MEASL'RES OF ENERGY 115 



of proteids, although provided witli al>undant fats and car- 

 bohydrates will starve quite as trulj' as if it liad 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- 

 bohj'drates? While it is indeed true that proteids may serve 

 as a source of energ^^ it has been found that the amount of 

 energy derivable from the food we eat is very nearlj' 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 eightj' 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 unnecessarj' burden upon the 

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

 harmful in other waj's before it could be eliminated from the 

 sj'stem. 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 ha^-e to depend for warmth and 

 strengtli mainly upon fats and carbohydrates, it becomes important 

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

 alreachr 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 carJDohydrates. We must 

 now try to undei'stand more fully what this means and at the same 

 time secure a more exact expression of the relation thus vaguely 



' It is of course assumed that the i-ations include a sufficient quantity 

 of water and of salts. 



2 Physiologists formerly estimated the daily need of nitrogen at twenty 

 grams, but recent e.xperiments indicate that ten grams is amply sufficient. 



