The various amino acids are specific in their functions. They are not interchangeable. Of the twenty-two known amino acids only ten or twelve are regarded as essential or indispensable. Tryptophan, tyrosin, lysin, cystin, glutamic acid, histidin and ornathin are among the essential amino acids. If the diet supplies the essential amino acids in adequate quantities, growth, maintenance and reproduction are normal. If one or more of these is lacking or deficient this is not true. Examples: A deficiency of valine in the diet of young animals stunts growth and development to a remarkable degree. If lysin is lacking in the diet there is more or less maintenance but no growth. No matter how much protein and other elements supplied in the diet, if lysin and tryphtophan are lacking, life soon comes to an end.

It is held that the amino acids other than the ten or twelve indispensable ones can be made by the tissues from the essential amino acids, apparently by oxydizing them. Glycine apparently can be manufactured in the animal body from the other amino acids if it is lacking in the diet. Prolin, which may be readily produced in the body by oxidation of histidin is, therefore, not considered an essential amino acid. Its production from histidin depends, however, upon an over supply of this latter acid. Glycocoll is also of such constitution that it may be produced in the body by the oxidation of several different amino acids. Casein of milk is devoid of glycocoll, but rats fed upon casein thrive.

By an essential amino acid, then, is meant, one that the body cannot produce by oxidation (reduction) of another amino acid. The animal body cannot synthesize amino acids out of the elements of earth, air and water, but must receive these from the plant, which, alone, has the power to synthesize these substances. The animal body is capable only of producing some of the less complex amino acids out of the more complex ones by a reduction process.

Since the lower grade amino acids are formed within the body out of the higher compounds, they are regarded as of no vital importance. This, in my opinion, is a mistake. The body does seem to require them so that they are actually essential, even if it is not essential that they be taken in as such, but can be produced from other amino acids. On the other hand they can be produced from the higher amino acids only if these latter are present in excess of need. It may also be true that a saving of energy is secured if the lower grade amino acids are taken in with the food stuffs and the body is not compelled to reduce the higher compounds to lower grade. There is another theoretical possibility. The older theories of nutrition overlooked the universal validity of the Law of the Minimum to be explained in a later chapter. Investigators ignored the extent to which every tissue builder is dependent upon all the others. As Berg puts it: "They failed to realize that what is decisive for development, is not so much the absolute quantity of the various nutritive elements, as their relative proportions. They did not understand that the bodily need in respect of any one constituent of a diet can be determined only when we simultaneously take into account all the other factors of nutrition." There is the possibility that when one of the "non-essential" amino acids is lacking in the diet and the body is forced to make it from one of the essential amino acids, an actual reduction, below normal requirements, of the essential amino acid takes place with a corresponding lag in development.