CARBOHYDRATES AND FATS. 929 



to these questions, but our present position in regard to them may 

 be indicated briefly. 



By observing the general principle that an excess of food over 

 the energy needs will be converted into body-fat we can change 

 at will the amount of this fat through alterations in the diet and 

 voluntary variations in the extent of the energy expenditure. Indi- 

 viduals whose daily occupation requires much muscular work, 

 farmers, laborers, etc., rarely, as Voit says, show any tendency to 

 adiposity. Their energy requirement is higher than the average, 

 and even an abundant diet furnishes little or no excess. In per- 

 sons whose mode of living is less active, excessive fat formation is 

 quite common. In such persons the amount of fat may be reduced 

 by cutting down the diet and by increasing the body metabolism 

 through muscular exercise. The usual procedure is to select a 

 diet containing from one-half to three-fifths of the calories 

 of the usual diet and to see that the reduction is made chiefly 

 in the fats and carbohydrates which are the fat-forming con- 

 stituents of the food. On such a regimen there is insufficient 

 energy for the body needs and the storage fat is called upon to 

 make up the deficiency. By consulting the tables giving the com- 

 position of the various common foods it is easy to construct such 

 dietaries and to vary them to suit the taste of the individual con- 

 cerned. On the other hand, it is possible, by reversing this pro- 

 cedure, to fatten up a thin person. Muscular and mental rest 

 combined with an abundant diet containing much fat and carbo- 

 hydrate in an easily assimilable form suffices to bring about an 

 excess of food material in the body which the tissues change over 

 into fat. 



These facts are true in general and can be demonstrated on man 

 and the domestic animals. We can reduce fat by muscular exer- 

 cise and partial starvation and we can increase body-fat by mus- 

 cular repose and stuffing. 



Nevertheless it seems to be true that individuals vary greatly 

 in the response that they make to such treatment. We must recog- 

 nize that there are other factors concerned about which we know 

 very little. As suggested above, it is possible that the energy re- 

 quirements of the body at rest may vary so that a diet which in 

 one person would just cover these requirements might in another 

 provide a considerable excess. And the level of these energy re- 

 quirements or energy expenditures may change, in the same person, 

 under different conditions of health or disease. Some experimental 

 investigations have been made upon this point. Calorimetric 

 studies on thin and fat people of the basal metabolism, that is to 

 say, the heat production of the resting individual eighteen hours 

 after a meal, show that there is substantially no difference between 

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