MEASURES OF ENERGY 115 



of proteids, although provided with abundant fats and car- 

 bohydrates Avill starve quite as truly as if it had no food what- 

 ever, whereas it may live indefinitely (although with danger 

 to health) on a proteid diet ^ from which all fats and car- 

 bohydrates are excluded. 



Since proteids alone will support life, we must conclude 

 furthermore that they are also sources of energy, and the 

 question may be asked, What need have we of fats and car- 

 bohydrates? While it is indeed true that proteids may serve 

 as a source of energy, it has been found that the amount of 

 energy derivable from the food we eat is very nearly propor- 

 tionate to the amount of carbon present, and largely inde- 

 pendent of the amount of nitrogen. It is estimated that an 

 average man at moderate work needs daily less than ten 

 grams of nitrogen and about two hundred and eighty grams 

 of carbon; that is to say about twenty-eight times as much 

 of the latter as of the former." Since in proteids there is 

 only about three and a half times as much carbon as nitrogen, 

 it is clear that in order to obtain from them the necessary 

 amount of carbon, a man would have to consume about 

 eight times as much nitrogen as he had any use for. Not 

 only would this impose an unnecessary burden upon the 

 digestive organs, but so large an excess of nitrogen would be 

 harmful in other ways before it could be eliminated from the 

 system. Hence we must conclude that although proteids 

 are absolutely essential as building material, their inadequacy 

 as sources of energy requires that they be supplemented by 

 carbonaceous and non-nitrogenous food-stuffs. 



42. Measures of energy. As we have to depend for warmth and 

 strength mainly upon fats and carbohj^drates, it becomes important 

 to inquire how these compare with each other in fuel value, for as 

 already shown, these substances are to our bodies essentially as coal 

 to a steam-engine. It was stated in the last chapter that fats af- 

 ford more than twice as much energy as carbohydrates. Wc must 

 now trj' to understand more fully what this means and at the same 

 time secure a more exact expression of the relation thus vaguely 



> It is of course as.sumed that the rations inchide a sufficient quantity 

 of water and of salts. 



'^ Physiologists formerly estimated the daily need of nitrogen at twenty 

 grams, but recent experiments indicate that ten grams is amply sufficient. 



