CHEMISTRY OF DIGESTION AND NUTRITION. 291 



proteid, Voit has devoted numerous researches to the purpose of demon- 

 strating that this is the main source of body-fat. His belief is that in the 

 course of metabolism the proteid molecule undergoes a cleavage, with the for- 

 mation of a nitrogenous and a non-nitrogenous part. The former, after further 

 changes, is eliminated in the form of urea, etc. ; the latter may be converted 

 into fat, or possibly into glycogen. The theoretical maximum of fat which 

 can arise in this way is 51.5 per cent, of the entire amount of proteid. Voit 

 attempted to demonstrate this theory by actual experiments. He showed that 

 dogs fed upon large amounts of lean meat did not give off as much carbon in 

 the excreta as they received in the food. The excess of carbon must have been 

 retained in the body, and, in all probability, in the form of fat. As corrob- 

 orative evidence he cites the apparently direct conversion of proteid material 

 into fat in such cases as the formation of fat-droplets in the fat-cells or cells 

 of the mammary glands, and in muscle-fibres and liver-cells undergoing fatty 

 degeneration ; but evidence of this latter character is not conclusive, since we 

 have no immediate proof that the fat arises directly from the proteid material 

 in the cells. Voit's experimental evidence has been questioned recently by 

 Pfliiger, his criticisms being directed mainly toward the calculations involved 

 in Voit's experiments. The result of this criticism has been to make us more 

 cautious in attributing the origin of body-fat solely or mainly to proteids, but 

 as regards the possibility of some proteid being converted into fat in the body 

 there can be no reasonable doubt. It has been proved (p. 268) that glycogen 

 may be formed from proteid, and since it is now generally accepted that fats 

 are formed from carbohydrates, the possibility of an indirect production of fats 

 from proteids seems to follow necessarily. 



The connection between the carbohydrates of the food and the fat of the 

 body has been a subject of discussion and investigation among physiologists 

 for a number of years. It was the original belief of Liebig that carbohydrates 

 are the source of body-fat. This view was afterward abandoned under the 

 influence of the work of Pettenkofer and Voit, but renewed investigations seem 

 to have re-established it upon solid experimental grounds. In some older 

 experiments of Lawes and Gilbert it was shown that the fat laid on by a young 

 pig during a certain period was greater than could be accounted for by the 

 total fat in the food during that period, plus the theoretical maximum obtain- 

 able from the proteid fed during the same time. Of more recent experiments 

 demonstrating the same point, a single example may be quoted from Rubner, 1 

 as follows: A small dog, weighing 6.2 kilograms, was fed richly with meat for 

 two days and was then starved for two days ; its weight at the end of this time 

 was 5.89 kilograms. The animal was then given for two days a diet of cane- 

 sugar 100 grams, starch 85 grams, and fat 4.7 grams. It was kept in a respira- 

 tion apparatus and its total excretion of nitrogen and carbon was determined : 



Total C excretion 87.10 grams C. 



176.6 " " 



89.5 " " 



Zeitschri/tfiir Biohgie, 1886, vol. 22, p. 272. 



C ingesta l? 6 -6 



89.5 " " retained in the body. 



