A good illustration of the problems which the animal meets in its protein nutrition may be had by comparing the digestion products of the protein molecule to the letters of the alphabet. The proteins of the food and of the tissues may be regarded as made up of the same letters arranged in different orders and present in different proportions. In growth the animal takes as food, proteins which are very unlike those of its tissues, splits these into the simple compounds, the amino-acids, and then, after absorbing these, puts together the fragments in new order, and in new proportions to form the tissue proteins.

If the muscle tissue of an animal be likened to a block of printer's type so arranged as to print the rhyme beginning "Jack Spratt could eat no fat, and his wife could eat no lean," the proteins of which the muscle consists are represented by the individual words, and the protein digestion products by the letters of which the words consist. Now if the animal should take food proteins which correspond to a block of type which would print the jingle beginning "Peter Piper picked a peck of pickled peppers," it is easy to understand that when the proteins of the food are resolved into their constituent letters, and an effort made to form the body proteins of the new and different type from the letters supplied by the food, the transformation cannot be made. In setting up the first line, "Jack Spratt could eat no fat and his wife could eat no lean," we need four of the letter t, but the food proteins contain but one. The first line of the Jack Spratt rhyme, which represents the muscle proteins, requires but one letter p, whereas the food proteins expressed by the Peter Piper rhyme yield nine in the first line. The first line of the Jack Spratt rhyme contains the letters j and n, whereas the Peter Piper rhyme contains none, so that even with the entire stanza:

Peter Piper picked a peck of pickled peppers, If Peter Piper picked a peck of pickled peppers, Where's the peck of pickled peppers That Peter Piper picked?

It is not possible to reproduce even the first line of the Jack Spratt rhyme, and in order that growth might become possible it would be necessary to take proteins of another character which would supply the missing letters.

Such a comparison between food proteins and tissue proteins gives a good illustration of the kind of problem which the animal meets in its protein nutrition. The most conspicuous protein of the corn kernel, zein, is wholly lacking in three of the amino-acids or digestion products which are obtainable from most tissue proteins. In accord with what we should expect on theoretical grounds, this protein is, when taken as the sole source of amino-acids, not capable of supporting growth, or of maintaining an animal in body weight. This illustration shows how we may have superior, good or inferior food proteins for the formation of body proteins in growth.