Is a vegan diet suboptimal for building muscle? Original paper

In this randomized controlled trial, a high-protein vegan diet and a high-protein omnivorous diet had similar effects on resistance-exercise-induced adaptations.

Whether a high-protein vegan diet supports resistance exercise-induced adaptations to the same extent as a high-protein omnivorous diet.

The following outcomes were assessed:

• Changes in muscle mass, including whole-body lean mass, assessed via DXA (primary outcome)
• Changes in muscle fiber cross-sectional area and thigh muscle volume, assessed via MRI
• Changes in strength, including 1-repetition maximum strength in the deadlift, barbell back squat, and incline bench press, as well as knee extensor peak isometric torque

22 young adults with no health issues (average BMI of 23; 11 men, 11 women) with at least 6 months of resistance exercise experience.

The participants performed a resistance exercise program and consumed a high-protein diet consisting of at least 1.8 grams of protein per kilogram of body mass per day. They were randomly assigned to eat either almost exclusively plant-based protein sources and avoid animal products at least 6 days per week (vegan group) or to eat mostly animal-based protein sources (omnivorous group) for 10 weeks.

In both groups:

• The macronutrient distribution of the diet was approximately 44% of energy from carbohydrate, 32% fat, and 24% protein.
• The participants were recommended a daily caloric target to place them in an energy surplus of 0%–10%.
• The participants supplemented with creatine monohydrate daily.
• The participants were provided with protein-rich foods and protein supplements to help them reach their daily protein target.

To monitor dietary adherence, the participants were required to record their dietary intake in an app at least 3 days per week, and they discussed their dietary habits weekly with the research team.

Compared to baseline, total lean mass, total thigh muscle volume, average muscle fiber cross-sectional area, and type II muscle fiber cross-sectional area increased in both groups, with no differences between groups.

Additionally, compared to baseline, strength increased across all exercises in both groups. However, incline bench press strength increased to a greater extent in the vegan group, and deadlift strength nonsignificantly (p=0.053) increased to a greater extent in the vegan group.

Differences in dietary intake are central to the design of this study, so it’s prudent to discuss the specifics of the dietary interventions, especially the vegan diet, because the omnivorous diet ultimately functioned as a control diet.

In the omnivorous group, 67% of daily protein intake was obtained from animal sources, whereas 99.4% of protein intake in the vegan group was obtained from plant sources. Although the participants in the vegan group only had to consume a vegan diet on 6 days per week, 7 of 10 participants adopted an exclusively vegan diet, and 1 participant consumed dairy only in coffee.

The participants in each group received 1 to 2 protein-rich foods per day in addition to protein supplements. The participants in the omnivorous group received chicken and beef and a milk protein supplement. In the vegan group, the participants received mycoprotein-rich foods, which also contained potato and wheat protein, and a mycoprotein supplement. Mycoprotein is rich in essential amino acids and derived from the fungus Fusarium venenatum.^[2]

The researchers stated that the “participants were advised upon a dietary intervention to support muscle hypertrophy with respect to sufficient daily energy intake and dietary protein amount, quality, and timing.” Specific details were not provided for the timing aspect, but the studies cited in support of optimal protein timing (technically protein distribution) had the participants consume approximately 20–30 grams of protein per meal to maximize daily MPS rates. Thus, it can be assumed that the mycoprotein-rich foods provided at least 20 grams of protein because they were to be used as the main protein source for some meals.

The mycoprotein supplement provided 46 grams of protein per day, and at least another 20–30 grams of mycoprotein was provided by the mycoprotein-rich foods. This means that, in sum, mycoprotein comprised at least half of the participants’ total daily protein intake (average of 127 grams per day) in the vegan group.

Previous evidence indicates that over a 4-hour period, the ingestion of mycoprotein can stimulate MPS rates to a greater degree than milk protein in young men.^[3] In addition, a 3-day study found that a high-protein vegan diet enriched in mycoprotein (57% of daily protein intake) stimulated daily MPS rates to the same extent as a high-protein omnivorous diet in older adults with no health conditions (average age of 66).^[4] Collectively, these data demonstrate that mycoprotein is a high-quality protein source.

Indeed, mycoprotein is fairly unique among plant-based protein sources because it contains a high content of leucine, which is on par with that of milk protein.^[2] Leucine is a particularly important essential amino acid because it’s known to trigger or “activate” the process of MPS. A lower leucine content is one of the reasons why plant-based protein sources are assumed to be less anabolic than animal-based protein sources.^[5]

Leucine content of plant and animal proteins

Mean leucine content (percent of total protein) of various dietary protein sources. Dashed line represents the requirement for adults (WHO/FAO/UNU Expert Consultation 2007).

Adapted from Gorissen et al., 2018.^[6]

It’s unlikely that the average vegan diet derives more than 50% of its protein content from mycoprotein, which means this study probably doesn’t represent the effects of a “real life” vegan diet, which is not enriched in mycoprotein.

Although the ingestion of 18–22 grams of soy protein has been shown to stimulate MPS less than the same amount of protein from whey or milk (but not casein),^[7][8] MPS rates were similar between groups when the same of amount of protein was combined with 45 grams of carbohydrate, which arguably is more indicative of the type of mixed macronutrient meal that people typically consume.^[9]

However, when the dose of protein is bumped up to 30 grams, plant-based protein — whether it’s derived from wheat, potato, corn, pea, or a combination of sources — has comparable effects on MPS rates and anabolic signaling pathways as whey or milk protein.^{[10][11][12][13][14]} Similar results were also reported in a study that compared chicken breast to a lysine-enriched meat alternative composed of wheat and chickpea protein.^[15]

Collectively, these data suggest that a high-protein vegan diet that contains a variety of protein-rich plant foods would stimulate daily MPS rates to a similar extent as an omnivorous diet that is rich in animal-based protein sources. However, a limitation of these studies is that they used plant-based protein isolates or concentrates, as opposed to whole plant foods, which improves protein digestibility and the absorption of amino acids that drive the MPS response.^[16]

It’s also questionable whether MPS rates over a several hour period can be used to predict long-term changes in muscle mass,^[17] which is what people ultimately care about. Fortunately, besides the summarized study, there is one other study that examined the ability of a high-protein vegan diet to support resistance exercise-induced adaptations.

In a 12-week study, untrained young men performed a resistance exercise program and consumed 1.6–1.7 grams of protein per kilogram of body weight per day, either exclusively from plant-based protein sources or predominantly from animal-based protein sources. Additionally, to help meet their daily protein target, the vegan group supplemented with soy protein (approximately half of their protein intake), and the omnivorous group supplemented with whey protein.^[18] In the end, both groups increased muscle mass and strength compared to baseline, with no differences between groups.

Because there are currently only 2 studies that assessed changes in muscle mass and strength over time between high-protein vegan and omnivorous diets, more research is needed to confirm that these diets are similarly effective for supporting resistance exercise-induced adaptations. A notable similarity between these 2 studies is that the vegan group consumed about half or more of their protein from plant-based protein sources (i.e., soy or mycoprotein) that are considered to be of high quality, meaning they have a well-balanced amino acid profile and adequate amounts of all the essential amino acids, as well as high digestibility.

Nevertheless, the evidence as a whole is shaping up to indicate that, in young adults, as long as total daily protein intake is sufficiently high and a variety of protein sources are consumed, a vegan diet is likely as effective as an omnivorous diet for supporting resistance exercise-induced adaptations.

Anything else I need to know? The greater increase in incline bench press strength in the vegan group was likely a consequence of a single participant whose strength increased dramatically.

Because the participants were young adults, the results may not be generalizable to older adults who are less sensitive to the anabolic effects of protein ingestion,^[1] and thus, the source of protein may have a larger effect.