The Dietary Reference Intakes (DRIs) for protein feature recommendations of 46 grams for adult females and 56 grams for adult males. These recommended intakes are designed to make certain that you get all of the amino acids you need to support your body systems. If you do not get all of the amino acids you need in the food you eat, your body will break down some of its own muscle tissue to support its need for amino acids. If inadequate intake of amino acids from protein continues over a long period of time, it can lead to stunting, poor muscle formation, thin and fragile hair, skin lesions, a poorly functioning immune system, and many other symptoms.
Although animal foods are typically rich sources of protein, they are not required for us to meet our protein requirements. Many of the World's Healthiest plant foods supply important amounts of protein, including beans and legumes, nuts and seeds, whole grains, and a good number of vegetables.
Many people are surprised by the amount of protein that can be obtained from plant versus animal foods. For example, an 8-ounce glass of milk typically provides about 8 grams of protein, and one cup of black beans provides about twice this amount (15.24 grams). There are some differences, however, between the type of protein provided by black beans and the type of protein provided by cow's milk. These differences involve the pattern of amino acids found in the two foods, and the role of what are called "essential amino acids" in our health.
There are approximately 300 different types of amino acids, but most of them do not play an essential role in our health. There are about 20 key amino acids found in the human body. Of those 20, only 8-10 have traditionally been classified as "essential amino acids (EAAs)."
In this case, "essential" means that our body cannot make the amino acid and must obtain it from food. The 9 essential amino acids are leucine, isoleucine, and valine (also called the branched-chained amino acids); methionine and cysteine (the sulfur-containing amino acids, that are sometimes combined as a group or represented only by methionine); and lysine, phenylalanine, threonine, tryptophan, and histidine. Since your body cannot synthesize these amino acids from any other molecules, they must be provided by your diet.
Many other amino acids, including glycine, arginine, taurine, and glutamine can be produced in the body from the EAAs or other substances. However, just because they can technically be produced from other EAAs does not mean that your body will always produce them. For this reason, many researchers like to refer to these other amino acids as "conditionally essential," since there may be times when your body does not produce them as needed.
Some foods provide all of the EAAs in a pattern that seems well matched to our human needs. These foods are almost exclusively animal foods, and well represented by the hen's egg, which is sometimes referred to as a food that can serve as a "reference protein" food. In this case, "reference protein" means a protein that supplies all of the EAAs in a balance that is well matched to our human needs.
When individuals do not consume any animal foods, they typically do not get any "reference proteins" that include all EAAs in an optimal balance. However, research studies have shown that an optimal balance of EAAS is not ever required in a single food, or even in a single meal. Through the course of the day, however, it is a very good idea to combine all foods in ways that meet all of your EAA needs. For this reason, persons who consume no animal foods whatsoever need to make sure that their overall daily diet provides them with adequate protein and adequate intake of all EAAs.