High quality protein is protein that contains all nine essential amino acids in sufficient amounts and is easily digested and absorbed by your body. Not all protein sources are equal: a chicken breast and a bowl of rice may both contain protein, but your body can use far more of what the chicken provides. The difference comes down to two things: the amino acid profile and how well your digestive system can actually extract those amino acids.
What Makes a Protein “High Quality”
Your body needs 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 essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. A protein source that delivers all nine in adequate proportions is called a “complete” protein.
But completeness alone isn’t enough. A protein could contain all nine essential amino acids yet pass through your gut without being fully absorbed. That’s why protein quality depends on two factors working together: having the right amino acids and your body’s ability to digest and use them. A food that scores well on both counts, like eggs or milk, is considered high quality. A food that falls short on either one, like raw wheat or peanuts, ranks lower.
How Protein Quality Is Measured
Scientists score protein quality using a system called the Digestible Indispensable Amino Acid Score, or DIAAS. It replaced an older method (PDCAAS) after a 2012 FAO Expert Consultation found that the previous system had significant limitations. The older method measured how much nitrogen showed up in feces to estimate digestibility, but that approach was skewed by bacteria in the large intestine that produce their own nitrogen. DIAAS fixes this by measuring amino acid absorption earlier in the digestive tract, at the end of the small intestine, which gives a much more accurate picture of what your body actually takes in.
DIAAS also removes a mathematical cap that the old system used, which had the effect of making some lower quality proteins look better than they really were. The result is a scoring method that more honestly reflects the differences between protein sources. A DIAAS score above 100 is considered excellent. Scores between 75 and 99 are good. Below 75, the protein is considered lower quality.
Animal Proteins vs. Plant Proteins
Animal proteins consistently score higher in digestibility. Eggs have a digestibility rate of 98%, casein (the main protein in cheese) hits 99%, and milk lands at 96%. Plant proteins are generally less digestible: black beans sit at 70%, peanuts at 52%, and whole grains at just 45%. Some plant sources do better, with chickpeas reaching 89% and yellow split peas hitting 88%, but they still trail most animal sources.
The gap isn’t just about digestibility. Many plant proteins are low in one or more essential amino acids. Grains tend to be low in lysine, while legumes are often low in methionine. This is why the classic pairing of rice and beans works: each fills in the amino acid gap the other is missing. You don’t need to eat them at the same meal, but getting a variety of plant proteins throughout the day matters if you’re relying on them as your primary source.
Cooking and processing help close the gap. Heat-treated plant proteins show about 18% higher digestibility than unprocessed versions. So canned lentils absorb better than raw ones, and tofu absorbs better than whole soybeans.
Why Plant Proteins Are Harder to Absorb
Plants contain compounds called antinutrients that interfere with protein digestion. Two of the most significant are trypsin inhibitors and phytic acid. Trypsin inhibitors, common in raw legume seeds and cereals, block the digestive enzymes your body uses to break down protein. They reduce how much protein you can extract from a meal and can increase stress on your pancreas. Phytic acid, found at concentrations of 1 to 5% by weight in cereals, legumes, nuts, and seeds, binds to minerals and can further reduce nutrient absorption.
Simple kitchen techniques reduce these compounds significantly. Soaking beans overnight, sprouting grains, and thorough cooking all lower antinutrient levels. Fermentation is particularly effective, which is one reason fermented soy products like tempeh are nutritionally superior to raw soybeans.
The Best Plant Protein: Soy
Among plant-based options, soy stands out. Across all soy products, the average DIAAS score is about 85, which puts it in the “good quality” range and closer to animal sources than any other common plant protein. Soymilk scores highest among soy products, while full-fat soy flakes score lowest (around 72). Soy protein isolate, the kind used in many protein powders and meat alternatives, falls in between. Research comparing soy-based burgers to pea-based burgers found that soy delivered significantly higher protein quality.
If you eat a plant-based diet, prioritizing soy products, combining complementary proteins like legumes and grains, and choosing cooked or processed forms over raw ones will get you closer to the amino acid profile and absorption rate of animal proteins.
Leucine and Muscle Building
One amino acid plays an outsized role in triggering your body to build muscle: leucine. It acts as a signal that flips the switch on muscle protein synthesis. Research estimates that you need roughly 2.5 to 3 grams of leucine in a single meal to maximally stimulate that process in younger adults, and 3 to 4 grams per meal for older adults. That translates to about 25 to 30 grams of total protein per meal when the source is high quality.
This is where protein quality has real practical consequences. To hit that leucine threshold from whey protein, you might need just one scoop. To reach it from a plant source lower in leucine, you’d need to eat a substantially larger portion. The total protein on a nutrition label doesn’t tell you how much of it your body will actually use.
Protein Needs by Age
The baseline recommendation for healthy adults is 0.83 grams of protein per kilogram of body weight per day. For a 70 kg (154 lb) person, that’s about 58 grams daily. But this is a minimum to prevent deficiency, not an optimal target for maintaining muscle.
Older adults face a challenge called anabolic resistance, where muscles become less responsive to the protein you eat. The same 20 grams of protein that triggers robust muscle building in a 25-year-old produces a weaker response in a 70-year-old. To compensate, research supports eating 25 to 30 grams of high quality protein at each meal, totaling at least 60 grams per day from high quality sources. Spreading protein intake evenly across meals matters more with age, since a single large serving at dinner won’t make up for protein-poor breakfasts and lunches.
Whey Protein: Isolate vs. Concentrate
Whey protein, derived from milk during cheese production, is one of the most studied and highest scoring protein supplements. It comes in two main forms. Whey concentrate contains up to 80% protein by weight, with small amounts of fat, carbohydrates, and lactose making up the rest. Whey isolate undergoes additional processing to reach 90% or more protein by weight, stripping out most of the fat and lactose.
Per 100 calories, isolate delivers about 23 grams of protein compared to 18 grams from concentrate. The amino acid profiles are virtually identical since both come from the same milk proteins. The practical difference is that isolate packs more protein per serving with less lactose, which matters if you’re sensitive to dairy sugars. Concentrate costs less and works perfectly well for most people.
Practical Takeaways for Choosing Protein
- Eggs, dairy, fish, poultry, and meat are the most efficiently absorbed protein sources, with digestibility rates above 80% and complete amino acid profiles.
- Soy products are the strongest plant option, with DIAAS scores averaging around 85. Soymilk and soy protein isolate rank highest.
- Legumes and grains work well when combined, since each compensates for the other’s missing amino acids. Cook them thoroughly to reduce antinutrients.
- Aim for 25 to 30 grams per meal if you’re trying to maintain or build muscle, especially past age 65.
- Spread protein across meals rather than loading it all into dinner. Your body can only use so much at once for muscle building.