The study of nutrition, or the problem of tracing the food products, after absorption from the alimentary canal, through the various changes which they undergo prior to elimination in the comparatively simple forms of waste matter - namely, water, urea, and C02 - has long baffled the science of physiological chemistry. There are, however, general principles that are established both by analysis and clinical observation which concern the effect upon the system of these different classes of foods.

Among the lower animals the effect upon the whole system of modifications in nutrition produced by special methods of feeding are much more striking than in man, as, for example, the feeding of the queen bee, and of draught horses as compared with racers.

Animal food is believed by many to make the blood rich in fibrin and corpuscles, and to increase the mineral salts, especially the phosphates; it both repairs the old and forms new tissue; it improves the condition of the muscles, which are made firmer than they are through a vegetable diet, and it favours the reduction of stored-up or surplus fat. Under some conditions it may produce a part of the body fat, although most of it is derived from other sources. (See Obesity.) It also increases the quantity of urates, phosphates, sulphates, and urea normally present in the urine, and tends to make the fluids of the body somewhat more acid or less alkaline.

Bauer (Dietary of the Sick) says: "The functional activity and resisting power of the organism seem to be essentially connected with the presence of an ample supply of albumin".

Animal food requires a considerable quantity of oxygen for its complete combustion, and a diet of this nature increases the demand for oxygen and favours its consumption. Meat in general has a more stimulating effect upon the system and is more "strengthening " than vegetable food, and it gives rise to sensations of energy and activity. A meal consisting of meat remains an hour or two longer in the stomach than a purely vegetable meal. It seems to satisfy the cravings of hunger, bulk for bulk, to a greater extent and for a longer time than vegetable food, and a man can live longer upon exclusive nitrogenous food than upon exclusive carbonaceous food. Animal food occupies less space in the stomach, and is more portable than vegetable food. Moreover, albuminous foods can be eaten longer alone without exciting loathing, as a rule, than can fats, sugars, or even some pure starches. In fact, there is a constant tendency to eat too much meat, and when its effects are not counterbalanced by free outdoor exercise, it produces an excess of waste matter which accumulates and causes biliousness, and sometimes lithiasis, gout, etc.

A carbonaceous diet taxes the excretory organs to a lesser degree than animal food.

Sir Henry Thompson says: "It is a vulgar error to regard meat in any form as necessary to life." Nitrogenous food man must have, but it need not necessarily be in the form of meat, which "to many has become partially desirable only by the force of habit, and because their digestive organs have thus been trained to deal with it.". This is but partially true, for the training has become so strongly a matter of heredity through many centuries that those who possess it are certainly in better health for a reasonable allowance of meat in their dietary, and many primitive savage tribes have always subsisted largely apon meat. Errors in diet are far more common on the side of excessive meat eating than the eating of too much vegetable food, especially among civilised communities. In the temperate zones an increase in prosperity, together with the improvements made in the methods of preparing and preserving meat as well as those in breeding cattle for market purposes, tend to increase the habit of meat eating.

The common estimate, in which meat should occupy one fourth and vegetable food three fourths of a mixed diet, is overstepped by many persons with whom the proportion may be two or three to four.

The proper association of different foods always keeps healthy men in better condition than too long continuance of any selected diet system.

Sir Henry Thompson, in speaking of the advantages of a well-proportioned diet, says: "A preference for the high flavours and stimulating scents peculiar to the flesh of vertebrate animals mostly subsides after a fair trial of milder foods when supplied in variety.... The desire for food is keener, the satisfaction in gratifying appetite is greater and more enjoyable on the part of the general light feeder, than with the almost exclusively flesh feeder.... Three fourths at least of the nutrient matters consumed are from the animal kingdom. A reversal of the proportions indicated - that is, a fourth only from the latter source, with three fourths of vegetable produce - would furnish greater variety for the table, tend to maintain a cleaner palate, increased zest for food, a lighter and more active brain, and a better state of health for most people not engaged in the most laborious employments of active life".

This comment is more applicable to the upper classes in England than in this country, where more attention is given to the cultivation and cooking of fresh vegetables and the preparation of vegetable products.

Letheby wrote: "The best proportions for the common wants of the animal system are about nine of fat, twenty-two of flesh-forming substances, and sixty-nine of starch and sugar." An average of eighty-seven practical dietary studies made by the U. S. Department of Agriculture showed that the food consumed was 45 per cent of animal and 55 per cent of vegetable origin.

Meats which are deficient in fat are usually eaten with added fat. Thus bacon is eaten with veal, liver, and chicken, and most fish are cooked with butter or oil. Similarly, butter, eggs, or cream are mixed with amylaceous foods, such as rice, sago, potatoes, etc., which are lacking in fat, and cheese containing fat is added to macaroni. Bacon is added to beans, and pork to greens.