CH. XXXIX.] 



FAT METABOLISM 



591 



too much nitrogen or too little carbon. The principle that underlies Banting's 

 method of treating obesity is to give meat almost exclusively : the individual then 

 derives the additional supply of carbon necessary for combustion from his own 

 adipose tissue. We have already seen that this may be and often is counteracted 

 by the laying on of fat which comes from the non-nitrogenous moiety of the proteid. 



Feeding with fat. If an animal receives fat only, the nitrogenous excreta are 

 derived from the disintegration of tissue without any corresponding supply of 

 nitrogen being supplied in exchange in the food. When fat only is given, or a large 

 excess of fat exists in the food, the respiratory quotient falls. F. Hofmann fed a 

 dog on a mixture of a large amount af fat and a small amount of proteid. After 

 death the quantity of fat found in the body was such that only a small part could 

 have been derived from the proteid, the greater amount being directly derived from 

 the fat of the food. The animal, moreover, lays on fat in which palmitin, stearin, 

 and olein are mixed in a definite proportion ; this proportion is often different in the 

 fat of the food. In addition to this an animal will fatten (laying on fat with its 

 usual composition) on fatty food, such as spermaceti, which contains no glycerides. 



Feeding with carbohydrates. The respiratory quotient approaches unity when 

 carbohydrates alone are taken. So far as regards nitrogen the animal is in a state 

 of inanition, as when fat alone is taken. If given in combination with other foods, 

 both carbohydrates and fat act as proteid-sparing foods. 



The following table is from Pettenkofer and Voit, and illustrates what happens 

 in a dog on a mixed diet of flesh and carbohydrates. 



Even when the diet consists wholly of carbohydrates, fat is laid on ; the fat laid 

 on when meat and starch are both present in the food comes partly from the proteid 

 and partly from the carbohydrate of the food. When no carbohydrate is given at 

 all, as in the last experiment, the nitrogenous metabolism is raised. Carbohydrate 

 food is thus when given with other foods both fat-sparing and proteid-sparing. The 

 formation of fat from carbohydrates was first observed in pigs by Lawes and Gilbert, 

 and has since been confirmed by numerous investigators. 



One of the most important instances of the carbohydrate origin of fat is the 

 formation of bees'-wax. 



Instances of the formation of fat from proteids are (1) the laying on of fat in 

 carnivorous animals ; (2) the formation of adipocere, a wax-like material which 

 forms in the muscles of corpses buried in damp soil, or allowed to remain in water ; 

 (3) the gradually increasing quantity of fat in old cheeses. 



