A complete protein is a source of protein that contains an adequate proportion of all nine of the essential amino acids necessary for the dietary needs of humans or other animals. Some incomplete protein sources may contain all essential amino acids, but a complete protein contains them in correct proportions for supporting biological functions in the human body.
The following table lists the optimal profile of the essential amino acids, which comprises a complete protein, as recommended by the Institute of Medicine’s Food and Nutrition Board:[2]
| Essential Amino Acid | mg/g of Protein |
|---|---|
| Tryptophan | 7 |
| Threonine | 27 |
| Isoleucine | 25 |
| Leucine | 55 |
| Lysine | 51 |
| Methionine+Cystine | 25 |
| Phenylalanine+Tyrosine | 47 |
| Valine | 32 |
| Histidine | 18 |
Nearly all foods contain all twenty amino acids in some quantity, and nearly all of them contain the essential amino acids in sufficient quantity. Proportions vary, however, and some foods are deficient in one or more of the essential amino acids. Though some vegetable sources of protein contain sufficient values of all essential amino acids, many are lower in one or more essential amino acids than animal sources.
Consuming a mixture of plant-based protein sources can increase the biological value of food. For example, to obtain 25 grams of complete protein from canned pinto beans requires consuming 492 grams (423 kcal), however if combined with 12 grams of Brazil nuts, requires only 364 g of pinto beans (391 kcal). These complementary protein sources do not need to be consumed in the same meal, but should be consumed within 4 hours to provide optimal benefit.
(Shout-out to Wikipedia for the perfectly-phrased info.)
