Proteins are made up of amino acids. Some of these amino acids are indispensable, others may be made from the essential amino acids. No two proteins have the same amino acid content. Some of them are very deficient in one or more of the essential amino acids. Either the amino acid is entirely absent or it is present in such minute quantity that one would be forced to consume enormous quantities of the protein to secure an adequate supply of the deficient amino acid. Proteins lacking in an essential amino acid are inadequate proteins. According to their adequacy, individual proteins are grouped as:

1. Complete: Those maintaining life and providing for normal growth of the young and reproduction in the adult when fed as the sole protein food. Examples of complete proteins are excelsin of the Brazil nut, glycinin, of the soy-bean, casein and lactalbumen of milk, ovalbumen and ovovitallin of eggs, edestin, glutenin and maize glutellin of cereals. Rose showed that the proteins most suitable for maintaining growth in dogs are lactalbumen (milk), ovalbumen and ovavitellin (eggs); that next in order of suitableness are glutenin (wheat), casein (milk), glutelin (corn) and glycinin (soy bean). Gliadin (wheat and rye) and legumin (peas) are capable of maintaining nitrogen balance, but not growth. Zein and gelatin can do neither.

2. Partially Complete: Those maintaining life but not supporting normal growth. Examples of these are gliadin, of wheat, hordein of barley and prolamin of rye, legumin of peas, legumenin in the soy bean, conglutin, in blue and yellow lupin, phaseolin in the white kidney bean, legumin and vignin in vetch.

3. Incomplete: Those incapable either of maintaining life or of supporting growth. Gelatin from horn and other hard parts of the animal is the most conspicuous example of an incomplete protein, Zein of corn (maize) is another example of this class.

Let us take a look at an incomplete protein. With zein as the sole source of amino acids, growth is impossible. In fact, experimental animals fed zein as their sole protein, lose weight. If tryptophan is added to the zein, weight is maintained but growth does not occur. Only after both lysine and tryptophan are added can normal growth take place. Zein is deficient in tryptophan, glycine, lysine and glycocoll.

Gliadin, found abundantly in wheat and rye, lacks sufficient lusine to maintain growth. Gelatin lacks tyrosin and tryptophan. Unless these are supplied to the animal fed on gelatin as its sole source of protein, it soon dies.

Thus it may be seen that since the nutritive value of proteins is determined by the kinds and quantities of amino acids they contain, all proteins are not of equal value to the body and cannot be used interchangeably. The nutritive value of foods cannot be determined by reference to a table of food composition. This fallacy was exposed by Prof. Huxley many years ago. Sophie Leppel followed him in protesting against the belief that tables of food analysis give reliable indexes to food values. All the fuss made about the need for 118 grams of protein daily, without specifying the kinds of proteins, does not amount to much.

While I have emphasized the fact that the various proteins are not interchangeable, it is necessary to distinguish between the various isolated proteins and the common protein foods. All protein foods contain two or more proteins. The deficiencies of one protein of a food are often made up by the other protein of the same food. For example, tryptophan may be lacking in one protein and one of the other proteins in the same food may be rich in this amino acid. Returning to zein of corn, which, as we have seen, will not maintain life; it is supplemented by glutelin, of which the corn possesses almost an equal amount, and these two proteins are capable of supporting a normal rate of growth. Gliadin of wheat and rye lack sufficient lysine to maintain growth. But wheat contains other proteins which supply liberal portions of this amino acid.

We do not eat isolated individual proteins and do not depend upon but one such protein as our source of amino acids. On the contrary, we eat whole foods which contain two or several proteins. We also eat several foods, all of which contain proteins. Just as one protein in a food may supplement another protein in the same food, so the protein of one food may supplement the protein of another food. Two inadequate proteins may prove adequate when supplied to the same individual. This can be so, of course, only when they are not both inadequate in the same amino acids. If each is abundant in what is lacking in the other, the combined proteins will prove adequate. The sum total of the various proteins in the diet, if the diet is varied, will prove fully adequate.

It is customary to use young rats in testing the value of the various proteins. It is obvious to everyone that young rats never attempt to live on isolated and single proteins. They eat the whole food and eat different kinds of protein foods so that they receive all of the needed amino acids. Most of the experiments with the different vegetable and grain proteins have been made with denatured proteins and may not prove all that they are supposed to prove. They have been performed with isolated, individual proteins and Hindhede aptly says of these substances that, far from being remarkable that these isolated proteins have so little value, "it is remarkable that such substances, isolated by complicated chemical processes, have any value at all."

It may be ideal for experimental purposes, in testing the value of the different proteins, to use only single isolated proteins, but it is a far cry from this experimental condition to the eating practices of man and animals. It is not only true that the diets of both man and animals commonly contain more than one kind of protein food, but it is also true that all protein foods contain two or more proteins. If only a single protein food were consumed, the diet would contain more than one protein. Note the different proteins in corn, wheat, milk and eggs. It frequently happens that the protein in one food is abundant in the amino acids in which the protein in another food is deficient. Thus the two proteins supplement each other so that, together, they constitute a complete protein. Often the deficiency in a protein is so small that a very slight addition of the deficient amino acids from another source suffices to support normal growth and maintenance. All proteins are, therefore, capable of supplying the body with important nutritive substances. The mere fact that a protein is inadequate is not sufficient reason for rejecting it completely.

It is true that some mixtures of protein foods have been shown to be inferior, even, to certain single articles of protein. This is especially true of the grains as compared to milk. Some of the cereal proteins are adequate, but only so when fed in large amounts. Glutenin from wheat may be made to supply a sufficiency of amino acids in which it is deficient only by separating this protein from the wheat and feeding it in concentrated form and in amounts one could not secure by eating wheat. Edistin of hemp is another example of this kind. In small quantities it does not supply sufficient lusine. The same thing is true of the casein of milk. It is low in cystine, hence in small quantities, does not supply sufficient of this amino acid. Thus it becomes apparent that some complete proteins may prove to be partially incomplete when fed in reduced amounts.

A mixture of grains will not suffice to maintain growth and repair. Rye and barley are about the only grains that are adequate for the adult body. Even a mixture of as many as ten varieties of grains does not provide adequate protein for growth due to the fact that all of them are poor in lysin and cystin and most of them contain too little tryptophan.

In regular practice we do not consume casein as our sole source of protein, nor do we live upon an exclusive grain diet. We regularly consume many other protein-containing foods. Hygienists, on the other hand, have long contended that grains form no normal part of man's diet and have long considered them to be inferior foods. Dr. Densmore was the first to point out the inferiority of grains as an article of human consumption. We are not surprised that the experimenters have fully verified most of his contentions. More of this in a later chapter.