60 FEEDING EXPERIMENTS WITH ISOLATED FOOD-SUBSTANCES. 



condition, eating well and exhibiting favorable nitrogen balances, 

 until the end of q months, when a gradual decline was observed. 

 When the animal, at the end of a total feeding period of 335 days 

 (42 days after the onset of the decline) was reduced to 162.5 grams 

 in weight and near death, an attempt was made to see whether 

 the decline was due solely to improper food or to the onset of old 

 age or disease. With mixed food realimentation took place at once 

 and the rat regained its weight in a week. A resumption of the 

 former glutenin food during 35 days gradually led to a second decline, 

 which was promptly checked by a change in the diet involving only 

 the non-protein components of the food mixture. Here, then, is a 

 record of the feeding of a full-grown rat, with the exception of 7 days, 

 during a period of 454 days on a diet of isolated food-stuffs and on a 

 diet containing a single protein, glutenin, for 371 days. This obser- 

 vation is remarkable because of the exceptional duration of the 

 experiment. It is apparent, therefore, that as a maintenance diet our 

 food lacked something other than protein and energy. 



It remains to be shown precisely what the lacking component 

 of our earlier diets is, whether some organic constituent or a peculiar 

 proportion of inorganic ingredients. In any event it is evident that 

 our original artificial food mixtures are incapable of supporting life 

 indefinitely. Aside from this, however, records like that of rat 71 

 living on glutenin as the sole source of protein (see Chart XXX) , or 

 rat 133 (Chart LXX) on edestin, in contrast with rats xi, xiv, 146. 

 and 157 (Charts CXXVI, CXXVII, CXXVIII, and CXXIX) on 

 zein indicate the possibility of nutritive inequalities among the 

 proteins themselves. Marked deficiencies tend to manifest them- 

 selves in comparatively short periods of time. In all of these cases 

 the food actually consumed supplied sufficient energy for the imme- 

 diate needs of the rats under investigation. 



In the continuation of our experiments we have tried to profit 

 by the first year's experiences. The methods have not been materially 

 altered, except that the determination of the nitrogen balance has 

 been omitted for the present. We learned from very numerous trials 

 that it runs parallel with gain or loss of weight, and that the food 

 intake varies closely with the weight of the animal, thereby making a 

 record of the nitrogen unnecessary for judging the nutritive status of 

 the rats employed. The same cages as heretofore have continued to 

 prove very satisfactory. Instead of being rested on glass funnels for 

 the collection of urine, they are now placed over a frequently changed 

 sheet of absorbent paper (paper napkin) upon an enameled tray or 

 pan. The fluid excreta thus promptly absorbed are frequently 

 removed. It has already been pointed out that the food mixtures, 

 prepared in paste form to prevent scattering by the animals and 

 make it possible to obtain accurate records of the quantities eaten, 



