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Exercise researcher Nicholas Burd says there was a long-held belief in his field that making gains in the gym required eating meat or other animal products.
This wasn't a matter of bro science — the idea was backed up by well-controlled studies.
In the lab, people who were fed animal-based protein like whey had better muscle protein synthesis than those consuming soy or other plant sources of protein.
"It was certainly ingrained in my mind that animal [protein] is better than plant," says Burd, director of nutrition and exercise at the University of Illinois Urbana-Champaign.
Meat has an abundance of the essential amino acids we need to make muscle and mirrors the composition of our own skeletal muscle.
Plant-based foods also have all the essential amino acids, the building blocks of muscles, but have lower concentrations of some that are important for protein synthesis.
But in recent years, more well-controlled studies have come out, overturning previous assumptions — showing that plant protein can be comparable for putting on muscle.
"On a gram-for-gram basis, animal protein is certainly of a higher quality than plant-based protein," says James McKendry, who studies nutrition and aging at the University of British Columbia. "But if you're consuming enough of [plant-based protein] and in the context of a whole meal, those differences really don't come out in the wash."
Protein synthesis on a vegan diet
Some of the newest data comes from a trial run by Burd, published recently in the journal Medicine & Science in Sports & Exercise. It enrolled 40 young adults and randomized them to either a vegan or omnivore diet.
The participants had three weightlifting sessions over nine days, and researchers provided all their meals to them. At the end of the study, Burd says, they biopsied the participants' muscles, which allowed them to calculate protein synthesis.
They found that the two diets offered the same "muscle-building potential," says Burd, and their data suggests long-term muscle gains would also be the same.
The trial also looked at whether timing mattered. Some people back-loaded their protein at the end of the day, as many people tend to do with their dinner, while others spaced out their protein intake.
Here, too, there was no difference.
"I think it's an important study, with respect to our attitudes towards where we get our protein from," says Benjamin Wall, a researcher at the University of Exeter.
Longer-term studies needed
The new data echoes what Wall found in his own trial several years ago that compared plant and animal protein for muscle building.
He says historically one of the main reasons that people believed plant protein was inferior was studies that analyzed the impact of a single meal, a single protein, over a few hours.
"We now have several studies which suggest there's far less difference," he says.
In Wall's trial, participants were fed a high-protein diet of about 2 grams per kilogram of body weight. In contrast, the new University of Illinois study lowered that to 1.2 grams, which is reflective of how much protein Americans tend to consume per day and is still 50% higher than the U.S. recommendations.
There are caveats to the new research.
You can infer only so much from a short-term study, which didn't measure muscle growth or changes in strength, and included only young people.
Based on what he and others have found, though, Wall believes the findings would have held true over a longer period of time.
Studies, like Burd's, that provide participants with well-balanced meals are more accurate because they don't rely on self-reports. On the other hand, they don't necessarily "address the real-world challenge that it's probably harder to consume a high-protein diet," Wall says.
Mindful plant-based eating can deliver a protein punch
Some plant-based foods have relatively high protein, like beans (18 grams for a cup of cooked lentils) and tofu (22 grams per ½ cup serving). Grains like quinoa (8 grams per cooked cup) have some, as do some vegetables like broccoli (2.6 grams per cup).
But meat is more protein dense, so you can end up eating more calories than you'd like, to get the same amount of protein as from plants. Plus you have to be mindful to eat a wide variety of plant foods so that all the essential amino acids your body needs to maximize muscle growth are covered.
"A lot of people around the world have kind of figured that out," says McKendry. "If you combine rice with beans, they have complementary amino acid profiles."
For example, peas are a good source of the amino acid leucine, which is critical for switching on protein synthesis, but they are low in methionine, which means you need to look for that somewhere else, like rice or soy.
"Vegan diets are just as good provided they're balanced and you're eating enough protein," Burd says.
Exactly how much protein you need is another — and sometimes divisive — question.
For people who are trying to build muscle, Wall says 1.6 grams of protein per kilogram of body weight is a "widely accepted figure to hit to optimize training adaptations," though much of this depends on your activity level, your goals, your age and other factors. (Thinking in pounds, this would be about 0.72 grams of protein per pound of body weight.)
"Beyond that point, there's diminishing returns," says McKendry, especially for the average person who's training and trying to put on some muscle.
But he says it's important to remember that "protein is really the icing on the cake" in the muscle-building equation. The bigger piece is the resistance training itself, which research shows has benefits for longevity, brain function and cardiovascular health, to name just a few.
"It doesn't matter how much you consume — if you're not progressively resistance training, you're not going to build much muscle at all," he says.
