Despite decades of rumors that an all-plant diet will shrivel up your muscles and turn you into a sissy, all those who go vegan and lift weights prove otherwise. You can get complete, muscle-building protein from combinations of foods and seeds like quinoa and hemp. Soy protein, the most popular vegan muscle supplement, has been shown to offer benefits on par with whey, and so has pea protein. In 2015, the Journal of the International Society of Sports Nutrition found that young lifters drinking pea shakes made the same gains in biceps thickness and strength as their whey-drinking counterparts.
One caveat though: I was vegan for two years, and while I actually got my absolute biggest with that approach (250 pounds), I also put on a lot of fat. Eating vegan means eating lots of carbs and fats by default, so you may experience unwanted weight gain if you don’t watch your calories. That’s why I stopped.