The protein requirements discussed so far were based on studies that used animal-based protein supplements, such as whey or egg protein supplements, or were conducted mostly in omnivores. There is no reason to believe that people who get their protein mostly or entirely from plants have inherently different protein requirements, but since plant-based proteins tend to be lower in quality than animal-based proteins, if you obtain most of your protein from plants you will need to pay attention not just to the amount of protein you eat but also to the quality of that protein.[92]

A protein’s quality is determined by its digestibility and amino acid profile.

Digestibility matters because if you don’t digest and absorb some of the protein you eat, then it may as well not have been eaten. Animal-based proteins consistently demonstrate a digestibility rate higher than 90%, whereas proteins from the best plant-based sources (legumes and grains) show a digestibility rate of 60–80%.[93]

Digestibility of various plant- and animal-based proteins

Reference: FAO. Protein Quality Evaluation in Human Nutrition. 2013.[94]

Plants contain anti-nutrients that inhibit protein digestion and absorption, such as trypsin inhibitors, phytates, and tannins.[95] While cooking does reduce anti-nutrient concentrations, it doesn’t eliminate them entirely. Plant-based protein powders, however, are mostly free of antinutrients and so have digestibility rates similar to those of animal-based proteins.[93]

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, which are therefore essential amino acids (EAAs) you must get through food.

Building muscle requires that, cumulatively, muscle protein synthesis (MPS) exceeds muscle protein breakdown (MPB), resulting in a net accumulation of muscle protein. All 20 AAs are required to build muscle tissue,[96] but MPS is stimulated primarily by the EAAs in the food you ingest.[97]

Plant-based proteins, whether from whole foods or protein powders, contain less EAAs than animal-based proteins.

EAA content of plant- and animal-based proteins

Reference: FAO. Protein Quality Evaluation in Human Nutrition. 2013.[94]

In particular, plant-based proteins are lower in the EAA leucine, which is believed to act as a signal to “turn on” anabolic signaling pathways and MPS,[98][99] although all EAAs are required for the effect to persist.[100]

Leucine content of plant- and animal-based proteins

Reference: van Vliet et al. J Nutr. 2015.[101]

The lower leucine and EAA content of plant-based proteins helps explain why several studies have reported lower rates of MPS from soy protein powders and beverages than from whey protein,[102][103] skim milk,[104] whole milk with cheese,[105] and lean beef.[106]

However, while differences in MPS do appear to translate to differences in lean mass when modest supplemental protein doses are used (about 20 g/day),[107][108] when higher doses are used (33–50 g/day), animal-based (whey) and plant-based (soy, rice) supplemental proteins appear to affect lean mass similarly.[109][110][111][112] In short, consuming more protein overall appears to offset the lower quality of the plant-based proteins.

Plant-based proteins also contain limiting amino acids, which are EAAs present in such small amounts that they bottleneck protein synthesis. Lysine is the most common limiting amino acid, especially in cereal grains, such as wheat and rice.[113] Nuts and seeds also tend to have lysine as a limiting amino acid. Beans and legumes, on the other hand, contain sufficient lysine but lack sulfurous amino acids, such as methionine and cysteine. Combining different plant-based proteins can help make up for their respective deficits.

Plant-based proteins are of lower quality (they are less bioavailable and contain less EAAs). If you get most of your protein from plants, you will need to consume more protein to achieve the same muscle growth as someone with a more omnivorous diet.

Bolstering plant-based proteins

The simplest method to overcome the EAA deficits of a plant protein is to eat more of it. As aforementioned, a handful of studies have shown that large doses (33–50 g/day) of animal-based (whey) and plant-based (soy, rice) supplemental proteins appear to increase lean mass similarly.[109][110][111][112]

Another way to overcome the EAA deficits of plant proteins is to combine complementary EAA profiles.[114] Historic examples of such combinations include beans with corn in the Americas and rice with soybean in Asia. These grain-legume combos work because legumes supply the lysine missing in grains whereas grains supply the methionine and cysteine missing in legumes.

Combining incomplete proteins

Adapted from: Woolf et al. PLoS One. 2011.[114]

Unfortunately, most plant proteins are low in leucine, meaning that combining different plant proteins will not have a large benefit unless one of those proteins is corn protein (whose leucine content rivals that of whey protein).

If your protein has less leucine, you need to eat more of it to maximize MPS — or you can take leucine as a supplement. MPS was increased similarly by 25 grams of whey protein (providing 3 grams of leucine) and by a combination of 6.25 grams of whey protein and 4.25 grams of supplemental leucine (5 grams of leucine in total).[115] A rodent study using plant proteins reported similar results.[116]

The EAA deficits of plant-based proteins can be overcome by eating more, combining complementary proteins, and supplementing with leucine.