Plant vs Animal Protein: What the Science Says About Quality and Absorption

A gram of protein from chicken and a gram of protein from lentils weigh the same. They contain the same amount of nitrogen. But they’re not equivalent for building muscle or meeting amino acid needs. The difference comes down to which amino acids are present, in what quantities, and how much actually gets absorbed.

This isn’t a reason to avoid plant protein. It’s a reason to understand what you’re working with.

What Makes a Protein “Complete”

Proteins are chains of amino acids. There are 20 amino acids used by the human body. Nine of them are essential: your body can’t synthesize them in adequate amounts, so they must come from food. These nine are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

A “complete” protein contains all nine in proportions adequate for human requirements. Animal proteins (meat, poultry, fish, eggs, dairy) are complete. Most plant proteins are not.

Common amino acid gaps in plant proteins:

• Lysine is low in grains (wheat, rice, corn, oats). This is the limiting factor in wheat-heavy diets.
• Methionine is low in legumes (beans, lentils, peas, soy is the exception).
• Leucine is lower across most plant sources compared to animal protein, which matters specifically for muscle building.

This doesn’t mean plant proteins are useless. It means individual plant proteins have a weak link, and relying on only one plant protein source long-term means your body has less of the limiting amino acid available for protein synthesis.

How Protein Quality Gets Measured

The field moved through two scoring systems over the past few decades.

PDCAAS (Protein Digestibility Corrected Amino Acid Score) was the standard for years. It scores protein quality based on the ratio of essential amino acids relative to a reference pattern, corrected for digestibility. The scale maxes out at 1.0. Eggs and milk both score 1.0. Soy protein concentrate scores about 1.0. Beef scores about 0.92. Wheat protein scores about 0.42 because of its lysine gap.

DIAAS (Digestible Indispensable Amino Acid Score) replaced PDCAAS as the FAO’s recommended method in 2013. It measures the true ileal digestibility of each amino acid separately (at the end of the small intestine, before colonic fermentation), which is more accurate. Scores above 1.0 are possible.

The Gorissen et al. 2018 study (PMID: 30167963) analyzed amino acid profiles across commercially available plant protein isolates and confirmed that soy is significantly superior to most other plant sources, with pea and hemp being the next best options.

The Leucine Threshold

Leucine deserves its own section because it’s the key trigger for muscle protein synthesis.

Leucine activates the mTOR signaling pathway in muscle cells. When leucine in the blood reaches a threshold (roughly 2-3g per meal in most research), it triggers muscle protein synthesis. Below that threshold, the signal is weak even if total protein is adequate.

Animal proteins hit the leucine threshold more easily because they have higher leucine density. A 30g serving of chicken protein delivers about 2.3g of leucine. A 30g serving of wheat protein delivers about 0.9g of leucine. To get the same leucine signal from wheat protein, you’d need roughly 75g of wheat protein, which is a huge portion.

Among plant proteins, soy is the best option. Pea and hemp are good. Rice protein is moderate. Wheat protein is the weakest for this purpose.

The van Vliet et al. 2015 review (PMID: 26224750) summarized the mechanistic and acute trial data: equivalent doses of plant protein produce lower rates of muscle protein synthesis than animal protein in most studies. But longer-term trials with adjusted (higher) plant protein intake show similar muscle-building outcomes. The difference is real in the short term. It’s manageable over time with adequate total intake.

Complementary Proteins

The “complete protein at every meal” rule was a widespread dietary teaching for decades. It’s not accurate.

Your body maintains an amino acid pool, drawing on circulating amino acids throughout the day. Eating rice at one meal (low lysine) and beans at another meal (adequate lysine) provides the body with all essential amino acids over the course of the day. You don’t need both in the same meal.

Classic complementary pairs work because they cover each other’s gaps:

• Rice + beans: rice lacks lysine, beans provide it. Beans lack methionine, rice provides it.
• Hummus + whole grain pita: chickpeas + wheat cover each other’s weak amino acids.
• Oats + soy milk: the soy provides the complete amino acid profile oats lack.

This isn’t magic. It’s just the math of covering nine amino acids across a day’s eating.

Digestibility Differences

The DIAAS numbers above include a digestibility correction, but what drives those lower scores for plant proteins?

Two main factors: antinutrients and food structure.

Legumes contain trypsin inhibitors that partially block the pancreatic enzyme that digests protein. Phytates bind not just minerals but also some proteins and digestive enzymes. Cooking destroys most trypsin inhibitors, which is why raw legume protein is much less digestible than cooked. The antinutrients article covers phytate and trypsin inhibitor chemistry in detail.

Food structure also matters. In whole plant foods, protein is often embedded in cell walls and fiber matrices. Your digestive system can’t fully extract all of it. Protein isolates (like soy protein isolate or pea protein isolate) have had the cell structure disrupted and fiber removed, so their digestibility is higher than whole food versions of the same plant.

The Broader Health Picture

Protein quality is only part of the comparison. Whole food plant protein sources come with fiber, phytochemicals, and minimal saturated fat. Legumes in particular consistently show favorable effects on LDL cholesterol and cardiovascular risk in observational and intervention studies.

Red meat protein, particularly processed red meat, has different associations in the epidemiological data. This doesn’t make meat protein nutritionally inferior for muscle building. It means the overall health context includes more than just the protein fraction.

For most healthy adults eating a varied diet, the protein source question is about optimization, not survival. Omnivores who include eggs, dairy, or modest amounts of lean meat have no shortage of high-quality protein. Plant-based eaters who understand leucine, complementary proteins, and total intake can achieve the same outcomes with more intentional choices.

This article is for educational purposes only and does not constitute medical advice. Consult a qualified health professional before making changes to your diet or health regimen.