Whether you gain or lose muscle depends on the long-term balance between your muscle protein synthesis (MPS) and your muscle protein breakdown (MPB), both of which are affected notably by exercise and the amount and quality of the protein you consume. The quality of animal proteins tends to be higher than the quality of plant proteins, due to greater bioavailability and better amino-acid profile. The amino-acid profile of a protein matters because all proteins, including the protein you eat and the protein in your body, are made from some combination of 20 amino acids (AAs). Your body can produce 11 of these AAs, making them nonessential amino acids (NEAAs). Your body cannot produce the other 9 — you need to get them through food, which makes them essential amino acids (EAAs).
There are various ways to compensate for the lower quality of plant proteins. For instance, you can simply eat more; you can also combine them to make sure you get enough of each EAA; or you can supplement with individual EAAs, if you know which ones your diet lacks.
Similarly, manufacturers can improve the AA profile of their plant-based products by adding one or more EAA. But can such products stimulate MPS as well as animal-based products can?
In this randomized controlled trial, 24 men (with an average age and BMI of 24 and 23) ate 40 grams of protein from either chicken breast (175 g) or a plant-based meat substitute (230 g, including 11.5 g of added lysine). The protein in the meat substitute came from wheat and chickpea. In order to eat as much protein as the meat group, the plant group had to consume 21% more calories (559 vs. 461 kcal)
For 8 hours, starting 3 hours before eating, the participants received an infusion containing an isotope tracer: L-[ring-13C6]-phenylalanine. In other words, they were injected with an EAA (an AA the body cannot synthesize) made radioactive so it could later be detected in blood samples and muscle biopsies.
The primary outcome was MPS, expressed as fractional synthetic rate (FSR). FSR was determined by measuring how much of the tracer was incorporated in plasma protein (3 hours before eating) and in three biopsies of the vastus lateralis muscle (just before eating, 2 hours later, and 5 hours later).
The tracer was also used to assess the increase in plasma levels of AAs and anabolic signaling proteins in blood samples. Blood samples were also used to measure glucose and insulin levels.
The increase in MPS was the same in both groups at 2 hours (second muscle biopsy) and 5 hours (third muscle biopsy).
To be more specific, MPS increased nonsignificantly more in the meat group during the first two hours, the last three hours, and the whole five hours.
Plasma lysine was higher in the plant group between the 15- and 90-minute marks. Plasma leucine and plasma BCAAs were higher in the meat group for all 5 hours, as was the area under the curve (AUC) of EAAs. The AUC of NEAAs and total AAs did not differ between groups.
Peak plasma glucose was higher and came earlier in the plant group (30 min) than in the meat group (140 min). Similarly, peak plasma insulin was higher and came earlier in the plant group (38 min) than in the meat group (78 min).
The plasma levels of anabolic signaling proteins didn’t differ between groups.
This study was conducted in young, healthy men; its results may not apply to older adults. As you age, your muscles’ anabolic resistance increases: you require more dietary protein to stimulate MPS.
The big picture
While all 20 AAs are required to build muscle tissue, MPS is stimulated primarily by the EAAs in your food. Unfortunately, plant proteins are usually lower in EAAs than animal proteins, though one study found the EAA content of potato protein (37%) and corn protein (32%) to be similar to that of casein (34%) and egg protein (32%). Of course, potatoes have more carbs than protein, but a potato protein isolate is a valid option to stimulate MPS.
Reference: van Vliet et al. J Nutr. 2015.
Plant proteins also have “limiting AAs” — EAAs present in such small amounts that they bottleneck protein synthesis. Lysine (which was added to the plant-based meat substitute in the present study) is the most common limiting AA, especially in cereal grains, such as wheat and rice.
Together, limiting AAs, lower EAA content, and lower leucine content help explain why plant proteins don’t appear to stimulate MPS as well as animal proteins do, though a couple of studies have observed no difference.
Plant proteins don’t all have the same limiting AAs, though. For instance, as shown in this table, pea and soy have a lot of lysine, and rice a lot of methionine; and as shown in the graphic above, corn has a lot of leucine. According to one study, per 100 grams of protein isolate, corn has more leucine (8.8 g) than do whey (8.6) and casein (5.8 g). Therefore, by combining different plant proteins, you can compensate for their individual weaknesses.
You can also compensate for the lower quality of plant proteins simply by eating more. In a 2016 randomized controlled trial (RCT) in healthy older men, when equated for leucine content (4.4 g), whey protein (35 g) increased peak plasma leucine more than did wheat protein (60 g), but the two proteins increased MPS rates similarly (wheat a little more, but the difference wasn’t statistically significant). Similarly, in a 2020 RCT, when equated for leucine content (2.5 g), mycoprotein (70 g, including 31.5 g as protein) increased resting and postexercise MPS rates more than did milk protein (31 g, including 26.2 g as protein).
So more is better … up to a point. Beyond a certain amount of protein, eating more won’t further increase your MPS. In other words, if your protein intake is high enough, it no longer matters if the protein is animal protein or plant protein. This would explain why, in another 2020 RCT (lasting 3 days), older adults on a high-protein diet (1.8 grams of protein per kilogram of body weight per day: 1.8 g/kg/day) saw similar increases in MPS whether the protein was mostly from animal sources or entirely from plant sources (57% mycoprotein).
Reference: Pinckaers et al. Sports Med. 2021.
We’ve been mentioning MPS a lot, but although MPS studies do provide useful information, MPS is just a proxy for what we actually care about: increasing muscle mass. As we said at the start, whether muscle is gained or lost depends on the long-term balance between MPS and MPB, so we need long-term trials that measure changes in muscle mass.
We summarized one such study in the May 2021 issue of Study Summaries. In this 12-week nonrandomized controlled trial, 38 young men performed resistance exercise twice per week. Their diet was high in protein (1.6 g/kg/day) and either omnivorous or vegan. In the end, there was no between-group difference in leg lean mass, rectus femoris cross-sectional area, or vastus lateralis cross-sectional area.
A potential limitation of this study is that the participants were untrained, and untrained people gain muscle easily when they start a resistance-training program. In other words, in this study, the effect of the resistance-training program may have drowned the effect of the difference in protein type. Then again, if that’s the case, it means that neither protein type proved to be greatly superior to the other.
Unfortunately, the above trial is the only one of its kind. Other long-term trials have compared plant-protein and animal-protein supplements (e.g., soy protein vs. whey protein), but the supplement typically made up only about a quarter of the participants’ daily protein intake, which means that the “plant protein” group could still have consumed a lot of animal protein. Moreover, plant proteins are much more bioavailable as protein powders (concentrates or isolates) than in whole foods, and the plant protein used is usually a high-quality protein, such as soy protein. So it comes as no surprise that changes in strength and body composition were similar between the “plant protein” and “animal protein” groups.
What data we have draw a consistent pattern: as long as we eat enough protein (around 1.6 g/kg/day), the type of protein (plant or animal) affects neither strength nor body composition. This is especially true in the real world, where vegans are likely to eat a wide variety of plant proteins on a regular basis, thereby ensuring that all their EAA needs are met.
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