Protein Quality: Why Not All Protein Sources Are Equal

When people talk about eating enough protein, they usually mean quantity. But protein quality — how well your body can actually use what you eat — matters just as much. Two foods can both deliver 20g of protein per serving while performing very differently in your body.

Complete vs Incomplete Proteins

Proteins are made of amino acids. Nine of these are essential — your body cannot synthesise them and must obtain them from food. A complete protein contains all nine in sufficient quantities. Animal proteins (meat, fish, eggs, dairy) are complete. Most plant proteins are incomplete, meaning they are low or lacking in one or more essential amino acids. Rice is low in lysine. Legumes are low in methionine. Combining rice and beans provides all nine — which is why this pairing appears across almost every traditional cuisine worldwide.

Measuring Protein Quality: DIAAS

The Digestible Indispensable Amino Acid Score (DIAAS) is the current gold standard for measuring protein quality. It accounts for both amino acid composition and digestibility. Whey protein scores around 1.09 (above 1.0 is considered excellent). Whole egg scores 1.13. Soy protein isolate scores 0.98 — the best plant-based option. Pea protein scores around 0.82. White rice scores 0.59.

Practical Implications

If your diet is primarily plant-based, you need to be intentional about combining protein sources to cover all essential amino acids. You also generally need to eat more total protein, since plant proteins are less digestible. A common recommendation is 1.6g per kg of bodyweight for plant-based athletes versus 1.2-1.4g for omnivores doing the same training volume.

The Leucine Threshold

Muscle protein synthesis requires reaching a leucine threshold — approximately 2-3g of leucine per meal. Whey protein reaches this threshold easily. Plant proteins generally require larger servings to hit the same leucine dose. This is relevant for people using protein supplements for muscle building: pea and rice protein blends are more effective than either alone because their amino acid profiles are complementary.