Short answer: In two trials that compared them directly with resistance training, gains in muscle and strength did not differ between pea and whey.12 Both trials were funded by or involved a pea protein maker. Whey is a complete milk protein with more leucine than casein or soy; milk is a major allergen.34
| Whey protein | Pea protein | |
|---|---|---|
| Source | Milk; one of its complete proteins3 | Peas |
| 12-week trial, 161 young men (25 g twice a day) | Biceps thicker in all groups; no difference from pea1 | Same; in the weakest men, +20.2% vs +15.6% whey vs +8.6% placebo1 |
| 84-day trial, sedentary adults | No difference in strength or muscle vs pea2 | No difference vs whey2 |
| Strength | Rose over time; no difference between groups1 | Same1 |
| Allergen | Milk is a major food allergen4 | Not a dairy product |
| Upper limit | No upper limit set for protein3 | Same3 |
In a 12-week trial, 161 men aged 18 to 35 did arm training and took 25 g of pea protein, whey or placebo twice a day. Biceps thickness grew in all groups, with only a trend toward differences. Among the weakest men at the start, pea did better than placebo, while whey did not differ from either. Strength rose with no differences between groups.1
In an 84-day trial in sedentary adults doing weekly resistance training, pea and whey did not differ in whole-body strength, muscle mass or how much people liked them.2
Both trials tested one company's pea protein (Roquette), and its staff were among the authors.12 These are study protocols, not personalized advice; confirm any dosing with your prescriber.
NIH cites an RDA of 0.8 g per kg of body weight a day for healthy adults, and 1.2 to 2.0 g per kg a day suggested by sports nutrition groups for athletes. It notes whey has more leucine than casein or soy.3
More detail: Whey vs casein protein: what is the difference?, Whey vs soy protein: which is better?, and the Whey Protein and Pea Protein guides.