Although experiments on food have been going on for ages, it is only within a few decades that the results of these experiments have been classified scientifically. To exemplify such a classification we may take a meal such as is quite common in this country. A man sits down to a steak or chop, attached to which is probably a piece of fat; he has along with it potatoes or bread, and if there is no fat upon the steak or chop he probably has a pat of butter extra. He seasons the meat with salt, and drinks a glass of water during or after the meal. On analysing such a meal we find it contains :

1. Proteins, represented by the lean of the steak or chop.

2. Fats, represented by the fat of the meat or butter.

3. Carbo-hydrates, represented by the potatoes or bread.

4. Mineral matters, like the salt.

5. Water.

Salt is not the only mineral matter which he takes at such a meal, for if the steak or chop were burned to ashes the residue would contain potassium salts, and rarely indeed is drinking water absolutely free from lime, so that in addition to the sodium contained in the salt, he obtains potassium along with small quantities of phosphorus, from the meat and potatoes, and lime from the water.

In order to understand the uses of these different constituents we may utilize the hackneyed comparison between a steam engine and a human body. The motor parts of the steam engine consist of iron; carbo-hydrates in the form of coal yield the motive force; water in the boiler is absolutely necessary for the working of the engine, and its movements are facilitated by the application of oil or fat. If it were not for the question of expense, fat would be the best fuel wherewith to feed the engine, and it is said that in the old days when there was so much racing between the steamers on the Mississippi, the furnaces were sometimes fed with bacon hams, while the captain sat on the safety valve, a proceeding which frequently ended in a tremendous explosion !

In the human engine the bones, which may be likened to the cranks of the locomotive, are mainly composed of phosphate of lime and protein matter. The driving machinery consists of the muscles, which are chiefly composed of proteins, and the energy is supplied partly by carbo-hydrates, and partly by fat, which has also a lubricating function in the body as well as in the locomotive.

Half a century ago it was thought that the energy exhibited by the muscles of the body was derived from combustion of the muscles themselves, and so a large amount of flesh was thought to be necessary, or at least advisable, in the dietary of those who had to undergo great muscular exertion. Men about to enter for athletic contests were then usually fed on beef steaks and beer. Fick and Wislicenus in their celebrated ascent of the Faul-horn in 1865, made some observations which led to the overthrow of the old ideas. The amount of urea in their urine, which served as an index to the quantity of protein waste in their bodies,, underwent such a slight increase even during their long continued and severe exertions as to show that the energy necessary for the work which they had done could not have been derived from the destruction of proteins, and that its source must be looked for elsewhere.

Following on these experiments came many other scientific researches which have shown that the protein waste in the body may be likened to the wear and tear of the cranks and wheels of the engine, and that quite a small quantity of protein in the diet is sufficient to supply the waste. But there is a difference between the engine and the body, for the cranks and wheels of a locomotive are quite incombustible under any circumstances, but the proteins of the body, whether they be in the muscles or elsewhere, are capable of combustion and undergo destruction to a great extent during exertion, especially if the supply of hydrocarbons and fats is insufficient for the energy necessary for the exertions.

A careful consideration of the dietaries in use amongst various peoples, in various classes of society, and engaged in various; occupations, has confirmed the results of scientific experiments, and it is now universally acknowledged that upon vegetable diet men are capable of very great and long continued muscular exertion. On the other hand the Indians and Guachos can live, as I have said, on an exclusively meat diet. But in both the vegetable and animal diets the five constituents of food which I have already mentioned (p. 4) must be present.

The Hindoo, who obtains most of his energy from the carbohydrates of rice, must add a certain amount of fat in the form of melted butter, and protein in the form of millet, a seed which contains a large proportion of albuminoid matter. A certain proportion of salt is also indispensable, and water is so necessary that death from thirst is a great deal worse than death from hunger. Nor is this to be wondered at when we consider that three-quarters of the weight of the body consists of water, and that all our tissues, as Claude Bernard has well put it, live in a fluid medium, namely, the lymph in which they are bathed, from which each tissue draws its nutriment, and to which it returns the products of its waste.

In the same way the Pampas Indians obtain from meat, and the fat which accompanies it, the carbo-hydrates which will yield the energy they need, although with this difference : that they have to consume a quantity of protein far above that which they actually need for the repair of albuminous waste.

The typical dietary is that which will give a proper proportion of the different ingredients to supply the needs of the body without any one of them being in excess. Frequently the circumstances under which men live render it impossible for them to obtain the typical dietary, and then they must make the best approach to it that is possible for them. The quantities in a typical dietary, such as the one I have already mentioned, would be :

13 oz. Beef Steak. 3 oz. Butter. 6 oz. Potatoes. 22 oz. Bread. This quantity is sufficient for twenty-four hours, and it is obvious that few men could eat it at one meal, but they can divide it into two, or perhaps still better, into three similar meals. But typical diet tables are made out on the assumption that the body is all to be worked equally in its different parts. There is, however, a very great difference between the work of the navvy and that of the railway director. The navvy works hard with his muscles, but very little with his brain, while the railway director has very little muscular work, but a great deal of thought and anxiety. The navvy's work consumes a great deal of energy, and his muscles produce much waste, while the brain, which is an incomparably finer instrument, requires much less energy to work it, and produces far fewer waste products. The two may be likened to the steam engines and the chronometer on board a ship. Both are equally requisite for a safe voyage, but, while the engines consume thousands of tons of coal, all the energy needed for the chronometer is that supplied by a few turns of the key which winds it up. The food required to supply energy to the muscles of the navvy is not only much larger in quantity, but different in kind from that demanded by the brain of the director. Muscular work, as I have already said, can be done, to a great extent, on a diet consisting chiefly of carbo-hydrates, with only a very small proportion of nitrogenous matter, but it would appear that brain work requires food in a larger proportion of a nitrogenous nature, and if associated with muscular inaction a great quantity of carbo-hydrates is inadvisable. The comparative effect of a non-nitrogenous and a nitrogenous diet in nervous energy has been well expressed (I think by Butler) in the lines :

Was ever Tartar fierce or cruel, Upon the strength of water gruel? But who withstands his rage and force, When first he kills, then eats his horse?

At the same time, it must be remembered that the amount of fuel required by the brain is very small, and that if much nitrogenous food be taken by a sedentary man, he is unable to utilize it properly and his brain becomes clogged by the products of waste, so that it may, on too rich a diet, be less active and vigorous than on one that is too meagre.1