How many essential amino acids are there and what are they? What foods provide complete protein? Read this article and learn about the right kinds of protein for a healthy diet.
Protein is a necessary part of the human diet. Not all proteins are created equal, however. When making food choices, it is important to choose the right kinds of protein in the right combinations.
Essential Amino Acids
Proteins are made of amino acids. Twenty different amino acids combine in various sequences to form all proteins. The human body is capable of manufacturing some of these amino acids, but the remaining ones must be obtained through the diet. These amino acids are called the essential amino acids. Protein is not stored in the body for later use, so all the essential amino acids must be consumed on a daily basis. Otherwise, the body is forced to break down necessary tissue, such as muscle, to obtain the missing amino acid (University of Arizona, 2003).
The number of essential amino acids differs for young children and adults. In adults, the essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. There is controversy about whether histidine is also essential in adults; as of 2007, the World Health Organization (WHO) has taken the conservative position that it is, bringing the total number of essential amino acids in adults to nine. For infants and young children, arginine is an also essential amino acid, although adult bodies can manufacture it (University of Arizona, 2003).
The non-essential (or "dispensable") amino acids are actually recycled by the body from other amino acids. One non-essential amino acid, tyrosine, can only be manufactured from the essential amino acid phenylalanine.
Complete and Incomplete Proteins
A complete protein is a protein that contains all the essential amino acids in the correct proportions. Some protein sources are incomplete, meaning they lack one or more of the essential amino acids. Incomplete protein sources differ in the missing amino acids and can be combined to form a complete protein overall. These combinations may be eaten in the same meal or in different meals in the same day.
Complete Plant Proteins: Soybeans (left) and Quinoa (right)
Sources of Complete Protein
Animal protein, such as meat, poultry, fish, and eggs, is considered complete protein. Not surprisingly, the protein in animals' bodies contains amino acids in the proportion needed by animals' bodies, including humans. Most vegetable protein sources are incomplete.
Pulses (peas, lentils, and beans other than soybeans) are low in methionine. Nuts and most grains are low in lysine. A combination of pulses with nuts or grains cancels out these deficiencies, resulting in a complete protein. The combination need not be consumed at the same meal, but should be eaten during the same day.
Soy is an exception to this rule; it provides a complete amino acid profile. Examples of soy protein include tofu, soy milk, tempeh, and miso. Another plant source of complete protein is the grain quinoa.
References and Credits
The Biology Project (University of Arizona), 2003. "The Chemistry of Amino Acids."
Veganhealth.org, 2003. "Where Do You Get Your Protein?"
World Health Organization, 2007. "Protein and Amino Acid Requirements in Human Nutrition" (PDF). WHO Technical Report Series.
Soybeans Image: National Renewable Energy Laboratory
Quinoa Image: Wikimedia Commons, licensed under the GNU Free Documentation License.