Plant Protein vs. Animal Protein: What the Research Says About Both

What Makes a Protein 'Complete' or 'Incomplete'?

Protein is made up of building blocks called amino acids. Of the 20 amino acids the body uses, nine are considered essential — meaning the body cannot produce them on its own and must obtain them from food. A protein source containing all nine in sufficient quantities is called a complete protein.

Animal proteins — including eggs, dairy, meat, poultry, and fish — are generally complete. Most plant proteins, with a few exceptions like soy and quinoa, are considered incomplete, meaning they are relatively low in one or more essential amino acids.

However, the incomplete label can be misleading. The body pools amino acids from everything consumed throughout the day, not just from single meals. Eating a varied diet that includes legumes, grains, nuts, seeds, and vegetables provides all essential amino acids without requiring precise meal-by-meal combinations. This is a well-established position among major nutrition organizations.

For a deeper look at how different protein sources fit into a well-rounded eating pattern, see our guide to building a balanced plate.

Digestibility and Bioavailability: How Much Actually Gets Used?

Having protein in a food is one thing; how much the body can actually absorb and use is another. This is measured by a metric called the PDCAAS (Protein Digestibility Corrected Amino Acid Score) or the more recent DIAAS. Animal proteins like eggs, dairy, and meat consistently score near the top of these scales.

Plant proteins are generally digested less completely, partly because plant foods contain antinutrients — compounds like phytates and tannins that can interfere with protein and mineral absorption. Cooking, soaking, fermenting, and sprouting legumes and grains can significantly reduce antinutrients, improving the effective protein yield from these foods.

The practical takeaway: someone eating a diverse plant-based diet likely gets adequate protein and amino acids, but may need to consume somewhat larger overall quantities than someone eating animal proteins to achieve comparable amino acid absorption.

What Long-Term Research Suggests About Each Pattern

Large observational studies and some randomized trials have examined dietary protein patterns and health outcomes over time. The picture that emerges is nuanced.

Plant protein intake is consistently associated with lower risks of cardiovascular disease, type 2 diabetes, and all-cause mortality in population studies. Researchers believe this reflects not just the protein itself, but the fiber, unsaturated fats, vitamins, and phytonutrients that come packaged with plant foods. This is one reason the processing context of food matters — whole food sources versus highly processed ones can change these outcomes substantially.

Animal protein research is more mixed. Unprocessed animal sources — fish, poultry, eggs, and dairy — are not consistently linked to increased chronic disease risk in the research literature. Processed red meats (like bacon and deli meats), however, have a stronger association with cardiovascular and colorectal risk. The protein itself may not be the driver; saturated fat content, sodium, and processing methods are thought to play significant roles.

For a detailed comparison of specific amino acid profiles and bioavailability metrics by food type, see our nutritional comparison of plant and animal protein.

This article provides general nutrition information and education. It is not a substitute for advice from a registered dietitian or other qualified healthcare professional. Individual nutritional needs vary.