The results of Slonaker's experiments were published in 1912, and just at the time when McCollum and Davis were securing the experimental data which revealed the differences in the growth-promoting power of fats from different sources, and which established the existence of a new and hitherto unsuspected dietary essential. They fed rats on relatively pure food-stuffs as described in Chapter II (A Biological Method For The Analysis Of A Foodstuff. 17. Mccollum'S Experiments Not Verified By Osborne And Mendel) with various fats of both animal and vegetable origin, and found that no fat which was derived from plant tissues could compare in growth-promoting properties with butter fat, egg yolk fats and the fats from the glandular organs. It seemed to them that the most probable explanation of the results of Slonaker was the absence or shortage in his vegetarian diet of the dietary essential furnished so abundantly by these animal fats, and which later came to be designated as fat-soluble A. A low protein intake was possibly another cause for Slonaker's failure. With this idea in mind they tried during the summer of 1914, an experiment similar to that of Slonaker, but so modified as to give the animals a much higher protein intake than his animals probably enjoyed. It seemed that if Slonaker's rats ate liberally of such leaves as cabbage and other leafy vegetables and of tubers and roots the protein content of which in the fresh condition does not as a rule exceed 2 per cent, the protein in the other components of the diet might not have been high enough to give the entire ration consumed a content in their food sufficiently high to promote growth at the optimum rate.

McCollum and Davis, therefore, fed their rats a diet which afforded a choice among the following list of foods: wheat, maize, rye, oat (rolled), wheat germ, maize gluten, wheat gluten, flaxseed oil meal, green clover, green alfalfa leaves, onions, peanuts, and cooked navy beans and peas. With the exception of the last two articles the foods were fed raw. It will be observed that in this list there are several vegetable foods having unusually high protein contents. Maize gluten, a by-product of starch manufacture, contains about 25 per cent; wheat gluten, prepared by washing ground wheat free from starch, about 85 per cent; flaxseed oil meal, 30 per cent and wheat germ 30-35 per cent. Since animals are known to grow well on many diets containing 15 to 18 per cent. of protein, it seemed that with this variety to select from one possible cause of failure in Slonaker's experiments, too low a protein intake, would be avoided. McCollum and Davis had at that time not discovered that the leaf of the plant is a moderately good source of the dietary essential fat-soluble A, although it was known that the leafy foods enable herbivorous animals to thrive on diets derived entirely from plant sources. It was then assumed that when both the leaves and so many different kinds of seeds and seed products were supplied, there could be little doubt that everything a herbivorous animal requires was present in the foods.

The rats fed this wide variety of vegetable foods, and supplied with liberal amounts of protein, duplicated in all respects the results described by Slonaker. They grew at about half the normal rate for the first few weeks, then became permanently stunted, none ever reaching a size much greater than half that of the average normal adult. The addition of butter fat to the diet of some of these animals failed to benefit them in any noticeable degree. The answer to the question as to why rats did not thrive on such strictly vegetarian food mixtures was not secured from these experiments. It was, however, soon after learned wherein lay the cause of failure.