McCollum (20) attempted to utilize Folin's discovery as a means of studying the problem whether an amino-acid supply which is inadequate for the support of growth, would be satisfactory for maintenance. A farm pig was given a nitrogen free diet of starch, salts and water until the exogenous metabolism disappeared, as shown by the composition of the urine. In a subsequent period a definite amount of an incomplete protein such as gelatin or zein was to be added to the experimental diet of carbohydrate, salts and water. From the increase in the nitrogen output in the urine and from the change in the forms in which it appeared, it was believed to be possible to decide to what extent, if at all, the amino-acids of the incomplete protein were utilized.

It was found that young swine, which were used as experimental animals, readily ate a sufficient amount of the nitrogen free diet to cover their energy requirements, and could easily be brought into the condition, in which they exhibited only the endogenous type of metabolism. When to pigs in this condition, a known amount of nitrogen in the form of urea, an end product of metabolism, was added to the diet it was all recovered in the urine in addition to the amount derived from endogenous sources. The percentage composition of the urine with respect to creatinin and urea showed the expected alterations the former representing less and the latter more of the total nitrogen of the urine.

When, instead of urea, gelatin or zein was administered in quantities sufficient to supply exactly the amount of nitrogen derived from endogenous metabolism, the nitrogen from these "incomplete" proteins was only in part excreted in the urine in addition to the amount derived from endogenous sources. The results showed that 60 per cent of the nitrogen of gelatin was excreted and 40 per cent retained for some purpose in the tissues, presumably for the partial repair of tissue waste. Under the same experimental conditions 73 per cent of the nitrogen of zein was retained and 27 per cent excreted in addition to the amount expected from endogenous sources.