The biological method for the analysis of a food-stuff was first developed with a view to discovering the nature of the deficiencies of individual natural food-stuffs. For this purpose the food under investigation is the principal component of the diet and is supplemented with small additions of one or more purified food substances (e.g., protein, inorganic salts, vitamins), in order to bring to light the nature of the additions which enhance its value. The method is applicable in another modification, however, which has yielded much valuable information concerning the relative values of many of our more important foods with respect to any one dietary constituent.

The last mentioned application of the method involves the conduct of feeding experiments in which a basal food mixture is employed which is entirely satisfactory as a source of nutriment for a growing animal except that it lacks entirely a single dietary essential. For example a mixture of purified protein, carbohydrate, an adequate salt mixture and a fat containing fat-soluble A, constitutes a diet which is complete for the rat with the exception of lack of the anti-neuritic substance, water-soluble B. If experiments are conducted with such a food, supplemented with minimal additions of some natural food whose value as a source of water-soluble B it is desired to estimate, it can be determined what is the smallest addition of the food under investigation which will furnish just sufficient of this vitamin to make possible the normal growth and satisfactory maintenance of the animals. This method was first employed by McCollum and Davis (16), who determined that wheat germ was approximately five to seven times as valuable as a source of water-soluble B as is whole wheat or other cereal. Their results indicated that the cereal grains are essentially on a parity with respect to their content of this substance, and are somewhat inferior to alfalfa leaves in this respect. In a similar manner, if the basal diet be made complete except for fat-soluble A, the method may be satisfactorily employed for comparing the content of this substance in a series of natural foods. To this end they are each fed with the basal ration, the amount added being adjusted in a series of experiments so as to find the least amount which completes the ration and induces normal nutrition.