Part 41. Food As A Fuel And Building Material

41. Food as fuel and building material. Before proceeding to compare vegetable with animal foods certain fundamental facts regarding food in general must be considered. We know that so long as a man is alive and active, the parts of his body are wearing out from daily use, and he is losing heat. If deprived of food, his weight and strength decrease, while, on the other hand, if he is properly fed, nutritive materials become incorporated with the various parts of his body as fast as these wear away, and he finds his strength kept up by a constant supply of energy. Were it possible to conceive of a steam-engine which could derive from the fuel it consumes not only heat and power but also material to replace that used up in action, we should have a machine to which we might liken the human body in its use of food. If we could imagine, furthermore, a small locomotive able to do all this, and also to increase the size of its parts by the addition of extra material, so as to grow into a large locomotive, such a marvelously endowed machine would be very like the body of a child. Thus we see that food answers the double purpose of supplying us with building material and with fuel. But as already intimated in the last chapter, proteids, fats, and carbohydrates are not equally useful as sources of substance and energy.

As the chief wear in our bodies comes upon the muscles and other parts that are composed largely of nitrogen, and as neither fats nor carbohydrates contain this element, it follows that proteids, being nitrogenous, must be of the first importance as furnishing building material. This enables us to understand why it is that an animal deprived entirely of proteids, although provided with abundant fats and carbohydrates will starve quite as truly as if it had no food whatever, whereas it may live indefinitely (although with danger to health) on a proteid diet ¹ from which all fats and carbohydrates 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 carbohydrates? 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 proportionate to the amount of carbon present, and largely independent 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.² 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.

¹ It 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.

Fig. 120.-Chart showing the chemical composition and fuel value (in Calories) of various foods.