What DIAAS actually measures
The Digestible Indispensable Amino Acid Score, or DIAAS, rates a protein by how much of its first-limiting essential amino acid is truly absorbed at the end of the small intestine. It improves on older scoring by measuring digestibility of each individual amino acid rather than crude protein, and by not capping high-quality proteins at a score of 1.0.
A higher DIAAS means the protein supplies more usable essential amino acids per gram for the body's needs.
Why dairy and eggs top the ranking and most plants trail
Whey, milk, and egg score at or above 1.0 because they are highly digestible and well balanced in essential amino acids, especially leucine and lysine. Soy sits in the middle. Many plant proteins fall below 1.0, typically limited by lysine (in grains) or methionine (in legumes), and by lower digestibility from fiber and antinutrients.
A lower score does not make a plant protein useless: combining complementary sources or simply eating enough total protein can cover the gaps. The diagram ranks quality per gram, which is general educational information rather than a verdict on any whole diet.