Soy protein isolate: what it is, downsides, and soy vs whey

Soy protein isolate is protein from soybeans with the starch and sugars separated out, so most of what is left is protein. People often ask whether it has downsides, whether it changes hormones, and whether it can match whey. This page answers from the research linked at the end.

What is soy protein isolate?

The U.S. Soybean Export Council (USSEC) fact sheet defines soy protein isolate as at least 90% protein on a moisture-free basis. It is made from defatted soy flakes: dissolved at pH 9–11, centrifuged to remove insoluble carbohydrate, then brought down to pH 4.2–4.5 to separate the protein from soluble sugars such as sucrose and oligosaccharides, before it is neutralised and dried.

Downsides with evidence

Soy allergy: the American College of Allergy, Asthma and Immunology (ACAAI) says soy is a common allergen in young children, and many outgrow it. Symptoms range from vomiting, cramps and hives to wheezing; anaphylaxis is rare but can be life-threatening. People with the allergy must avoid every product containing soy.
Less leucine than whey: the abstract of a 2018 meta-analysis notes that whey has more leucine and is therefore thought to stimulate muscle protein synthesis better in short-term studies, but over training of 6 weeks or more there was no practical difference (next section).

Does soy change men's hormones?

A meta-analysis by Reed and colleagues (2021) pooled 41 clinical studies in men, with testosterone measured in 1,753 of them. Soy protein and isoflavones did not change total or free testosterone, estradiol or estrone, whatever the dose or study length.

Soy or whey for muscle?

Messina and colleagues (2018) pooled 9 studies with 266 people doing resistance training: those taking soy protein gained strength and lean mass no differently from those taking whey or other animal protein. What really differs, then, is allergies, taste and price per gram of protein.

Soy protein in our store

By label figures and current prices:

BAAM ISO-SOY: 30 g protein per scoop · 0.44 baht per gram

For comparison, the best-value whey in our store right now is Ultimate Nutrition Prostar 100% Whey at 1.17 baht per gram. Every plant protein is on plant and soy protein; for another brand's soy isolate such as Matell, read Matell whey protein.