584 METABOLISM 



addition of the ash of the lipoid extract failed to maintain the mice, so 

 that the lacking substance could not be inorganic in nature. 



More recent and extended observations, however, have shown that neutral 

 fat is also necessary for the adequate and continued growth of the 

 animal. For a period of two months or so an animal may, as we have 

 seen from Osborne and Mendel's experiments, grow in apparently nor- 

 mal fashion on an artificial fat- and lipoid-free diet composed of casein, 

 carbohydrate and inorganic salts, but sooner or later the great majority 

 of these animals begin to show failure of adequate growth. The in- 

 adequately growing animals often manifest indications of malnutrition 

 other than the failure to increase in weight; for example, inflammation 

 of eyes, roughening of the fur, etc. When certain fats are added to 

 the inadequate diet, normal growth is immediately resumed. Fats pro- 

 ducing this normal growth are such as butter fat, or the fat extracted 

 from egg yolk, or cod-liver oil, added to the extent of 5 per cent of the 

 ration. On the other hand, vegetable oils, such as olive oil or almond 

 oil, are inefficient in promoting growth. That all oils or fats do not 

 suffice to produce growth, and that one dose of an adequate oil or fat may 

 be sufficient to stimulate it, indicate that something other than the mere 

 presence of the comparatively simple fat molecule that is, some acces- 

 sory material must be the agency responsible for the growth. 



This conclusion is further supported by the interesting observation of 

 McCollum and Davis that vegetable oils can be rendered efficient for 

 growth by shaking them with a solution of soap prepared by com- 

 pletely saponifying butter fat with potassium hydroxide in the absence 

 of water. 



ACCESSORY FOOD FACTORS, VITAMINES 



In searching for the nature of the accessory food factors, the im- 

 portant observations which have been made in recent years concerning 

 the so-called vitamines must be considered. These are substances essential 

 in the diet for the proper maintenance of nutrition in adult animals. 



The existence of such substances was suggested by observations on 

 the disease beriberi, which is caused by exclusive feeding on polished 

 rice; that is, on rice from which the pericarp had been removed by the 

 process of polishing. When patients suffering from this disease were 

 given unpolished rice, the symptoms immediately disappeared. Further 

 investigation of the exact nature of these substances was greatly facil- 

 itated by the discovery that a similar condition is readily induced by 

 feeding fowls on polished rice. The birds develop a polyneuritis, from 

 which, however, they very promptly recover if some rice polishings or, 



