animal research.
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Allergy
Soy is a major allergen
Soy is one of the most common food allergens and is declared as such in most jurisdictions. Soy protein isolate contains the relevant proteins and is unsuitable for anyone with a soy allergy.
Interaction
Thyroid medication and iodine status
Interacts withLevothyroxine
Soy protein interferes with absorption of levothyroxine, and soy isoflavones have been reported to affect thyroid function where iodine intake is marginal. Separating soy from thyroid medication by several hours is the usual advice.
Worth knowing
Isoflavones and hormone-sensitive conditions
Soy isoflavones bind oestrogen receptors weakly. Human evidence in breast cancer survivors has been broadly reassuring and in some cohorts favourable, but the amount of isoflavone in a protein isolate is rarely stated, and anyone advised by an oncologist to limit them cannot check the label.
Worth knowing
Heavy metals in plant proteins
Independent testing has found plant-based protein powders averaging higher lead and cadmium than dairy-based ones, because plants take up soil metals. Third-party contaminant testing is the practical safeguard.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.