ENERGY, relative values of Food as sources of

Chemists and physiologists, although they agree that muscular power is derived from the action of the oxygen supplied during respiration upon the digested portions of the food, differ in their conclusions as to whether the nitrogenous or non-nitrogenous principles of the food, form the chief source of this power or not. The opinion of Liebig, Play fair, Ranke, and others, that the oxidation and metamorphosis of the nitrogenous ti ue is the fountain of muscular force has of late years been contested, and on the opposite view adduced, viz. that it is principally to the oxidation of the carbonaceous or non-nitrogenous constituents of the food, that animal dynamic power is due. This latter view has received support from the experiments of Frank land, Lawes, and Gilbert (from their observations on the feeding of cattle), Edward Smith, Meyer, Pettenkofer, Voit, Wislicenus, Fick, Parkes, and others. The data upon which it is based are those derived from the observation of the amount of heat generated by the combustion of a definite quantity of food out of the body; which, it is affirmed with certain deductions, represents the quantity of heat evolved by the oxidation of the same food within the body; and as heat is the equivalent of muscular force or energy, that aliment which, in burning, gives off the most heat, must, it is supposed, nece arily be the richest in the production of animal motive power. Of course these conditions will, amongst others, be very considerably modified by the extent to which the proce es of the animal economy, such as digestion, a imilation, ., can liberate the elements of the food so as to become available as sources of this energy. Were these proce es perfect, all the carbon of the carbonaceous, as well as that of the nitrogenous constituents of the diet, after deducting the carbon which pa es off as urea (one part of dry nitrogenous matter yielding about a third of its weight of urea) would be utilised and converted into heat-producing power. But even under these circumstances a considerable portion of this thermotic power would be expended in sustaining the internal movements of the body, such as respiration and the heart’s action, which it has been computed are daily maintained by a force capable of raising 600,000 pounds a foot high. No wonder if, with such varying factors introduced into the problem, physiologists and physicists should differ so widely in their calculations; and that, whilst one inquirer believes that food practically yields only about half the force which, according to theory, it actually contains; another estimates it at only one fifth. The following table by Dr Frank land shows the amount of force which different foods yield when burned. The results agree very closely with those theoretically given by Play fair and others. Energy developed by one gramme, or one ounce of the following substances, when oxidised in the body. | Name of Substance | Per cent. of Water | 1 gramme will equal kil.-metres of energy. | 1 ounce will equal foot-tons of energy, or in other words, would raise the under-given number of tons, 1 foot high. [276] | | Beef (lean) | 70·5 | 604 | 55·0 | | Veal (lean) | 70·9 | 496 | 45·3 | | Ham (lean, boiled) | 54·4 | 711 | 64·9 | | Bread crumb | 44·0 | 910 | 83·0 | | Flour |... | 1627 | 148·5 | | Ground rice |... | 1591 | 145·3 | | Oatmeal |... | 1665 | 152·0 | | Pea meal |... | 1598 | 146·0 | | Potatoes | 73·0 | 422 | 38·5 | | Carrots | 86·0 | 220 | 20·0 | | Cabbage | 88·5 | 178 | 16·2 | | Butter |... | 3077 | 280·9 | | Egg (white of) | 86·3 | 244 | 22·3 | | Egg (yolk) | 47·0 | 1400 | 127·0 | | Cheshire cheese | 24·0 | 1846 | 168·5 | | Arrowroot |... | 1656 | 151·3 | | Milk | 87·0 | 266 | 24·3 | | Sugar (lump) |... | 1418 | 129·5 | | Ale (Ba ’ bottled) | 88·4 | 328 | 30·0 | | Porter (Guinne ’ stout) | 88·4 | 455 | 41·5 | [276] The amount of work done is generally estimated in this country as so many lbs. or tons lifted 1 foot. In France it is expre ed as so many kilogrammes lifted 1 metre,—and called ‘the kilogramme metre,’ as above. “A table of this kind,” says Dr Parkes, “is useful in showing what can be obtained from our food, but it must not be supposed that the value of food is in exact relation to the energy which it can furnish. In order that the force shall be obtained, the food must not only be digested and taken into the body properly prepared, but its energy must be developed in the place and in the manner proper for nutrition. The mere expre ion of potential energy cannot fix dietetic value, which may be dependent on conditions in the body unknown to us. For example, it is quite certain, from observation, that gelatin cannot take the place of albumen, though its potential energy is little inferior, and it is easily oxidised in the body. But, owing to some circumstances yet unknown, gelatin is chiefly destroyed in the blood and gland-cells, and its energy, therefore, has a different direction from that of albumen. So also of the potential energy, it is quite po ible that all is not usefully employed. The tables of energy give broad indications, and can be used in a general statement of the value of a diet; but at present they do not throw light upon the intricacies of nutrition.”