Proteins are made up of amino acids. Amino acids are the building blocks of protein. There are 20 different amino acids that join together to make all types of protein. Some of these amino acids can’t be made by our bodies, so these are known as essential amino acids. It’s essential that our diet provide these.
Humans can produce 10 of the 20 amino acids. The others must be supplied by food. If we don’t get enough of even 1 of the 10 essential amino acids the body’s proteins degrade. Unlike carbohydrate and fat, the human body does not store excess amino acids for later use—the amino acids must be in the food every day.
The 10 amino acids that we can produce are alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. Tyrosine is produced from phenylalanine, so if the diet is deficient in phenylalanine, tyrosine will be required as well. The essential amino acids are arginine (required for the young, but not for adults), histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids are required in the diet.
In the diet, protein sources are labeled according to whether or not they have all the essential amino acids or not. They are called complete or incomplete.
A complete protein source is one that provides all of the essential amino acids. Animal-based foods such as meat, poultry, fish, milk, eggs, and cheese are complete protein sources.
Plant sources of protein are incomplete, with the exception of soybeans and quinoa. If a vegetarian has a limited diet, they can potentially not be getting their daily protein needs met. They must be careful to eat a wide a variety of protein rich plant foods.
Complementary proteins are two or more incomplete protein sources that together provide adequate amounts of all the essential amino acids.
Lentils for example are low in certain essential amino acids; however, these same essential amino acids are found in greater amounts in barley. Similarly, barley contain lower amounts of other essential amino acids that can be found in larger amounts in lentils. Together, these two foods can provide adequate amounts of all the essential amino acids the body needs. A soup that contains both lentils and barley is an excellent example of a complete protein.
In the past, it was thought that these complementary proteins needed to be eaten at the same meal for your body to use them together. Now studies show that your body can combine complementary proteins that are eaten within the same day.