First, let’s get some protein terminology under our belts.
Proteins are made up of amino acids. There are 20 dietary amino acids that may be components of protein. Each protein has a distinct number of amino acids and a specific sequence in which they are linked together. Nine of these aminos your body cannot make on its own. These are called essential amino acids, and you need to get them from the protein you eat or the essential amino acid supplement you take.
A normal diet includes a variety of protein food sources. Some protein foods are considered complete proteins, which contain all the essential amino acids the body needs…and other protein foods are considered incomplete proteins, which have low amounts of just some of the essential amino acids.
Combine two incomplete proteins together and you have a complementary protein that contains a sufficient amount of essential amino acids. Or does it?
There are many websites that provide “examples” of complementary proteins:
- Legumes with grains, nuts, seeds, or dairy
- Beans and rice
- Peanut butter sandwich
- Yogurt with nuts
However, a big cautionary note about complementary plant-based protein foods is warranted. Let’s take the example of red beans and rice, which is often cited as a good example of complementary proteins. While neither is a high-quality protein food source, together they provide a balanced mixture of essential amino acids, in theory. Beans are deficient in methionine. Grains generally are relatively high in methionine, so the combination of beans and rice provide at least some of all the essential amino acids. However, just because two proteins are complementary doesn’t necessarily mean that the combination becomes a high-quality protein.
The Incomplete Nature of Complementary Proteins
The notion that the combination of a grain and a legume provide a complete protein is widely promoted on vegetarian websites. Take the example of the peanut butter sandwich. Technically a peanut butter sandwich provides a complete mixture of all the essential amino acids, because the combo of peanut and wheat protein provide at least some of all the essential aminos. However, the quality of peanut protein is low, and the quality of wheat protein is even lower. A peanut butter sandwich also has low protein density, which means the protein calories are fewer than the non-protein calories. The caloric intake required to consume sufficient peanut butter to meet all of your essential amino requirements exceeds the total caloric requirement for the day. The bottom line is that although peanut and wheat proteins are technically complementary proteins, a peanut butter sandwich is still a very low-quality protein food source.
Complementary proteins must have truly complementary profiles of essential amino acids. Unfortunately, the quality of most plant-based proteins is limited by the availability of lysine. Therefore, it is unlikely that two plant-based proteins will be complementary. This can become a real challenge to vegetarian diets and vegan diets that don’t allow for dairy. In contrast to the difficulty of finding complementary plant-based proteins that result in a high-quality protein source of essential amino acids, a typical omnivore diet that combines animal protein and plant-based protein foods can be quite effective. Most plant-based proteins are limited by lysine and animal-based proteins are generally high in lysine. The abundant lysine in the animal protein can improve the quality of the plant-based protein.
A practical alternative to combining animal and plant-based proteins to enhance protein quality is to incorporate essential amino acid supplements into the diet. Amino acid supplements can address any imbalance in the essential amino acid profiles of dietary protein. Furthermore, since an essential amino acid supplement has minimal non-protein components, the protein density of the meal will be greatly increased by consuming essential amino acids with dietary proteins. Adding an essential amino acid supplement to vegetarian and vegan diets is particularly appealing, as it will greatly improve the ratio of essential amino acids to nonessential amino acids in the diet without contributing much to the total caloric intake. And you won’t need to put as much thought into matching the “right” protein foods to make high-quality complementary proteins!