Amino Acid
General Description
Amino acids are one of the three major sources of energy in the human body, the other two being fatty acids, and monosaccharides such as glucose.
Amino acids are linked together in construction of the body's proteins. Most amino acids are incorporated into proteins which are either structural or regulatory in nature. Structural proteins such as collagen and elastin, make up the muscles, tendons, ligaments and bones. Regulatory proteins, called enzymes, control the function of all of the metabolic pathways within the cells of the body. Some enzymes are general in their activity and help breakdown food. Class-specific enzymes regulate larger-scale processes.
Amino acids therefore maintain the integrity of bone and connective tissue, act as transmitters of electrical activity along the peripheral and central nervous systems, and are components of hormones such as insulin and glucagon. Amino acids may also help regulate substances influencing our emotions, impact upon the level of serum cholesterol, and help reduce pain. They have these impacts as regulatory agents for certain hormonal or messenger substances produced within the body.
Approximately twenty amino acids are incorporated into the body's protein. These are classified into two groups: essential and nonessential.
Essential amino acids cannot be produced by the human body and must be obtained through the diet.
Nonessential amino acids can be manufactured in the liver from other amino acids or food sources.
Some amino acids are not incorporated into the body's proteins and work as isolated amino acids. These amino acids include taurine, carnitine and glycine.
Protein Metabolism and Degradation
When dietary protein is consumed, it is broken down into amino acids and small protein fragments in the digestive tract. These molecules are absorbed into the blood stream and carried through the circulation on transport proteins (Albumin, the primary transport protein, is produced in the liver and rich in essential amino acids). The blood stream carries this amino acid/transport protein complex to the cells of the body. The cells then take up the amino acids and utilize them in the construction of their own proteins.
Cellular proteins may in turn be catabolized (broken down) into the individual amino acids and waste products. The body excretes protein waste products as urea in urine. Inability to properly regulate the excretion of urea can result in the buildup of toxic metabolites from protein and produce serious illness. Regulation of this urea cycle is thus extremely important for overall good health.
Dietary Protein Requirements
Cell proteins are synthesized according to messages contained in an individual's genes. The principle of "biochemical individuality" holds that, due to differences in their genetic heritage, no two individuals will have identical protein structures or requirements. Some of these differences are so profound as to lead to the inborn errors of amino acid metabolism producing disease (See phenylketonuria for an example).
In other cases the genetic differences may be more subtle and not produce overt disease, but rather may dictate distinct needs for various amino acids to promote optimal protein synthesis and health.
It has been suggested humans should receive at least 0.8 grams of protein in the diet for every kilogram (2.2 pounds) of body weight.
Excessive dietary protein may lead to an increased risk of bone loss or the production of toxic metabolites dangerous to the kidney. In cases where individuals have kidney disease, it may be beneficial to maintain them on a low protein diet. Individuals who are undergoing muscle wasting or starvation may require a protein enriched diet.
Effects of Amino Acid Deprivation
In cases of protein deficiency (amino acid deprivation), one of the first systems of the body to be adversely affected is the immune system. Inadequate levels of the essential amino acids in the diet suppress the immune function and increase the individual's risk of infection and malignancy.
This suppression is partly related to the maintenance of proper body protein stores. Evidence is accumulating which indicates another factor also may be involved, one in which amino acids are no longer confined to their roles as pieces of a puzzle combining to make protein.
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