Essential- and non-essential amino acids
As mentioned above, amino acids are required for protein synthesis in growth, maintenance and repair. Some of the amino acids can be synthesized in the body as the need arises (nonessential amino acids), while others must be supplied by the ingested food (essential amino acids). The nonessential amino acids are not biologically less important, but if there are essential amino acids missing in the diet, they may be synthesized from those available in the amino acid pool. There are 22 different natural amino acids. Eight are essential for adults : Phenylalanine, Methionine, Valine, Lysine, Leucine, Isoleucine, Threonine, Tryptophan. Nine are essential for growth and maintenance in children (also histidine). Fourteen are nonessential amino acids: e.g. Glycine, Alanine, Serine, Cystine, Tyrosine, Proline, Aspartic acid, glutamic acid etc.
If one or more of the essential amino acids is not present with protein synthesis, the synthesis is limited quantitively by the availability of essential amino acid lowest in supply.
When there is a marginal supply of external sources, urea, ammonium salts and glutamic acid, have a sparing effect to the nonessential nitrogen for human growth and maintenance. With severe nitrogen deprivation, nonessential amino acids can be synthesized in the body from suitable multicarbon precursors in the metabolic pool and nitrogen fed in the form of urea and ammonium salts.
An essential amino acid which is present in inadequate amounts in a food substance, is called a limiting amino acid. It will be the first to be used up in protein synthesis. In vegetable protein, lysine and methionine, are limiting amino acids e.g. legumes are lower in methionine, higher in lysine; grains are lower in lysine and higher in methionine. Therefore if you consume only corn as a protein source, the body’s stores of lysine will be depleted. Grains and legumes together, balance each other out. Each compensates for the other’s limiting amino acid. By eating them together in the right ratio (2 parts of grain to 1 part of legumes), you have a vegetable protein with all eight essential amino acids, which is comparable in quality to an animal protein.
Population groups where protein intake is mainly from cereals like wheat, will benefit from supplemental lysine, if their lysine intake is lower than desirable.
It is not necessary to consume grains and legumes simultaneously, but they have to be eaten within a brief time interval of one another to be effective. Therefore the time factor is very important in mutual supplementation. There exists no physiological mechanism for the storage of individual amino acids in the body, for periods when they are required for the synthesis of a special tissue protein.
Therefore effective supplementation occurs only when the deficient and supplementary proteins are fed simultaneously or within a brief time interval.
This also implies that in feeding proteins of low biological value, should always be supplemented with high-quality proteins simultaneously.
BIOLOGICAL VALUE of food proteins:
The biological value (percentage of absorbed nitrogen for maintenance and growth) of food proteins, depends on their amino acid composition. Proteins are classed as having a high or low biological value on the basis of their ability to supply all the amino acids required (structural completeness) for the formation of body tissues, enzymes and hormones. Another factor that influence the value of a given protein, is it’s digestibility. A protein must be digested completely and hydrolyzed into amino acids, before it can be absorbed into the bloodstream and made available to the body’s amino metabolic pool. With incomplete hydrolysis of the protein, only part of the amino acids become available, while the rest remain undigested and are eliminated, without contributing to the nutrition of the individual. The method of preparation (cooking, baking, toasting etc.) of the protein food, determines it’s digestibility. Overheating (dry heat/frying) reduces the nutritive value, by destroying the amino acid, lysine or by tying them up in new chemical linkages not susceptible to digestion (e.g. trans fats). Although, cooking with water increased the digestibility and improves the nutritive value of wheat protein e.g. methionine and other amino acids are liberated more rapidly. Egg protein has the highest biological value. Second, is milk protein (albumin and especially casein). Egg and milk supply all the essential amino acids for normal growth and healthy life (provided in adequate amounts). Red meat provides the complete amino acid-balanced protein that is readymade for human needs. Soy protein is a good substitute for animal protein – it lowers cholesterol levels more than a low-fat, low cholesterol diet, that includes small servings of lean meat!.