CHAMPAIGN, Ill. — A new study asked three questions about muscle protein synthesis in response to a nine-day diet and weight training regimen: First, does the source of protein — plant or animal-based — make any difference to muscle gain? Second, does it matter if total daily protein intake is evenly distributed throughout the day? And third, does a moderate but sufficient daily protein intake influence any of these variables? The answer to all three questions is “no,” the researchers found.

Their findings are reported in the journal Medicine and Science in Sports and Exercise.

“The longstanding belief or the current dogma was that animal-based protein sources were better, particularly for the muscle-building response,”  said Nicholas Burd, a professor of health and kinesiology at the University of Illinois Urbana-Champaign who led the new study with former graduate student Andrew Askow. This belief was rooted in science: Previous studies that took muscle biopsies after a single feeding found that an animal-based meal provided more of a stimulus for muscle protein synthesis than a vegan meal, Burd said. “And so, our general hypothesis based on these previous studies was that the animal-based eating pattern would be more effective at supporting the muscle-building response.”

But measurements taken after a single meal might not reflect the effects of consuming a balanced vegan diet over time, Burd said.

One previous clinical trial had looked at muscle responses in vegans and omnivores who ate a laboratory diet and engaged in weight training for 10 weeks. That study found no significant differences in muscle protein synthesis over time. However, volunteers in that study consumed 1.6-1.8 grams of protein per kilogram of body weight per day, which is much higher than what is needed to maximize muscle protein synthesis and build bigger muscles with weight lifting, Burd said. It also gave those on the vegan diet the bulk of their plant protein in supplements, which is not a realistic recreation of how vegans normally eat, he said.

Burd and his colleagues wanted to know whether the habitual consumption of a varied vegan or meat-based diet of whole foods — rather than ingestion of just a single meal or getting one’s protein from limited sources — would influence the rate of muscle protein synthesis over time. They also wanted to test the hypothesis that a moderate protein intake — in the range of 1.1-1.2 grams of protein per kilogram of body weight per day — should be distributed evenly throughout the day to maximize muscle growth.

A previous study from Burd’s lab found that protein intakes higher than 1.1 g/kg per day make no difference to the rate of muscle protein synthesis when weight training. This amount of protein also is more in line with a typical American diet, and testing what people normally eat is important, he said.

For the new study, the team recruited 40 healthy, physically active 20-40-year-old adults. The participants underwent a seven-day “habituation diet” to standardize their nutritional status prior to the clinical trial. Then they were randomly assigned to either a vegan or omnivorous diet. The research team provided all meals, some of which were eaten in the lab while most were consumed at home. Roughly 70% of the protein for the omnivorous meals was obtained from animal sources: beef, pork, chicken, dairy, eggs. The vegan diet balanced the amino acid content of the meals, ensuring that participants consumed complete proteins.  

The vegan and omnivorous groups were each divided again into those who ate roughly the same amount of protein at each of three meals and those whose protein intake varied across five meals throughout the day, with a larger proportion of protein consumed toward the end of the day.

All participants engaged in a series of muscle-strengthening activities in the lab every three days. They also wore accelerometers to keep track of their activity levels when not in the lab.

Each day, participants drank “heavy” water, which was labeled with deuterium, a stable isotope of hydrogen. The deuterium atoms “exchanged with hydrogen atoms within amino acids to make them heavy and served as tracers” that allowed the team to trace their incorporation into muscle tissue, Burd said. Biopsies of tissue from a leg muscle were taken at the beginning and end of the trial.

Burd was initially surprised to see that there were no differences in rates of muscle protein synthesis between those eating vegan or omnivorous diets. He also was surprised to see that protein distribution across the day had no effect on the rate of muscle building given results from past studies of acute responses to dietary interventions and weight training.

“It was thought that it was better to get a steady-state delivery of nutrients throughout the day,” he said. “I also thought that if you’re getting a lower quality protein — in terms of its digestibility and amino acid content — that perhaps distribution would make a difference. And surprisingly, we showed it doesn’t matter.”

Now, Burd says, if anyone asks him what’s the best type of food they should eat for muscle building, he’ll tell them: “It’s the kind you put in your mouth after exercise. As long as you’re getting sufficient high-quality protein from your food, then it really doesn’t make a difference.”

The Beef Checkoff program, overseen by the National Cattlemen’s Beef Board, supported this research.

Editor’s note:  

To reach Nicholas Burd, email naburd@illinois.edu.  

The paper “Impact of vegan diets on resistance exercise-mediated myofibrillar protein synthesis in healthy young males and females: A randomized controlled trial” is available online.

DOI: 10.1249/MSS.0000000000003725