920 NUTRITION AND HEAT REGULATION. 



experiments have satisfied physiologists that the tissues can pro- 

 duce fat from sugar. The chemistry of the change is not under- 

 stood and cannot be imitated in the laboratory, but it is evident 

 that in the long run it involves a series of important reductions, 

 since in the end the oxygen-rich sugar is transformed to an oxygen- 

 poor fat. In the sugar the oxygen constitutes 53 per cent, of the 

 molecule, while in fat it forms only 11.5 per cent. The oxygen split 

 off in the series of changes should appear as H2O and CO2, and in 

 accordance with this view it is found experimentally that when 

 carbohydrate is being stored as fat there is an increase in the respi- 



CO 



ratory quotient, that is, in the ratio -z^. 



The Source of Body Fat in Ordinary Diets. — For the pur- 

 poses of demonstration the experiments made to prove the origin 

 of body fat from carbohydrate or the fat of food have made use 

 of abnormal diets and conditions. It would be a matter of practical 

 interest to ascertain whether upon normal diets the fat of the 

 body arises more easily from the fat or from the carbohydrate of 

 the food. While the question is one to which a positive answer 

 cannot be given, it seems to be probable that the result varies with 

 conditions and the nature of the animal. Experience seems to 

 show that carnivorous animals can be fattened more easily on a 

 fat diet, herbivora on a carbohydrate diet. In animals, like our- 

 selves, there is reason to believe that the carbohydrates are more 

 easily and more quickly destroyed in the body than the fats, and 

 that, therefore, the latter may be more readily deposited in the tis- 

 sues, although an excess of carbohydrate beyond the actual needs 

 of the bodv will also be preserved in the form of fat or glycogen.* 



The Cause of the Deposit of Body Fat — Obesity. — From a 

 nutritive standpoint fat constitutes a storage material which can 

 be called upon as a source of food-energy during starvation or when 

 the diet is inadequate. It is to be assumed, therefore, that body-fat 

 is formed and deposited only when food is ingested in excess of the 

 actual expenditure of energy by the tissues. 



Our experience shows that individuals vary greatly in the 

 amount of body-fat formed under the ordinary conditions of living. 

 The question arises whether a tendency to lay on fat is an indica- 

 tion of a marked excess in eating, or whether in such cases the 

 metabolizing capacity of the tissues is less than usual, so that even 

 a moderate diet furnishes an excess of material which the adipose 

 cells store as fat. It is certainly a general belief that on one and 

 the same diet some individuals lay on fat and others do not. Our 

 knowledge is not sufficient to furnish wholly satisfactory answers 



* Consult Rosenfeld, "Ergebnisse der Physiologie," vol. i., part i, 1902. 

 Complete literature. 



