Protein Synthesis: How does the Body Use and Break Down Protein?
A close look at exactly how the body uses amino acids
There are 22 amino acids important to human nutrition. Nine of these are essential amino acids, meaning the body cannot make them. Instead they need to be provided in the diet. The body can produce the other 13. The role of protein in food is not to provide our bodies with proteins directly, but to supply the amino acids from which the body can make its own proteins. When we eat a diet that supplies each essential amino acid in adequate amounts, our body supports protein synthesis.
The following lists the amino acids, including the nine essential amino acids, that make up proteins in human nutrition.
Essential Amino Acids:
Other Amino Acids:
To make protein, cells must have all the needed amino acids available simultaneously. Therefore, the first important characteristic of protein in our diet, with respect to protein, is that it should supply at least the nine essential amino acids for the synthesis of others, to make proteins. If one amino acid is supplied in an amount smaller than needed, the total amount of protein that can be synthesized from others will be limited. It is impossible to produce a partial protein. Only complete ones can be made. A diet that contains an imbalance of amino acids is a diet containing poor protein quality. When the body attempts to use the amino acids supply from such a diet, it wastes many amino acids. In the absence of one, it can't use the others and it has no place to store them.
Each food has its own characteristic amino acid balance, and when foods are combined, they almost invariably supply plenty of essential amino acids. In countries where protein is scarce and/or only one protein rich food is eaten regularly, the quality of that particular food's protein is crucially important to people's health and particularly important to children's development.
A complete protein is one that contains all of the essential amino acids in about the same amount the human body requires--and it may or may not contain all of the other amino acids the body can make. People generally associate complete protein with such foods as meats and eggs, but not with plant foods. Generally, proteins derived from animal foods such as meat, fish, poultry, eggs, and milk are a complete source of protein. Those derived from plant foods such as vegetables, grains, and beans vary more than meat and dairy. If you derive most or all of a day's food energy from rice and/or potatoes, you will obtain all of the needed essential amino acids. In addition, when two plant proteins, each containing the amino acids that the other lacks, are eaten at the same meal, they can make up an acceptably complete protein.
However, completeness is not the only issue with respect to protein quality. For the highest quality, proteins must not only be complete but also digestible, so that the sufficient numbers of amino acids reach the body's cells to permit them to make the proteins they need. Although the proteins of rice and potatoes are of high quality, perhaps the best form of protein comes from eggs. Egg protein tends to be retained in the body, which indicates that it is utilized with little waste. In fact, egg protein has been designated the reference protein for the purpose of measuring protein quality.
To summarize, for the body to use proteins with maximum efficiency, they must contain the essential amino acids, must be digestible, and must be consumed with sufficient energy from other sources, such as complex carbohydrates, so that amino acids will not be used for energy, but rather to help build and repair muscle tissue. They must also be accompanied by the vitamins and minerals needed to facilitate their use, and must be received by a healthy body able to use them.
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