MEASURES OF ENERGY 115 
of proteids, although provided with abundant fats and ear- 
bohydrates will 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 nadequacy 
as sources of energy requires that they be supplemented by 
carbonaceous and non-nitrogenous food-stufts. 
42. Measures of energy. As we have to depend for warmth and 
strength mainly upon fats and carbohydrates, 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. We must 
now try to understand more fully what this means and at the same 
time secure a more exact expression of the relation thus vaguely 
1 Tt is of course assumed that the rations include a sufficient quantity 
of water and of salts. 
2 Physiologists formerly estimated the daily need of nitrogen at twenty 
grams, but recent experiments indicate that ten grams is amply sufficient. 
