Most plant proteins are less bioavailable than animal proteins, but the gap varies dramatically depending on the source. Soy protein scores nearly as high as whey on standard quality metrics, while wheat protein scores less than half as well. The real answer depends on which plant protein you’re eating, how it’s prepared, and whether you’re combining it with other sources.
How Protein Quality Is Measured
Two scoring systems dominate the conversation around protein bioavailability. PDCAAS (Protein Digestibility Corrected Amino Acid Score) has been the standard for decades, rating proteins on a scale where 1.00 is the highest. The newer DIAAS (Digestible Indispensable Amino Acid Score) measures how well individual amino acids are absorbed in the small intestine, making it a more precise tool. The UN’s Food and Agriculture Organization currently recommends DIAAS as the preferred method, though both systems are still widely used.
Here’s how common protein sources compare across both systems:
- Whey: PDCAAS 0.97 to 1.00, DIAAS 0.90
- Soy: PDCAAS 0.93 to 1.00, DIAAS 0.92
- Pea: PDCAAS 0.78 to 0.91, DIAAS 0.66
- Rice: PDCAAS 0.53 to 0.64, DIAAS 0.52
- Wheat: PDCAAS 0.45 to 0.54, DIAAS 0.39
Eggs, beef, milk, and casein all score at or very near 1.00 on PDCAAS. The takeaway: soy holds its own against animal sources, pea protein sits in a middle tier, and grain-based proteins fall significantly behind.
Why Plant Proteins Score Lower
Two factors drive the bioavailability gap: amino acid profiles and digestibility. Your body needs nine essential amino acids from food, and plant proteins tend to be short on specific ones. Lysine and methionine are the biggest weak points. Plant proteins average about 3.6% lysine and 1.0% methionine, compared to 7.0% and 2.5% in animal proteins. Wheat and corn are particularly low in lysine (1.4% and 1.5%), while pea and soy fall short on methionine (both around 0.4%).
The second issue is that plant foods contain compounds that interfere with digestion. Trypsin inhibitors in soybeans and other legumes can reduce protein digestibility by up to 50% when present at high levels. Tannins in sorghum, cereals, and some legumes can cut digestibility by up to 23%. Phytic acid, common in both cereals and legumes, reduces protein and amino acid digestibility by up to 10%. These compounds physically block the enzymes your gut uses to break protein into absorbable amino acids.
Leucine and Muscle Building
Leucine is the amino acid that triggers muscle protein synthesis, the process your body uses to repair and grow muscle tissue. It also helps transport other amino acids into muscle cells. On paper, some plant proteins contain respectable leucine levels: brown rice protein has 5.8 grams per 100 grams, pea has 5.7, and soy has 5.0. But the overall essential amino acid content in plant proteins is lower, and on a gram-for-gram basis, plant proteins average about 7.1% leucine compared to 8.8% for animal proteins.
When researchers compare soy and whey protein head to head at the same total protein dose, soy stimulates less muscle protein synthesis in the hours after exercise. However, a 12-week resistance training study that matched soy and whey supplements for leucine content found no significant differences in muscle growth or strength gains between the two groups. This suggests that the acute difference in muscle protein synthesis can be overcome by hitting the right leucine threshold. That threshold is roughly 2 grams of leucine per meal. A well-planned plant-based diet can reach 2.9 grams of leucine per meal across four meals, totaling about 11.7 grams per day, which exceeds the amount needed to maximally stimulate muscle growth.
How Preparation Changes Digestibility
Cooking, soaking, sprouting, and fermenting can substantially reduce the antinutrient content of plant foods and improve how much protein your body actually absorbs. Fermentation is the most effective of these methods. Raw lentils have a protein digestibility of about 45%, but after fermentation that jumps to 84%. Chickpeas go from roughly 59% to 85%. Fermentation nearly eliminates trypsin inhibitors in cowpeas, outperforming soaking, sprouting, and cooking for that purpose.
The improvements aren’t limited to legumes. Fermentation increased the digestibility of grass-pea seed protein by about 25%. Even whey protein, already highly digestible, saw its digestibility climb from 88% to 94% after five days of fermentation. Traditional fermented foods like tempeh, miso, and sourdough bread aren’t just cultural preferences. They’re practical solutions to a real nutritional limitation.
Protein isolates and concentrates (the powdered forms used in supplements and processed foods) also have higher digestibility than whole food versions because much of the fiber and antinutrient content is removed during processing. True ileal digestibility of amino acids in soy protein isolate ranges from 89% to 97%, depending on the specific amino acid, which is close to animal protein territory.
Combining Plant Proteins
The classic principle of combining grains and legumes exists because their amino acid weaknesses are mirror images: cereals are low in lysine while legumes are low in methionine. Rice and beans, corn tortillas and black beans, peanut butter on whole wheat bread. These pairings have been dietary staples across cultures for thousands of years, and the nutritional logic is sound.
Researchers using mathematical modeling have found that optimized plant protein blends can closely mimic animal protein profiles. The best plant combinations achieved similarity scores of 98.8% to cow’s milk protein, 94.2% to egg white, and 92.4% to whey. A blend of pea albumin and alfalfa (90% and 10%) produced the richest overall essential amino acid profile among plant combinations tested. You don’t need to combine proteins at every single meal. What matters is getting a variety of plant protein sources across the day so your body has access to all the essential amino acids it needs.
Practical Doses for Plant Protein
If your goal is muscle maintenance or growth, the simplest way to compensate for lower bioavailability is to eat slightly more total protein. A modeled plant-based diet for adult male rugby players hit all protein and leucine targets at 1.68 grams of protein per kilogram of body weight per day. For a 180-pound (82 kg) person, that’s about 138 grams of protein daily, spread across four meals.
Choosing higher-quality plant sources makes this easier. Soy, pea, and potato protein all have lysine levels that meet WHO requirements, while brown rice, hemp, and corn protein do not. If you rely heavily on grain-based proteins, pairing them with legume-based sources becomes more important. And if you use protein supplements, a blended plant powder (typically pea and rice together) covers more amino acid bases than either one alone.