Soy protein keeps pace with whey for muscle

Protein supplements are a staple for athletes and active people looking to repair and rebuild skeletal muscle and connective tissue, and whey has long been the default choice. Soy protein has grown in popularity as a plant-based alternative, but it carries a reputation problem: lingering doubts about whether it builds muscle as effectively as animal protein, and persistent worries about its effect on hormones. This review set out to gather every randomised controlled trial of soy protein in active individuals and athletes, covering muscle adaptations, metabolic and antioxidant status, hormonal responses and exercise performance, and to compare soy against whey where possible.

The authors systematically searched PubMed, Embase and Web of Science on 27 June 2023, alongside manual searches in Google Scholar and EBSCO. They included randomised controlled trials of soy protein supplementation in athletes and physically active adolescents and young adults aged 14 to 39, and assessed risk of bias using Cochrane's revised tool. Nineteen trials qualified in total. Nine examined muscle adaptations, six looked at metabolic and antioxidant status, six at hormonal responses and six at exercise performance, with some trials covering more than one outcome.

In some trials, soy protein produced increases in lean mass identical to whey. Soy consumption also reduced exercise-induced circulating markers such as triglycerides, uric acid and lactate, and longer-term studies showed better antioxidant capacity against oxidative stress compared with whey. Every study on endurance pointed to potential benefits at doses of 10 to 53.3 g, including improved high-intensity and high-speed running, greater maximal cardiac output, delayed fatigue, better isometric strength and improved endurance in recreational cyclists. Results on VO2max conflicted, and effects on testosterone and cortisol were inconsistent, with the authors calling for more research on hormones.

This review challenges the assumption that whey is automatically superior for people who train, and it adds to a growing body of evidence that plant proteins can support serious training goals. It stops well short of declaring soy the better option: the lean mass parity appeared in some studies rather than all, the aerobic capacity data disagreed, and the hormonal picture remains genuinely unsettled. The open questions are the familiar ones: more direct head-to-head trials against whey, clearer dosing guidance for different types of training, and longer follow-up on hormonal outcomes.

For anyone who prefers a plant-based protein, this is a reason to think soy is a credible alternative to conventional supplements rather than a compromise. The evidence suggests it may support lean mass comparably to whey in some contexts, may improve antioxidant status, and looks potentially useful after muscle damage or repeated bouts of strenuous exercise. The inconsistent testosterone and cortisol findings cut both ways: they neither confirm the old fears about soy and hormones nor fully close the question. Dose and training type appear to matter, and the review treats those as live variables.