Benefits
Muscle and strength gains with resistance training
A meta-analysis of 9 trials (266 people) found that soy and whey or other animal proteins, taken with resistance training, gave similar strength gains, while the 5 head-to-head whey and soy trials showed no significant lean mass effect of either. In a 9-month trial in untrained adults, whey added more lean mass (3.3 kg) than soy (1.8 kg) at the same protein and calorie intake.
LDL cholesterol levels
In a meta-analysis of the 46 trials FDA identified for its health claim review, a median 25 g a day of soy protein for a median 6 weeks lowered LDL cholesterol by 4.76 mg/dL versus other proteins, about 3 to 4 percent. An American Heart Association advisory called an average 3 percent drop very small for the roughly 50 g a day tested, and found no lipid effect from isoflavones.
Complete plant protein
Soy supplies all the essential amino acids. In a laboratory analysis of three soy protein isolates and one concentrate, protein quality scores (PDCAAS) were 1.00 in one lab and 0.95 to 1.00 in a second, at or near the top of the scale. The analysis was done by scientists at a soy ingredient company.
Plant-based alternative to dairy protein
Soy protein contains no milk protein or lactose, so it is an option for people who avoid dairy because of a milk allergy, lactose intolerance or personal choice. In the 9-trial meta-analysis cited here, soy gave training results similar to whey and other animal proteins.
Mechanism of action
Leucine and muscle protein synthesis
Like other complete proteins, soy supplies leucine, which switches on the mTOR signal that starts muscle protein synthesis. In young men, soy raised short-term muscle protein synthesis more than casein but less than whey after exercise; the authors linked the differences to digestion speed and possibly to small differences in leucine content.
Cholesterol effect of the protein
The cholesterol trials compared soy protein with other proteins in otherwise similar diets. The American Heart Association advisory found that isolated soy isoflavones had no average effect on LDL cholesterol, and how soy protein itself lowers LDL slightly has not been settled in the studies cited here.
Isoflavones and hormone levels
Soy foods and some soy protein products contain isoflavones, plant compounds that bind weakly to estrogen receptors. In a meta-analysis of 41 clinical studies in men, neither soy protein nor isoflavones changed total or free testosterone, estradiol or estrone, regardless of dose or study length.
Clinical trials
Meta-analysis of trials lasting 6 weeks or longer comparing soy protein supplements with whey or other animal proteins during resistance training. (Messina et al. 2018, Int J Sport Nutr Exerc Metab)
266 participants across 9 trials (5 whey versus soy, 4 soy versus beef, milk or dairy protein).
Strength rose with both whey and soy, with no difference between them (bench press p=0.90, squat p=0.64). There was no significant lean mass effect of whey or soy in the 5 head-to-head trials, and no difference between soy and other proteins across all 9. The authors concluded soy gives similar strength and lean mass gains to whey.
Randomized trial of daily isocaloric supplements of whey protein, soy protein or carbohydrate during about 9 months (96 workouts) of supervised resistance training. (Volek et al. 2013, J Am Coll Nutr)
63 untrained men and women (22 carbohydrate, 19 whey, 22 soy).
Lean mass rose more with whey (3.3 kg) than with carbohydrate (2.3 kg) or soy (1.8 kg), even though daily protein intake was the same with whey and soy (1.4 g/kg). Fat mass fell slightly with no difference between groups. A result that favored whey over soy.
Meta-analysis of the 46 controlled trials FDA identified when reviewing its soy protein health claim, comparing soy protein with non-soy protein. (Blanco Mejia et al. 2019, J Nutr)
Adults with baseline LDL cholesterol of 110 to 201 mg/dL in 43 trials with usable data (41 with LDL data).
At a median 25 g a day for a median 6 weeks, soy protein lowered LDL cholesterol by 4.76 mg/dL and total cholesterol by 6.41 mg/dL versus non-soy protein, about 3 to 4 percent. About 75 percent of trials showed a reduction, but only a minority were significant on their own. There was no dose response and no sign of publication bias.
Random-effects meta-analysis of controlled trials comparing soy protein with animal protein diets. (Anderson et al. 1995, N Engl J Med)
38 controlled clinical trials; soy protein intake averaged 47 g a day.
Soy protein lowered total cholesterol by 9.3 percent, LDL cholesterol by 12.9 percent and triglycerides by 10.5 percent, with larger drops in people who started with higher cholesterol; HDL rose a nonsignificant 2.4 percent. A later meta-analysis of the trials FDA reviewed found an LDL drop of only about 3 to 4 percent.
Acute study of three groups given a drink with 10 g of essential amino acids as whey hydrolysate, micellar casein or soy protein isolate, at rest and after one-leg resistance exercise. (Tang et al. 2009, J Appl Physiol)
18 healthy young men (6 per group).
Whey raised blood leucine and essential amino acids more than casein or soy. At rest, muscle protein synthesis was about 93 percent higher after whey than casein; after exercise it was about 122 percent higher than casein and 31 percent higher than soy. Soy raised synthesis more than casein at rest (64 percent) and after exercise (69 percent). Small groups and a few hours of measurement.
Meta-analysis of clinical studies in adult men eating soy foods, soy protein or isoflavone extracts, measuring testosterone, estrogens and SHBG. (Reed et al. 2021, Reprod Toxicol)
Up to 1753 men across 41 studies (numbers varied by hormone).
No significant effect of soy protein or isoflavones on total or free testosterone, estradiol, estrone or SHBG, in any statistical model and regardless of isoflavone dose or study length.