Protein Quality: Essential Amino Acids, Digestibility and Complete Proteins

Overview

Not all proteins are built the same. Protein quality describes how well a food's amino acids match what the body needs and how much of them is actually absorbed. This page explains the terms you see on labels and in articles, and where the science is still unsettled. It supports Protein: What It Does and the powder pages in Amino Acids and Protein.

Essential amino acids and "complete" protein

Nine amino acids are essential: the body cannot make them, so they must come from food. [1] A "complete protein" is a popular phrase for a food that supplies all of them. The British Heart Foundation says animal foods and three plant sources (soy protein, quinoa and the meat alternative Quorn) contain all the essential amino acids needed for good health. [2] Other plant proteins lack some of them, which is why animal protein is sometimes called complete. [2] The same source says this does not put a vegetarian diet at risk, because eating a variety of plant proteins across the day supplies all the essential amino acids you need. [2]

MedlinePlus agrees that a diet based on a single plant item will not be adequate, but says we no longer worry about pairing proteins such as beans and rice at one meal. [1] What matters is the overall pattern across the day. [1]

How protein quality is scored

Two scoring systems appear in the literature.

• PDCAAS (Protein Digestibility Corrected Amino Acid Score): the limiting (lowest) amino acid score is multiplied by protein digestibility. [3]
• DIAAS (Digestible Indispensable Amino Acid Score): an FAO expert consultation recommended this newer measure to replace PDCAAS. It can have values below or above 100 percent. [3]

The FAO report also says dietary amino acids should be treated as individual nutrients. [3] It states that the data on true ileal amino acid digestibility were, at the time, insufficient to support applying DIAAS fully in practice, so the measure is still being refined. [3]

Example scores

One 2020 overview calculated DIAAS for 5 animal and 12 plant proteins. Pork meat, casein, egg and potato proteins averaged above 100; whey and soy averaged 75 or more; pea, rice, hemp and several others fell below 75. [4] The same paper found considerable variation within the same protein source. [4] Its authors are employees of a potato-protein company, so read the potato findings with that in mind. [4] A low score for one plant protein does not mean a plant-based diet is inadequate, because proteins complement each other: soy, pea and potato proteins raised scores when mixed with other plant proteins. [4]

Why this matters in real life

A 2019 review concluded that protein-rich foods such as legumes, nuts and seeds are sufficient to meet protein adequacy in adults on vegetarian or vegan diets, and that worry about amino acid deficiency has been overstated. [5] It also notes the question is more complex in older adults. [5] The lead author reports a research contract with a plant oil and protein industry body. [5] See Protein for Older Adults and Vegetarian or Vegan Diets.

For muscle-building, quality is one reason people choose whey. One review points out that most plant proteins have relatively low leucine content, and that combining protein sources can give a more balanced amino acid profile. [6]

Safety note

Protein quality scores say nothing about allergens, contaminants or kidney disease. See Protein Powder Safety, and talk to a clinician if you have kidney disease. [7] Supplements can also interact with medicines; see How Interactions Work.