510 THE HUMAN BODY 



Bodies than absolutely necessary, the amount of protein is cut 

 down. The very large protein intake of Eskimos probably serves 

 to insure for them a heat production adequate to the extreme 

 climate in which they live. 



The Nutritive Value of Albuminoids. These proteins, as stated 

 above (p. 503), lack some of the essential constituents of cell pro- 

 teins, and cannot, therefore, serve as tissue-restorers. We can 

 imagine, however, that they ought to satisfy the Body's demand 

 for protein fuel, and so be substituted for the major part of the 

 protein of the diet. Various attempts have been made to substitute 

 gelatin for proteins in this way, and it seems to be highly efficacious 

 in satisfying the Body's protein-fuel demand. But curiously 

 gelatin can be used thus for only a few meals; presently there 

 is a revolt of the appetite against it and no more can be eaten. 

 Experiments have shown that dogs will starve rather than take 

 continuously a diet whose chief constituent is gelatin. 



The Special Metabolism of Fats. Fats are very useful fuel 

 foods. Their energy content is twice that of the other nutrients. 

 As we saw in an early chapter (p. 106) there is no present reason 

 to suppose that they have to be changed to sugar before they can 

 be used as sources of muscular energy. There is, however, a 

 feature of their metabolism which negatives their consumption 

 in large excess. In the process of oxidation of fats there is a stage 

 in which certain organic acids are formed. These, if produced in 

 amounts so large that the alkalies of the Body cannot neutralize 

 them successfully, bring about a condition known as acidosis, 

 which is harmful and, when pronounced, fatal. The acid forma- 

 tion is kept in check if there is an accompanying metabolism of 

 carbohydrates. Acidosis is not so likely to occur on a mixed diet, 

 therefore, as on one in which fat is the chief item. 



A practical difficulty that arises in prescribing a diet in diabetes 

 (p. 496) is due to this feature of fat metabolism. The diabetic, 

 as we have seen, cannot utilize carbohydrates. To feed him upon 

 a carbohydrate diet is, therefore, not only wasteful but positively 

 harmful, since it involves the constant presence in his body fluids 

 of injurious quantities of sugar. The same difficulty inheres, al- 

 though in less degree, in a diet of protein, since the fuel residue 

 of this substance is, as we have noted (p. 504) essentially carbo- 

 hydrate. The most feasible source of energy to the diabetic is, 



