Nutrition

Not all protein is equal: what protein quality really means

Protein quality depends on how many essential amino acids a food provides and how well you digest them — and it is easy to compensate for.

Protein quality describes how well a food's protein meets the body's needs for building and repairing tissue. Two things matter most: the amounts of the nine essential amino acids, which the body cannot make, and how well the protein is digested and absorbed. Modern scoring systems such as DIAAS, the Digestible Indispensable Amino Acid Score, rate foods by the essential amino acid that is in shortest supply after digestion. Milk, eggs, meat and fish tend to score very high, soy scores fairly high, and most other plant proteins score lower because they are short in one or more amino acids such as lysine or methionine.

For muscle growth, leucine is particularly important, because it acts as a trigger for muscle protein synthesis. Animal proteins and some plant isolates contain more leucine per gram, so a smaller serving reaches the amount that seems to maximize the response after a meal. Plant proteins can still build muscle effectively, but people relying on them may need slightly larger servings or a mix of sources to get the same amount of essential amino acids and leucine.

In everyday life, protein quality is mostly a problem only when total protein is low or the diet is very narrow. Combining different plant sources across the day, such as legumes with grains, nuts and seeds, covers the gaps, and soy or blended plant protein powders are useful options for vegans. Aiming for a total of roughly 1.6 grams of protein per kilogram of body weight per day, spread over several meals with about 20 to 40 grams each, matters more for muscle than chasing the single highest-scoring protein.

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