High quality proteins are foods that contain all nine essential amino acids in proportions your body can actually use, and that your digestive system absorbs efficiently. The two factors that determine protein quality are the amino acid profile (what’s in it) and digestibility (how much your body can extract). Animal proteins like eggs, dairy, and meat consistently rank highest on both measures, but several plant sources come close, especially when combined strategically.
What Makes a Protein “High Quality”
Your body uses 20 amino acids to build and repair tissue. It can manufacture 11 of them on its own, but the remaining nine must come from food. These nine are called essential amino acids, and they each serve distinct roles: one helps produce histamine for your immune system, another drives muscle growth and tissue repair, and another creates serotonin, which regulates mood, appetite, and sleep.
A protein source is considered “complete” when it supplies adequate amounts of all nine essential amino acids. But completeness alone doesn’t make a protein high quality. The second piece is digestibility: how much of that protein your gut can break down and absorb. A food could technically contain all nine amino acids but deliver very little of them into your bloodstream if your body struggles to digest it. Whole grains, for instance, have a digestibility rate of roughly 45%, meaning more than half the protein passes through unused. Eggs, by contrast, hit about 98%.
How Scientists Measure Protein Quality
For years, the standard measurement was a score called PDCAAS, which rated proteins on a scale where 1.0 was the maximum. The problem was that any protein scoring above 1.0 got capped at 1.0, making it impossible to distinguish between good and exceptional protein sources. Whey protein and chicken breast both scored 1.0, even though they differ in how efficiently your body uses them.
The Food and Agriculture Organization of the United Nations now recommends a newer system called DIAAS (Digestible Indispensable Amino Acid Score). Instead of measuring digestibility based on what comes out in your stool, DIAAS measures how much of each individual amino acid gets absorbed in the small intestine, before gut bacteria have a chance to consume undigested protein in the lower intestine. This gives a far more accurate picture of what your body actually receives. Scores are not capped, so you can meaningfully compare one high quality protein against another.
Animal Proteins and Their Advantages
Animal sources dominate protein quality rankings because they consistently deliver all essential amino acids in near-ideal proportions and are highly digestible. Here’s how common sources compare on digestibility:
- Whey protein: 104% digestibility (the score can exceed 100% because the reference standard is whole casein)
- Eggs: 98%
- Casein (the main protein in cheese and milk): 99%
- Milk: 96%
These sources are also rich in leucine, the amino acid that acts as the primary trigger for muscle building. Research estimates that older adults need about 3 to 4 grams of leucine per meal to maximally stimulate muscle repair, which translates to roughly 25 to 30 grams of protein from a high quality source. Getting that same leucine threshold from lower quality proteins requires eating significantly more total protein.
Where Plant Proteins Fall Short
Plant proteins are not missing essential amino acids entirely. A more accurate way to think about it: they contain certain amino acids in lower proportions than your body ideally needs. Grains are low in lysine. Legumes (beans, lentils, peanuts) are slightly low in the sulfur-containing amino acids methionine and cysteine. This means relying on a single plant source for all your protein leaves gaps.
Digestibility varies widely among plant foods. Soy protein isolate reaches 98%, and pea protein concentrate hits 99%, putting them on par with animal sources. But less processed plant foods score lower: raw black beans sit around 70%, wheat gluten at 64%, and raw peanuts at just 52%. Cooking and processing generally improve these numbers. Cooked black beans jump to 83%, roasted peanuts to 98%, and cooked peas to 89%.
Plant proteins also have a weaker anabolic effect, meaning they stimulate less muscle building per gram than animal proteins. This is partly the digestibility issue and partly the lower leucine content. If your goal is muscle maintenance or growth, you typically need to eat a larger volume of plant protein to match what a smaller serving of animal protein delivers.
Combining Plant Proteins Effectively
The classic pairing of rice and beans exists for a reason. Rice is low in lysine but adequate in methionine. Beans are the opposite. Eaten together, or even over the course of the same day, they complement each other and provide a full amino acid profile. Other effective pairings include lentils with whole wheat bread, hummus with pita, or peanut butter on toast.
You don’t need to combine complementary proteins at every single meal. Your body maintains a pool of free amino acids that it draws from throughout the day. As long as you eat a variety of plant protein sources across your meals, the amino acid gaps in one food get filled by another. The people most at risk of falling short are those who rely heavily on a single plant food group, like grains alone, without regularly including legumes, nuts, or soy.
Soy stands out among plant proteins as a complete protein with high digestibility. Tofu, tempeh, edamame, and soy protein isolate all deliver a full amino acid profile without needing a complementary food.
Why Protein Quality Matters for Fullness
Higher protein intake correlates with feeling fuller after meals. In research measuring how satisfying different foods are calorie for calorie, protein content was one of the strongest positive predictors of satiety, alongside fiber and water content. Fat content, by contrast, was negatively associated with fullness, meaning high fat foods tended to be less satisfying per calorie.
This has practical implications for weight management. If you choose protein sources your body absorbs efficiently, you get more usable amino acids per calorie consumed. A serving of eggs or Greek yogurt delivers nearly all its protein into your system, while the same calorie count from a low-digestibility plant source may leave more protein unabsorbed, potentially offering less of the fullness signal that protein provides.
Practical Takeaways by Diet Type
If you eat animal products, hitting your protein quality targets is straightforward. Eggs, dairy, poultry, fish, and meat all score high on both amino acid completeness and digestibility. Spreading your intake across meals, aiming for at least 25 to 30 grams of protein per meal, helps maximize the muscle-building response at each sitting.
If you eat mostly or entirely plant-based, focus on three strategies. First, include soy regularly, as it’s the most consistently high quality plant protein. Second, pair grains with legumes across your daily meals to cover amino acid gaps. Third, consider processed plant protein sources like pea protein concentrate or soy protein isolate, which have digestibility rates comparable to animal proteins. The processing removes fiber and other compounds that otherwise slow digestion and reduce absorption.
For older adults, protein quality becomes especially important because aging muscle becomes less responsive to the building signal from amino acids. Choosing highly digestible, leucine-rich sources, or simply eating more total protein if relying on plant sources, helps compensate for this reduced sensitivity.