NATURE AND DESTINATION OF FOOD. S9 



It is not to be supposed, however, that any table of this kind, founded simply 

 upon the Chemical composition of the various substances, can indicate their 

 respective fitness as articles of diet; since this depends also upon the facility 

 with which they are reduced by the digestive process, and afterwards assimilated. 

 Thus an aliment, abounding in nutritive matter, may be inferior to one which 

 really contains a much smaller proportion, if only a part in the first case, and 

 the whole in the second, be readily taken up by the system. The calorific 

 powers of the substances above enumerated, however, are not precisely in the 

 inverse ratio to their histogenetic value ; for, as the amount of heat given off in 

 their combustion depends, not simply upon the amount of carbon and hydrogen 

 they may contain, but upon the amount of their hydro-carbon over and above 

 that which is already combined with oxygen, substances that are alike deficient 

 in nitrogen may differ considerably in this respect. Thus in ordinary fat, the 

 proportion of oxygen is only about 10 per cent., whilst that of hydro-carbon is 

 at least 90 per cent.; in alcohol, the proportion of oxygen is nearly 35 per cent, 

 to 65 per cent, of hydro-carbon; in starch, the oxygen is 49 per cent., the 

 hydro-carbon 50f per cent. ; in cane-sugar, the oxygen is 51 ?, the hydro-carbon 

 48 i ; and in grape sugar, the oxygen is 53 i, the hydro-carbon 46f . According 

 to the estimate of Prof. Liebig/ the following are the relative calorific powers 

 of these substances, the numbers expressing approximately the weights of each 

 which must be taken in as food, in order, with the same consumption of oxygen 

 to keep the body at its proper temperature during equal times; fat, 100; starch, 

 240 ; cane-sugar, 249 ; grape-sugar, 263 ; spirits (containing 50 per cent, of ab- 

 solute alcohol), 266. The equivalent of lean flesh required to produce the same 

 calorific effect with the foregoing, would be no less than 770. 



402. It is obvious that the most economical diet will be that in which there 

 is the most perfect apportionment of each class of constituents to the wants of 

 the system; and these, on the principles formerly explained ( 374-6), will 

 vary with the amount of muscular exertion put forth, and the lowering of the 

 external temperature. Thus, for a man of ordinary habits, and living under a 

 medium temperature, a diet composed of animal flesh alone is the least econo- 

 mical that can be conceived; for, since the greatest demand for food in his sys- 

 tem is created by the necessity for a supply of carbon and hydrogen to support 

 his respiration, this want may be most advantageously fulfilled by the employ- 

 ment of a certain quantity of non-azotized food, in which these ingredients pre- 

 dominate. Thus it has been calculated, that, since fifteen pounds of flesh con- 

 tain no more carbon than four pounds of starch, a savage with one carcass and 

 an equal weight of starch, could support life for the same length of time, during 

 which another restricted to animal food would require five such carcasses, in 

 order to procure the carbon necessary for respiration. Hence we see the im- 

 mense advantage as to economy of food, which a fixed agricultural population 

 possesses over those wandering tribes of hunters, which still people a large part 

 both of the old and new continents. The mixture of the azotized and non- 

 azotized compounds (gluten and starch), that exists in wheat flour, seems to be 

 just that which is most generally useful to Man; and hence we see the explana- 

 tion of the fact that, from very early ages, bread has been regarded as the " staff 

 of life." There are particular conditions of existence, however, under which 

 life may be advantageously supported upon animal food alone. Thus the Gua- 

 chos of South America, who pass the whole day in the saddle, and lead a life of 

 constant activity resembling that of a carnivorous animal, scarcely ever taste 

 anything but beef; and of this their consumption is by no means great; for the 

 temperature of the surrounding atmosphere is so high, that the body has no 

 occasion to generate more heat than is supplied by the combustion of the hydro- 



1 "Familiar Letters on Chemistry," 3d edit., p. 380. 



