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Protein quality measures how well a dietary protein supplies the essential amino acids the body needs in digestible form. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) evaluates the amino acid profile of a protein relative to human requirements, corrected for digestibility, with a maximum score of 1.0. The newer Digestible Indispensable Amino Acid Score (DIAAS) improves on PDCAAS by measuring ileal (small intestine) digestibility of each individual amino acid rather than overall fecal digestibility, allowing scores above 1.0 for exceptionally high-quality sources. Complete proteins (eggs, dairy, meat, soy) contain all essential amino acids in adequate proportions; incomplete proteins (most grains, legumes, nuts) are limiting in one or more. Complementary protein combining — eating foods whose amino acid profiles offset each other's limitations, such as rice and beans — achieves a complete amino acid profile without requiring animal sources, and the combinations need not be consumed in the same meal.
Calculate the PDCAAS for two or three protein sources by identifying the limiting amino acid, computing its ratio to the reference pattern, and multiplying by digestibility. Then design a day's meals using only plant proteins that achieve amino acid complementarity.
You already know that proteins are built from 20 amino acids, nine of which are essential—meaning the body cannot synthesize them and must obtain them from food. Protein quality is the question of how well a given food actually delivers those nine essentials in amounts and forms the body can use. The answer depends on two things: the amino acid profile (does this food contain the right amino acids in adequate proportions?) and digestibility (can the body actually access and absorb them?).
The PDCAAS (Protein Digestibility-Corrected Amino Acid Score) formalizes this. First, identify the limiting amino acid—the essential amino acid present in the lowest ratio relative to the reference pattern established for human needs. Lysine is commonly limiting in grains; methionine is commonly limiting in legumes. The limiting amino acid sets the ceiling: you cannot use the other amino acids more completely than the worst-supplied one allows. PDCAAS then multiplies this ratio by the protein's overall digestibility and caps the score at 1.0. Eggs and dairy score near 1.0; most plant proteins score between 0.5 and 0.9.
The newer DIAAS refines this in two important ways. First, it measures digestibility of each individual amino acid at the end of the small intestine (ileal digestibility) rather than estimating from fecal measurements—a more accurate picture of what the body actually absorbs. Second, it removes the 1.0 cap, allowing exceptionally high-quality sources to score above 1.0, reflecting that they supply more than the minimum requirement per gram. DIAAS is considered more physiologically accurate, but PDCAAS remains common in regulatory contexts because the data requirements for ileal digestibility measurements are more demanding.
Complementary protein combining is the practical application of this framework for plant-based diets. Legumes (beans, lentils) are rich in lysine but limiting in methionine; grains (rice, wheat) are the reverse. Eating them together across a day creates a combined amino acid profile that covers all essentials—the classic example is rice and beans. Crucially, you do not need to eat complementary proteins at the same meal: the liver maintains a free amino acid pool that buffers intake over roughly 24 hours. The key insight is that protein quality is a property of the overall diet, not of individual meals. A well-designed plant-based diet can achieve excellent amino acid coverage, but it requires conscious attention to which foods offset each other's deficiencies.