Also known as: Soy protein isolate, SPI, Isolated soy protein, Glycine max protein
Muscle & RecoveryStrong evidence
The only common plant protein that is complete on its own, with a decent leucine content and decades of research behind it. Also the one carrying a persistent myth about hormones that the evidence does not support.
Typical dose
25.0–40.0 g per day
Any time; total daily protein matters more than timing
What it is
Soy protein is extracted from defatted soybean flour. Isolates are typically 90 per cent protein or higher, concentrates around 70 per cent.
Unlike pea or rice, soy supplies all nine essential amino acids in reasonable proportion without needing to be blended. Leucine sits around 8 per cent, comparable to pea and below whey.
Soy also contains isoflavones, plant compounds structurally similar to oestrogen, which is the source of the persistent concern about it. Isolates contain far fewer isoflavones than whole soy foods, since processing removes much of them, and the concern itself does not hold up — see the evidence section.
How it works
Soy protein supports muscle protein synthesis by the standard route, supplying leucine to trigger it and a complete essential amino acid profile to build with.
Digestion is reasonably fast, sitting closer to whey than to casein.
On isoflavones: they bind oestrogen receptors weakly, and importantly they show preference for a receptor subtype that behaves differently from the one classical oestrogen acts through. That distinction is why binding a receptor does not translate into an oestrogenic effect in the body, and it is the mechanistic reason the hormone concern fails.
What the evidence shows
Soy protein has strong evidence for supporting lean mass and strength alongside resistance training. Comparisons with whey generally show whey slightly ahead on acute muscle protein synthesis, with differences narrowing considerably over longer training studies.
The hormone question has been examined thoroughly. Meta-analyses of controlled trials in men have found no significant effect of soy protein or isoflavones on testosterone or oestrogen levels. This has been tested repeatedly and the finding is consistent.
The myth persists largely on the back of a handful of case reports involving genuinely extraordinary intakes, far beyond what anyone consumes from a protein shake. Soy protein does not feminise men, and repeating that claim is not a cautious position but simply an inaccurate one.
Dosing
Aim for 25-40 g of protein per serving, similar to pea protein and slightly above what you would use with whey, given the leucine content.
Isolate supplies more protein per scoop and fewer isoflavones than concentrate. For muscle purposes the isolate is the more sensible choice.
Soy protein mixes reasonably well and has a milder taste than pea. It is a legitimate primary protein for anyone eating plant-based, and does not need blending to be complete. Soy allergen declaration is mandatory and soy is one of the more common food allergens.
At a glance
Common forms
Soy protein isolate (90%+), Soy protein concentrate, Textured soy protein, Within plant protein blends
Half-life / duration
Amino acids peak in blood within 60-90 minutes
Timing
Any time; total daily protein matters more than timing
Competition status
Permitted — not on the WADA Prohibited List
Category
Muscle & Recovery
Safety and side effects
Soy protein is well tolerated. Bloating and gas are the usual complaints and are generally mild.
Soy allergen declaration is mandatory. Soy is among the more common food allergens, and this is the genuine safety consideration rather than the hormone question.
Isoflavones may interact with thyroid hormone handling where iodine intake is low, so anyone whose thyroid function is being monitored should mention soy intake to their doctor. At adequate iodine status this does not appear to be a practical issue.
Who should avoid it
Soy allergen declaration applies; avoid entirely with a soy allergy. Speak to your doctor if your thyroid function is being monitored, particularly if your iodine intake is low. Speak to your doctor about protein intake if you have been told your kidneys are not working normally.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
