The diets were strictly controlled. The participants were provided with enough food to cover 95% of their calorie needs, and they were given a list of foods to choose from for the remaining 5%, along with instructions to spread their protein intake evenly throughout the day. They also ate lunch at the research facility during the week.

Protein comprised 15% of daily calorie intake, resulting in an intake of approximately 1.2 to 1.3 grams of protein per kilogram of body weight per day. In the omnivorous diet, animal-based foods covered 60% of daily protein intake.

Muscle protein synthesis was evaluated as daily mixed muscle fractional synthesis rate, which was determined using a D₂O dosing protocol and muscle biopsies.^[1]

Although the summarized study provides strong evidence that a well-formulated vegan diet doesn’t increase the risk of sarcopenia in older adults compared to an omnivorous diet, further research is needed to support these findings. Specifically, longer-term studies that measure changes in muscle size and strength are needed.

The capacity of a protein source to stimulate muscle protein synthesis is generally determined by its content of essential amino acids,^[2] all of which are required to fuel this metabolic process;^[3] and its digestibility, or how much of the amino acids from the protein are made available to peripheral tissues like skeletal muscle.^[4]

Plant-based protein sources are considered to be less anabolic than animal-based protein sources because they contain fewer essential amino acids overall and are often deficient in at least one essential amino acid.^[5]

The essential amino acid content of various foods

The dashed line represents the recommended essential amino acid content for adults set by the WHO/FAO/UNU. Adapted from Pinckaers P, et al., 2021, Sports Med.

Additionally, protein-rich plant foods tend to contain “antinutritional factors” (e.g., tannins) that impair the protein’s digestibility,^[2] and evidence suggests that a higher proportion of amino acids from some plant foods, like soy, are taken up by the gut and liver,^[6][7] meaning that less are available for muscle protein synthesis.

The process of amino acid absorption

Adapted from Trommelen et al., 2021, Clinical Nutrition Open Science

For these reasons, it’s thought that a greater amount of protein is needed from plant sources than animal sources to provide an equivalent anabolic stimulus,^[2] especially in the context of older adults. This is because older adults are less sensitive to protein’s anabolic effects than younger adults, as evidenced by a lower muscle protein synthesis rate in response to protein ingestion.^[8][9][10]

Evidence from acute studies support these ideas:

• Ingesting 40 grams of soy protein (but not 20 grams of soy protein) after exercise significantly increased muscle protein synthesis, whereas ingesting 20 grams of whey protein significantly increased muscle protein synthesis.^[11]
• Ingesting 60 grams of wheat protein (but not 35 grams of wheat protein) significantly increased muscle protein synthesis, whereas ingesting 35 grams of casein protein significantly increased muscle protein synthesis.^[12]
• In participants who ate 1 of 2 meals that provided 0.45 grams of protein per kilogram of body weight, a beef meal increased muscle protein synthesis more than a vegan meal containing quinoa, soy, chickpeas, and broad beans.^[13]

In contrast to this evidence, studies in younger adults have reported that when at least 30 grams of protein is consumed, plant-based protein — irrespective of whether it’s derived from wheat, potato, corn, pea, or a combination of sources — has effects on MPS rates and anabolic signaling pathways that are comparable to those of whey or milk protein.^{[14][15][16][17][18]}

Consequently, protein source may be a more influential factor in older adults, particularly because they often have a limited appetite,^[19] and so simply consuming more total protein to offset the inferior anabolic potential of plant proteins would be impractical.

Before writing off vegan diets as a risk factor for sarcopenia, it’s important to consider the limitations of these acute studies. For starters, they evaluate the effect of a single meal or protein source. Combining plant proteins to achieve a more optimal amino acid profile improves its anabolic potential,^[2] so the results from these studies may not be generalizable to a whole diet comprising protein from a variety of plant foods.^[20]

Second, muscle protein synthesis is typically only measured for a few hours. This glimpse into the anabolic response may not translate to muscle protein synthesis rates measured over an entire day.

Accordingly, a study published in 2021 that had older adults consume a high-protein (1.8 grams of protein per kilogram of body weight per day) omnivorous or vegan diet for 3 days found comparable daily muscle protein synthesis rates between groups.^[21]

However, in the vegan diet group, 57% of protein intake was derived from mycoprotein, which is a high-quality protein source with an amino acid composition similar to that of milk protein.^[22]

Besides the inclusion of this unique protein source, the vegan diet likely also has limited real world-applicability due to its high protein content. Eating 1.8 grams of protein per kilogram of body weight per day may not align with the dietary preferences of the average older adult, or such a diet may be infeasible due to protein’s satiating effect^[23] and the prevalence of reduced appetite in this population.^[19]

It’s also possible that the lack of difference between groups was due to a protein content that was high enough to overcome the inferior anabolic potential of the vegan diet, which is related to why the summarized study is a notable addition to this body of literature: The vegan diet in the summarized study was practical because it provided 1.3 grams of protein per kilogram of body weight per day from an array of commonly available sources, namely, soy-based dairy alternatives, legumes, nuts, cereals, and plant-based meats.

However, the participants in the study were physically active, with an average daily step count of about 12,500 steps per day. Physical activity increases the sensitivity of skeletal muscle to the anabolic effects of protein intake,^[24][25] and previous evidence indicates that walking can increase daily MPS rates.^[26] As such, the results may not be generalizable to sedentary older adults.