Soy and pea kept pace with whey across five strength-training trials. So why won't the authors call it a tie?

Whey has long been treated as the protein to beat in the gym, largely because it is rich in leucine, an amino acid that helps switch on muscle building. Plenty of people would rather not use it, whether for ethical, environmental or digestive reasons. A team at São Paulo State University in Brazil asked a narrow question: in adults new to resistance training, does a plant protein supplement change the body as much as whey? The answer appeared in the Journal of the International Society of Sports Nutrition on 29 October.

The filters were strict. The trials had to be randomized, last at least six weeks, compare a plant supplement directly with whey, involve healthy adults aged 18 to 59 who had not trained with weights, and measure the body with a DEXA scan. From 70 records, five trials survived, with 372 people enrolled in all. Two used soy isolate, one soy concentrate, one pea and one chia flour. They ran from eight weeks to nine months. The pooled lean mass comparison counted 88 people on plant protein and 93 on whey.

No trial's interval ruled out zero in this analysis, and neither did the pooled result. Plant protein came out 0.35 behind whey for lean mass, with a 95% confidence interval from 2.46 behind to 1.76 ahead. Fat mass fell 0.21 further on plant protein across four trials, with an interval from 2.31 further to 1.90 less. The paper does not print a unit in these figures; DEXA body mass is normally given in kilograms. The trials agreed with each other closely, with no measurable spread beyond chance. The one that stands out is the longest: over nine months with about 22 g of supplement protein a day, that trial's authors reported whey beat soy for lean mass. In this analysis the gap was 3.20, with a wide interval that crossed zero.

The review's authors are careful about what this means, and readers should be too. Intervals this wide can hide a real difference, so 'no significant difference' here does not mean 'the same', and they say so several times. Doses were not the same across trials, only one trial reported matching the leucine, and chia and pea had one trial each. The team also searched just three databases and did not test for publication bias with so few trials. Everyone was a beginner, so this says little about experienced lifters. For someone starting out who prefers to avoid whey, the authors see soy or pea as a practical option alongside structured training. Whether they are truly equal is still an open question.