Test Your Knowledge

Animal sources and soy are considered complete proteins because they provide all nine essential amino acids. These are considered higher quality proteins. Plant sources other than soy are considered incomplete proteins because there are lacking in one or more of the essential amino acids. They are considered lower quality proteins.

Incomplete proteins lacking in one amino acid can be eaten with another incomplete protein that is missing a different amino acid. This way, all nine essential amino acids are consumed. For example, eating legumes (which contain the essential amino acid lysine, but not methionine) together with grains (which contain methionine, but not lysine) provides all essential amino acids. When consumed together, these food combinations are called complementary proteins.

A measurement called the protein digestibility-corrected amino acid score (PDCAAS) can be used to identify high-quality (complete) proteins. It compares the amino acids contained within a particular food with the amino acids required by the human body. It also adjusts  for ease of digestion because proteins from animal sources tend to be more easily digested than those from plant sources.

However, foods considered sources of complete proteins are not necessarily healthier than those considered to be incomplete proteins. This is because complete proteins are often accompanied by high amounts of saturated fat. The complete nutritional package of a food must be considered. For example, lean cuts of meat provide complete protein with much less fat than other cuts of meat.

Soy is the only plant source of protein that provides all nine essential amino acids. This makes soy an important food choice for vegetarians, especially vegans, who do not consume any animal products. Soybeans can be processed into a number of different food products (e.g., tofu, soy milk, soy flour, tempeh), many of which take on the flavor or texture of the foods they are served with. Soy may also have additional, specific health benefits.

Amino acids are also available in supplement form. However, these are not used by the body as easily as those that come from dietary protein. Amino acid supplements can also lead to an imbalance of amino acid levels in the body. Patients are urged to speak with their physician or dietitian before taking amino acid supplements.

According to the U.S. Department of Agriculture (USDA), protein levels of some common foods include: