NUTRITION. 375 



taining only 18 per cent of protein. Whether this is due to the failure of the 

 adult animal to construct fat, or to an actual interference with tissue growth 

 as the result of the unique diets used, has not been determined. It is worthy of 

 note, however, that when the larger animals which have been brought to a 

 body-weight of 250 grams, or thereabouts, on these high-protein diets are 

 given food containing carbohydrate and fat and only 18 per cent of protein, 

 as a rule they make considerable additional gains. 



When the protein in the diet does not exceed 80 per cent of the food mixture, 

 and is supplemented by small amounts of starch or fat in addition to the in- 

 organic salts and vitamine-bearing components, growth as a rule has extended 

 to larger size than was the case in the first series of animals referred to. 

 Wherein the superiority of this lower concentration of protein lies, if it proves 

 to be a real one, remains to be ascertained. In any event, it is surprising to 

 find how readily the rat can use such extremely large quantities of protein 

 for growth. It is also interesting to note that the amount of these high 

 protein foods eaten by the rats is no greater than of those of equal calorific 

 value containing a liberal proportion of carbohydrate and very much less 

 protein, thus showing that protein meets the energy requirements of the 

 rat almost, if not quite, as well as does carbohydrate. 



Another extreme type of diet recently studied involves the feeding of a 

 ration containing at least 75 per cent of fat and devoid of more than insig- 

 nificant amounts of preformed carbohydrate. On such mixtures some ani- 

 mals have grown to 280 grams, although the rate has usually been somewhat 

 slower than the normal. This represents about the upper limit of possible 

 fat-content in the diet of growing animals; for the food intake seems to be 

 determined, when the food mixture is qualitatively satisfactory, by their 

 calorific needs. On such diets, relatively very rich in energy, the total weight 

 of food eaten is so small that the absolute intake of protein is too low to permit 

 a normal rate of growth. Thus, with as much as 80 per cent of fat in the 

 diet and a protein-content not exceeding 15 per cent, animals do not grow, 

 but are maintained for long periods at constant weight until the fat-content 

 of the diet is materially reduced. Apparently the failure to grow in such 

 cases is not due to any toxic effects of the large intake of fat, but rather to the 

 enforced reduction of the other essential factors of the diet. Our experiments 

 with diets having such unusual relative proportions of the nutrients have a 

 bearing upon the current discussions of the balance of ketogenic and anti- 

 ketogenic products in intermediary metabolism. 



Several years ago we showed that after rats have been maintained without 

 growth for very long periods — in some cases beyond the age of 500 days — 

 they have not lost the capacity to grow, even though they have long passed 

 the time at which adult life is normally reached. It was concluded that 

 the capacity to grow is not lost until it has been exercised, a conclusion some- 

 what at variance with some of the then current theories. At that time, 

 however, our experiments were stopped before the realimented animals had 

 reached full adult size. 



Since it is believed that malnourished children do not develop into such 

 large men and women as do the well nourished, it has been suggested that 

 had these experiments been continued it would have been found that the 

 stunted animals would not have grown to be as large as the average normal 



