The protein, or amino acids, ingested by an animal provides the nitrogen required for the body's growth, tissue maintenance, reproduction, and lactation. When a young animal gets too little protein, its growth will be limited and its energy intake will be thwarted. Protein-calorie malnutrition can result. Protein intake requirements vary with the kind of animal and performance expected of it. Adult animals have a lower protein requirement than young animals. (Approximately 4 kcal of energy are metabolized from each gram of protein.)
Approximately 10 amino acids are considered essential for the growing animal. An essential amino acid can be defined as one which cannot be synthesized at a sufficiently rapid rate to permit optimum growth of the young animal. Sources of protein vary in their nutrient value, depending upon the content and availability of the essential amino acids.
Vegetable seed proteins contain more essential amino acids than cereal grains or by-products. Among them, soybean meal is perhaps the best source of amino acids and is also a major source of protein for non-ruminants. Animal proteins derived from fish meal, meat and bone meal, and dried milk products are usually excellent sources of essential amino acids. If processed properly, these sources have a high level of amino acids and are very useful as supplementary proteins to complement the amino acid balance of cereal grain products.
On the fact sheets for TestDiet® products, a number of amino acids are listed with their level in the diet. One should be aware that the sum of the levels of individual amino acids usually will not equal the level of protein in the diet. One reason for this is that protein is determined by assaying for the amount of nitrogen in the product and then multiplying this nitrogen value by a factor. This generally gives a good approximation of the actual protein level, but it is not exact. Another reason for the difference between the sum of the amino acids and the level of protein is that not all of the amino acids are listed. Although the list we have included is extensive, it is not complete. A third reason for the discrepancy is that individual amino acids contain molecular groups which are lost to form water when they combine into protein molecules.
The water hydration values of individual amino acids mean that a group of individual amino acids equivalent to their content in a protein molecule will actually weigh more than the protein molecule by an amount equivalent to these water molecules. Some amines, such as glutamine and asparagine, are measured and reported as glutamic acid and aspartic acid. The amines contain a higher level of nitrogen than the amino acid, thus leading to a falsely elevated protein level when reported.
Traditionally and most commonly, milk casein is the source of protein in purified diets, often supplemented by one (or more) amino acids. Vitamin tested casein contains approximately 90-91% protein.
A number of purified diet manufacturers use in their diets less-expensive milk casein that is not vitamin tested; consequently, unknown and random amounts of various vitamins are introduced into each batch of diet without being recognized or reported. The protein content of this less expensive casein can also fluctuate significantly. This cost-cutting practice defeats the fundamental purposes of a purified diet—full knowledge of all ingredients, complete open formulas, and repeatable consistency, resulting in accurate, reliable data.
Consistent with the value of your research, at TestDiet® we use only the more expensive, more precise vitamin tested milk casein in our purified diets; and, we routinely assay our casein to confirm that the protein content is within tight tolerances.
Occasionally, soy protein is used as a protein source where the research protocol requires, e.g. when a diet from purely vegetarian or non-animal ingredients is desired. The researcher must be aware of the presence of phytoestrogens in all soy protein products and the estrogenic activity that may result in test animals.