How plant and animal proteins differ in quality
Protein sources differ in amino acid profile and digestibility. Most animal proteins (meat, eggs, dairy, fish) are complete, supplying all essential amino acids in useful proportions, and tend to be high in leucine, the amino acid that most strongly triggers muscle protein synthesis. They also score high on digestibility measures such as DIAAS (the Digestible Indispensable Amino Acid Score). Many individual plant proteins are lower in one or more essential amino acids, commonly lysine in grains and methionine in legumes, and can be somewhat less digestible because of fiber and antinutrients.
Combining complementary plant proteins across a day, such as legumes with grains, closes most of the amino acid gap.
Where plant sources have the advantage
Plant proteins arrive packaged with fiber, polyphenols, and other phytonutrients, and they lack the saturated fat and cholesterol found in some animal sources. Soy is a notable plant protein that is essentially complete and scores well on quality measures. For most people, total daily protein intake and overall diet pattern matter more than the source of any single meal. This is general nutritional information, not medical advice; specific needs, especially for athletes, older adults, or those with health conditions, should be discussed with a dietitian or physician.