THE ABSORPTION AND USE OF FOODS 513 



taking sufficient nourishment to insure a supply of protein for 

 tissue building. 



Carbon Equilibrium. For an animal to be in carbon equilibrium 

 only needs that all the fuel taken in be burned, and that no reserve 

 store be called upon. Aside from the temporary storage of carbo- 

 hydrate food as glycogen all the fuel taken into the Body must 

 look forward to one of two fates, either to be oxidized promptly or 

 to be stored in the form of fat for future use. Just as nitrogen 

 equilibrium may be established on a high or a low level so carbon 

 equilibrium can be maintained in the face of variations in the in- 

 take of fuel. It is easily seen, however, that the limits of carbon 

 equilibrium must be narrower than of nitrogen equilibrium. The 

 actual protein requirement of the Body is so much less than the 

 usual protein intake that considerable variations in the protein 

 consumed can be made without affecting the nitrogen equilibrium; 

 but the energy requirement of the Body is quite definite, varying 

 with the work done rather than with the food eaten. Thus it 

 follows that the fuel intake and the energy requirement are harder 

 to keep balanced than are the nitrogen intake and outgo. It may 

 easily be a matter of astonishment how successfully the Body, un- 

 der the guidance of the appetite, manages to make its fuel con- 

 sumption balance its fuel need. 



There is a difference of opinion among Physiologists as to 

 whether every accidental excess consumption of fuel results in 

 the normal individual in the deposition of the surplus in the form 

 of fat, or whether the Body has the power to carry on oxidations 

 in excess of the normal basal metabolism and of the amount of 

 muscular exercise. 



Such positive information as we have on this point (see next 

 paragraph) is based on observations on abnormal individuals and 

 cannot be taken as necessarily applying to persons in normal 

 health. 



The Influence of the Thyroid Hormone upon Metabolism. 

 Whether excess fuel shall be stored as fat or be burned, has been 

 shown to depend, to a large extent, at least, on the amount 

 of the thyroid hormone that is produced. When the hor- 

 mone is abundant the bodily oxidations are so vigorous that 

 no surplus of fuel remains to be converted into fat. An 

 inactive thyroid gland, on the other hand, signifies a likelihood 



