In preceding chapters an effort has been made to give what we actually know about what constitutes in chemical terms a satisfactory diet, and the type of experimental procedure by means of which this knowledge was gained. For the purpose of placing this knowledge in proper perspective and correlating it with the older views of problems in nutrition a brief account of certain earlier investigations and the deductions made therefrom were presented. With the object of broadening the vision of the present day investigators so as to enable them and their successors in this field to appreciate how carefully experimental work must be planned and controlled in order to avoid conclusions which tend to confuse rather than clarify, a critical examination of the technic of experimental methods has been included.

It will be appreciated how confusing was the task of interpreting the cause of failure or success with experimental diets, when it is recalled that a few years ago no definite conception was possible of the existence of at least three dietary factors which are indispensable in the food supply of man or certain animals. Fortunately for students in this field the rat requires but two of these, and this species therefore assumes special value, in gaining a satisfactory working hypothesis regarding the number of essential factors in its nutrition. Even with this species, however, in addition to the two unknown factors there were two variables, either of which might determine success or failure in the development of this species. These were the quality of the proteins, and the amounts of certain mineral elements contained in the food mixture. The solution of such a problem demanded first the accumulation of a considerable amount of data based upon the outcome of a series of experiments, each of which was not possible of interpretation except in the light of all the others. It must be admitted that success was actually in no small measure the result of active imagination and successful theorizing as to the true meaning of the effects of the experimental diets on the animals observed.

In order to understand how foods should be combined so as to make good each other's deficiencies, it is necessary to describe the dietary properties of each of the more important natural foods. This will form the subject of the present chapter.