Normally the metabolisable calories yielded by carbohydrates and fats supply the energy to keep the body warm and drive the processes of life, but in the case of a deficiency of calories from carbohydrates and/or fats part of the protein will be used for energy, too.
Although protein may serve as a source of energy, it is important primarily as a source of amino acids, substances used in making new protoplasm and cells and therefore essential for growth and replacement. An organism builds its own structural and functional proteins from the amino acids. Neither carbohydrates nor fat can fulfil this function, as neither contains the necessary element nitrogen.
Excess protein cannot be stored in the body like fats and carbohydrates, and as the body uses up protein all the time, a deficiency soon shows serious effects. Without a fresh supply the total body protein is reduced by half in about 80 days, for example, and the half-life period of liver protein is only 10 days.
This means first of all that enough protein must be supplied by the diet to replace the used body protein. As 70% – 75% of the dry matter in muscles is made up of proteins, and such tissues as hair, wool, feathers, nails, horns and hooves are largely protein in nature, an even greater supply is necessary at times of growth and muscle building because of the rapid protein consumption at these times. This means that baby and young animals automatically have a higher protein requirement than adult animals.
Up to now we have mainly spoken of protein and protein requirements, but for the nutritionist the most important factor of protein is its amino acid make-up. Every animal owner today knows that not all proteins are suitable for his particular species. This means that the value or quality of the diet should not be judged on the amount of protein in the food but on the amino acids it contains. Animal protein usually contains a higher concentration of the essential amino aicds than plant protein.
For a long time it was not known why the biological value of the various proteins was different, but now the term quality or biological value as applied to a food protein refers to the assortment and proportions of its amino acid chain: the more complete the assortment and the more nearly the proportions approach the physiological needs of a species in amino acids the higher the quality of the protein.
The proteins in the diet are broken down into amino acids by the enzymes in the stomach, duodenum and ileum (small intestine) and as such soluble compounds pass through the intestinal lining into the blood stream where they are carried to the cells to be turned into body protein.
Proteins are made up of one or more chains of amino acids.
The following table shows over 20 of the 20 – 25 known amino acids.
Not all the amino acids listed above are necessary as dietary components as some can be synthesised in the body from a suitable carbon source and amino groups from other amino acids. These are called dispensable or non-essential amino acids, but this only means they are dispensable in the diet and not to the animal. Indispensable = essential amino aicds are those that cannot be synthesised in the body fast enough to meet the physiological needs of the body or cannot by synthesised at all by the body and must therefore be furnished preformed in the diet.
A well-balanced diet must contain high-quality proteins supplying an abundance of the various indispensable amino acids.
As already mentioned, there are 20 – 25 different amino acids in existence, and the particular way they are combined in each protein determines its bio-chemical characterisitics, but the amounts of the various amino acids present in a protein determine its nutritional value. The nutritive value of a particular protein, however, must be considered in relation to all the other proteins in the diet, because although a protein can consist of 50 – 5000 different amino acid combinations, it is seldom the case that a single isolated protein contains all the essential amino acids needed by a certain species and in the right balance. The proteins of cereal grains, for example, are likely to be deficient in lysine and threonine, whereas the proteins of seeds of legumes, such as soybeans, are relatively well supplied with lysine and threonine but lack the sulphur-containing amino acids such as methionine and cystine. If just one of these amino acids is missing or under the requirement level the body cannot make its own body protein. Even if the others are present in excess of requirements, the “chain” is incomplete and this one amino acid would limit protein utilisation. In such a case we call this the limiting amino acid.
An apparently satisfactory level and balance of amino acids in dietary protein does not always guarantee, however, that ingestion of the diet will satisfy the amino acid requirements of an animal. Under certain conditions some amino acids may by unavailable because the proteins in the diet are incompletely digested or the construction of the plant cell walls also renders the proteins in the cell inaccessible to the digestive enzymes. The presence of inhibitors of the digestive enzymes, such as the trypsin inhibitor in soybeans, may impair the digestion in some cases and, of course, illness or medical reatment can reduce the amino acids’ availability in others.
With all this attention to the amount, quality and type of protein needed as a source of amino acids in the diet, we must not forget the three different, interrelated metabolic pathways amino acids may follow within the body: (1) used for protein synthesis; (2) serving as precursors in the synthesis of such nitrogen-containing compounds as choline and thyroxine; (3) as a source of energy. Balance among these pathways is constantly regulated so that changes in the rate of disposal along one route are compensated by changes along one or both of the other pathways. This means that in our efforts to provide a diet well-balanced in protein we must not forget to provide a supply of carbohydrates and fats in ratio to the protein, for a sub-optimum in calorie/protein ratios results in the degradation of amino acids to meet the energy needs of the body, whereas calorie/protein ratios much higher than the optimum result in an inadequate intake of protein.
Thus the protein nutrition is a somewhat complicated subject, and there is a lot more to be said about it. We would just like to point out that over 90% of the protein in our products is already in its digested form, amino acids, and in the natural L-form, the only form the body can use. This means it can start fulfilling its different functions in maintenance, growth, reproduction and as an energy source for the body at once, without the strain of digesting.