"Whenever one kind of food is wanting in any particular constituent, we invariably associate it with another that contains an excess of it" (Letheby).

If a labouring man is allowed an ordinary meat ration he requires in addition, to obtain his requisite carbon, of fat 346 grammes, and of starch 596 grammes (Voit). But he would soon tire of such a quantity of either of these two food classes alone, and he does better to eat some of each. As pointed out by Voit more than thirty years ago, the available vegetable nutrients are so enclosed in cellulose or woody fibre as to render their absorption difficult. Vegetable food eaten in excess is liable to induce intestinal fermentation, which excites peristalsis, and as the intestine of man is much shorter than that of herbivores, this class of food does not have time for complete absorption.

The carnivorous animal fed upon lean meat alone requires a daily supply equal to one twentieth of his body weight, but if a little fat be added he needs much less.

Animal food is often said to be more stimulating to the passions than vegetable food, and the general character of carnivorous animals is more savage than that of herbivorous, although the angered bull is as fierce as any carnivore. It is doubtful, however, whether this is attributable so much to their different diet as to many other conditions which have contributed through the survival of the fittest to their differentiation. Carnivorous animals are obliged to work and fight for their food; they obtain it at uncertain and infrequent intervals, often when made ravenous by long periods of fasting. Herbivorous animals, on the contrary, always have their food at hand, obtainable with no labour other than that of eating. The Polynesian warrior is more ferocious at times on a diet of plantains than the Eskimo, who eats nothing but fish, meat, and fats.

Formerly nitrogenous foods were believed to furnish the chief supply of energy for the body, especially in the production of muscular force; but the experiments of Parkes and others made some thirty years ago did much to modify this theory, and it is now believed that the chief value of nitrogenous food stuffs lies in tissue formation in distinction from force production, which is maintained by the hydrocarbons. Some energy is undoubtedly derived from nitrogenous material, for no molecular rearrangement or chemical reaction can take place without altering the balance of energy, but the force produced by combustion of proteids is insignificant in comparison with their other uses. In support of this statement, the following facts may be cited as abundantly proved: I. Severe and long-continued muscular exercise does not cause the quantity of urea in the urine to be materially increased. Urea represents the ashes of proteid material, and increase in its quantity is due to a more active oxidation of animal food, not necessarily to tissue waste. 2. Muscular exercise, on the contrary, does result in a great increase in the amount of CO2 eliminated from the lungs, which is primarily derived from the combustion of hydrocarbons. 3. Isolated muscles made to contracfoy artificial stimulation when contained in vacuo under a bell glass have been shown to produce by their activity large quantities of CO,, but no urea. 4. Prolonged muscular exercise or extra' ordinary exertion can be maintained upon a diet of starchy foods and fat without meat; and during the period following the exertion the urea is not increased beyond a degree which is readily accounted for by a slight wasting of the nitrogenous tissues of the body.

A man cannot perform more actual muscular labour upon an exclusive diet of animal food than of starchy food. He requires abundant animal food to replace the general wear and tear of muscular tissue, but the energy for muscular contraction is not derived from nitrogenous food, but from carbohydrates, the former being used merely to keep the muscles in a state of healthful equilibrium. He who is physically feeble and who lacks muscular power cannot restore that power by an exclusive nitrogenous diet. A man fed upon nitrogenous diet without vegetable food may not work as well in daily labour as when given a fair proportion of the latter; but, on the other hand, he is better fitted for sudden arduous exertion than are exclusive vegetable feeders.

A mixed diet is therefore the only rational one for man, and it is useless to reason otherwise from analogy with the lower animals. The ox is strong and performs daily labour upon a grass diet, but he extracts a large proportion of nitrogen from such food which man's feebler digestive organs do not enable him to do. The proportion of proteid to carbohydrate is 1 to 4.7 in such food. The horse is fed upon oats when it is desired to increase his spirit and activity, and he derives more nitrogen from them than does man from his oatmeal. The lion derives great strength from purely nitrogenous food, but even when trained he is wholly unfit for the continuous exertion that herbivores can endure.

The northern Eskimo, who has absolutely no starchy or saccharine food, eats fat with his proteid diet, and is therefore enabled to acquire energy to resist extreme cold and to take very long and fatiguing sledge journeys; yet he is no stronger, nor as strong, as the Central African negro, who lives upon a diet of manioc and plantains, without meat.

"Some food rich in protein will be found in the daily diet of all peoples. The Mongol eats with his rice, which is largely starch, small quantities of fish, fish eggs, and goose livers, but for his supply of proteid material he relies on his different preparations of bean cheese and on soja sauce made from the soy bean. The Mexican, whose supply of meat is scanty and of a poor quality, uses the native bean or frijole at almost every meal, made into a stew with vegetables and perhaps shreds of sun-dried beef, well spiced with the chili or red pepper " (Mary Hinman Abel).

Man is an animal of extraordinary adaptability to his environment, and one must be consequently guarded in making dogmatic statements in regard to his diet, and such observations as the above must be taken as applying only generally, for it seems possible for either class of foods to supplement to some extent the functions of the other. This fact is illustrated in disease as well as in health. For example, in the earlier stages of diabetes sugar is formed from farinaceous and saccharine food almost exclusively, but in the advanced stages, when these substances are withheld, it can undoubtedly be made from proteid material.

The combinations of foods which are by analysis shown to contain quantities of proteids, starch, and fat, have a very different effect in overtaxing the digestive organs according to the particular form in which their ingredients exist.

It is a popular belief that meat requires more effort for digestion than starchy food, but in health this is probably not true, provided both varieties of food are taken in correct proportion, for it certainly would be a strain upon the digestive system to be obliged to derive all the carbon needed from an exclusive meat diet, just as it overtaxes the alimentary canal to obtain sufficient nitrogenous material from an exclusive vegetable diet. It cannot be said didactically that a piece of beef is more or less digestible than a potato, and that it taxes the energies of the digestive organs to a greater extent, although it is more completely assimilated and leaves less waste in the intestine. The whole question devolves upon a true balance of the ingredients of a mixed diet. As regards the actual complexity of the digestive processes of the several classes of food, there is but little difference: both starches and proteids pass through intermediate stages on the way to the formation of their completed products, and fermentation cannot be said to present greater difficulty in the one case than in the other.

For man, certainly, Nature never intended that all the nutrition of the body should be derived from any one class of food stuff which would require the use of certain digestive juices, and imply the disuse of others which are normally present.

A diet of animal food is much less fattening than a vegetable regimen or than carbohydrates with a fair proportion of fats, but a stout man does not endure fatigue, or even starvation, better than a lean man. On the other hand, to increase the proteid substances of the body an albuminous diet with but little carbohydrate is necessary. Men, unless greatly emaciated, have a reserve store of energy in their bodies sufficient to maintain their animal heat and keep them alive for from seven to nine days, and this is true whether they have been meat eaters or vegetarians. Storage of fat will help them out in emergencies, but if it has been overdone - i. e., if there is too much fat in the tissues - they may be weakened by it and, although they have the material for force production on hand, they are unable to utilise it, and are worse off than if they were spare. (See Obesity and Leanness).

Bauer says: " The material effects of albumin and of fat in the system are in a certain sense opposed, for the former increases the tissue waste and secondarily the oxidation, while fat induces the opposite effects." When the organism is in health albumin constitutes upon the average about 10 per cent of the body weight and fat about 16 per cent, but these proportions may vary within wide limits.

Animal food is easily cooked, requiring less fuel than vegetable food, and in the process develops more flavour than does vegetable food.