Diet

Diet. The different classes of food-stuffs, and the value or potential of different foods, are described in the article FOOD. In the article DIGESTION the changes which the food undergoes in the alimentary canal are treated. It is here proposed to describe the quantity and quality of food which experience has shown to be required in order to preserve the body in health and vigour. Of course, no two individuals are alike, and the assumption will in the first place be made that an average healthy man of ordinary height and weight, and doing ordinary work, is alone under consideration.

In order to understand the requirements of the body in respect of food, it is necessary to bear in mind how complex the body is. Its chief activities are muscular and nervous, and not only have the brain and the muscular system to be fed, but in addition the various glands, cartilages, bones, and other tissues are all constantly requiring nourishment. If we examine all these tissues under the microscope, we see that although all are alike in some respects, yet that they differ one from another considerably in others. Their work, too, is different; and we should expect, and experiment shows, that they require different food.

Just as the life of the whole body is the sum of the lives of the different parts (cells) of the body, so the food must contain that which is necessary for these parts. The body has frequently been compared to a steam-engine, but, as we have seen, the body is complex, and the comparison should rather be between it and a collection of steam-engines, each one differing from the other in its pattern, use, and the kind of fuel required to stoke it. A steam-engine requires a store of fuel (containing the potential-power), by the combustion of which it moves. It also requires its parts to be repaired, and sometimes a new boiler or piston is necessary. In addition, it requires other things, so that it may work well, such as oil, which is neither burned nor used to build up the machine. In the same way the body, and each part of it, requires first, a store of fuel, from the combustion of which its energy or activity is obtained; secondly, material by which its wasted parts may be repaired; and, thirdly, substances such as water, salt, &c., which are required by the body, but which are neither burned nor used entirely in forming tissue. The term 'food' may then be applied to all that the body requires from without; and following out our comparison, corresponds with the stoking, mending, and oiling of a number of steam-engines.

Let us consider in the first place the food, which, like the fuel for the engine, the body requires in order to manifest its own peculiar activities. The coal of an engine contains carbon and hydrogen, which become oxidised by the oxygen of the air. As a result of this oxidation, we have heat and mechanical motion. In like manner the body receives food containing large quantities of carbon and hydrogen. At the same time oxygen is absorbed by the blood as it passes through the lungs. The digested food and the oxygen are taken by the blood to the tissues—muscles, brain, &c.—and within these tissues a slow oxidation takes place, giving rise to the heat of the body as well as mechanical and other activities. Just as the steam-engine requires a sufficiency of coal, so the body requires a certain quantity of food. In both cases the supply of oxidisable material must have what may be termed a sufficient 'potential'—that is to say, it must have the power of producing a certain amount of heat and mechanical motion. Some coals and some foods burn better than others, and their potential in that case is greater. Fat among the food-stuffs has the greatest potential, carbohydrates and albumens have less. It is obvious then that one cannot speak of the body requiring a certain weight of food per diem. This would mean nothing, for the value of different food-stuffs varies considerably, a pound of one kind of food being equivalent in potential, say, to twenty pounds of another. In arranging a diet one must carefully consider not only the quantity of food given, but in addition one must calculate its potential. The steam-engine may be driven at full or at half power, and in the latter case the amount of fuel required will be diminished. In the same way an individual doing hard work requires food with more potential, or, as we say, more nutritious food, than another living an easy and uneventful life, and still more than one confined to the house or to his bed.

There is yet another important point to be considered. In a steam-engine the production of heat is not what is sought as a result of the combustion within the steam-engine; the engineer constructs the machine with a view to obtaining the result of the combustion not in heat, but in mechanical motion. In the best machines, one-tenth of the potential is converted into work, while the rest, ninetenths, is rejected into the atmosphere as heat. The human mechanism can convert as much as one-fifth of the potential of its food into work, and changes four-fifths into heat. In the case of the human machine, however, heat is very necessary, especially if the body be ill clothed or surrounded by a cold medium. The powerlessness of the hands when numbed with cold will illustrate the necessity of a requisite temperature. Warm clothing actually saves food. Not only is the temperature of the body maintained in a cold climate by preventing its conduction and radiation from the body by the use of warm clothing, but appetite prompts the consumption of a large quantity of food, and especially those kinds that contain great potential. The Eskimos devour great supplies of fat blubber, and they are even said to prefer the taste of rancid oil to that of the less combustible sugar.

It has been found by experiment that a man of average height and weight, well clothed, in a temperate climate, and doing a fair day's work, requires such quantity of food that the combustion of it, were it all converted into mechanical work, would raise 4000 tons one foot high, or, as it is expressed, 4000 foot-tons. This result has been arrived at by varying the diet until the minimum but sufficient quantity has been arrived at which will preserve the health and vigour. The potential of the diet can be calculated by an instrument called the calorimeter, which will give the heat produced during the burning of any combustible substance, and from the heat produced the potential in respect to mechanical work can be calculated. In the subjoined table the potentials of one ounce of each of three important food-stuffs are given:

It will at once be seen how readily one could calculate the amount of food-stuff capable of furnishing the body with fuel for twenty-four hours. All that is necessary will be to divide 4000 by the number of foot-tons which correspond with the article selected; the quotient is the number of ounces required. Thus 11 ounces of fat, or 24 ounces of albumen, or 26 ounces of starch would contain the necessary potential.

From what has already been said regarding the complexity of the bodily mechanism, it may be surmised that a single food-stuff (even if a quantity be taken which provides a sufficient potential) is incapable of stoking the body. Numerous experiments, such as those conducted by the Gelatine Commission, by Dr Hammond, and others, indicate that the body is unable to maintain its natural vigour on a diet consisting solely of gelatine, or of starch, or of albumen, or of fat, &c. Animals fed on a single food-stuff waste rapidly, and will refuse their food after a few days with marked dislike, dying eventually of starvation. At least two staple articles of food must be given, mixed, even in this case, with many accessories, such as various salts, aromatic substances, &c., in order that health may be maintained.

Liebig taught that the muscular work of the body results from the oxidation of nitrogenous matter (albumen, both animal and vegetable), but it is now known that all the classes of food-stuffs except the mineral matters are used by the muscles. Nevertheless proteids (nitrogenous food) are essential, as we shall see on experimental grounds, and from the fact that in the dietary of every nation, proteid matter is always present, combined at any rate with fat, generally with carbohydrates as well.

In many parts of the world man subsists with- out the use of carbohydrates. The Eskimo and the North American Indian live entirely on the produce of the chase. The flesh of animals is rich in proteids and fats, but contains practically no carbohydrate. In the same way the Arab of the desert lives upon the flesh and milk of the camel, in districts where the date is not to be found. The use of carbohydrates is extensive in most temperate and tropical climates. Many of the inhabitants of India and Ceylon live chiefly on rice. Wheat, potatoes, and other vegetable foods, rich in carbohydrates, are staple articles of food both in Europe and America. In fact, within certain limits, man, like every other animal, is capable of adapting himself to the food produced by the district in which he lives. The French and Spanish peasants eat little meat, living on carbohydrates and vegetable proteids, and supplying necessary potential by an abundance of oil. The Sussex labourer consumes his beans, rich in vegetable proteids, and bacon, and does a good day's work on them. The Scotch labourer of a former generation lived on porridge and skimmed milk, with meat but once a week. The Arab of the Sahara, together with his family and his horse, subsist almost entirely on the date, while the Arab of the Nubian Desert hardly ever touches vegetable food. The diet of every man, under whatever circumstances, invariably contains proteids, although this may vary in its quantity. The universal use of proteid matter in some form or other suggests its importance, and this is fully borne out by the evidence of direct experiment. If proteid food be withdrawn from the diet altogether, nitrogenous matter continues to be eliminated from the body by the urine, although its supply is cut off. At the same time the body wastes, and death finally occurs. The experiments of Fick, Wislicenus, and Parkes show that during muscular exertion the body does not excrete any excess of nitrogen; and the writer's own researches show that urea—the most important excretion which contains nitrogen—is not found in abnormal extent in muscular tissue after exhaustive exercise. It would appear, therefore, that nitrogenous matter is not used by the tissues for fuel, and that when proteid food is consumed, the muscles burn a non-nitrogenous substance which is formed from it. The nitrogen, in the form of urea, &c., is carried by the blood to the kidneys, and at once excreted. We must look elsewhere than in the ordinary combustion of a muscle or brain cell for a use for the nitrogen which, as we have seen, is so necessary an article of food in the form of a proteid substance. In the child, the tissues which contain nitrogen in definite proportion are continually growing, and it will readily be seen that here, at any rate, we have an important use for proteid diet—namely, to furnish the necessary nitrogen for the new tissue. Even during adult life we have tissue change and growth. The skin is continually growing in its deeper layers to replace its surface layers worn away by constant friction. In the same way the cells of the blood are always being broken down, and their places taken again by new ones continually forming. Although it is hardly probable that such complete changes take place in all the tissues of the body, yet, without the entire breaking down of individual cells, there are no doubt changes both integrating and disintegrating, which are constantly taking place. As a result of disintegrating changes nitrogenous matter in an effete form is being given off by the system, and it has to be replenished from without in a form which the body can use. The proteid food-stuff provides nitrogenous pabulum for the tissues which they assimilate, and build up new tissue material. For this reason proteid matter is indispensable. At the same time the system obtains from the proteid food non-nitrogenous fuel to be burned by the tissues; this property of supplying tissue fuel is shared with the carbohydrate and fatty food-stuffs. It has been found by experiment that an ordinary diet should contain one part of nitrogenous matter (proteids) to about four of non-nitrogenous diet (fat and carbohydrates). If less nitrogenous matter be given, the tissue consumption of nitrogen will not be supplied, and the body will waste. If a much larger quantity of nitrogenous food be taken, a nitrogenous surfeit occurs, and the body is called upon to digest this excess, and to eliminate an unnecessary quantity of the useless nitrogenous compounds which result.

We have seen then that an ordinary diet must contain sufficient potential, and in addition one part of proteid to four of non-proteid material should be present. The following table from Dr Parkes shows the proper proportions of solid water-free food-stuffs in ounces required as daily food by an adult man:

Taking the diet table for a man performing ordinary work, we have 4.6 of proteid, and 17.4, or nearly four times as much non-proteid matter. It contains sufficient potential, for if the number of foot-tons which result from the combustion of an ounce of proteid, of fat, and of a carbohydrate be multiplied by the number of ounces of these substances given in the ordinary diet, it will be seen to amount to 4000. This, as we have already seen, indicates a sufficient potentiality.

It now becomes an easy matter to construct a diet table of articles of food. Most of these have been analysed, and the amount of water, proteid, carbohydrate, or fat in each calculated.

TABLE FOR CALCULATING DIETS (Parkes).

Many articles of food owe their chief importance to their action as stimulants. Such are alcohol, tea, coffee, and beef-tea. Alcohol is no doubt burned within the body, and is a source of energy. Only a small quantity, however, can be so utilised, perhaps one ounce in every twenty-four hours in the case of an ordinary individual. As a source of energy, therefore, it is of little value, and such substances as fat, meat, bread, are capable of supplying the potential at a very much smaller cost. Alcohol taken with other food stimulates the secretion of gastric juice, assisting in that way the digestive process. At the same time, however, the gastric juice is unable to act quite so readily upon the food, though this is hardly the case with diluted spirits, which form more wholesome beverages than wines or beer. No two individuals are the same, and while alcohol, in moderate doses, promotes digestion in most persons, others suffer from its use. (See the article on the action of alcohol, Vol. I. p. 135). It should never be given in collapse and weakness without giving at the same time easily digested food when that is possible. It excites the body to great and often unnecessary activity, the potential for which it does not supply. Afterwards greater weakness ensues from want of the necessary fuel, which has not in the meanwhile been forthcoming. One should never drink without eating. Tea and coffee are both nervous stimulants, and at the same time they retard both gastric and salivary digestion.

Beef-tea is generally regarded as a food-stuff of high nutritive value. This is, however, not the case. It contains nothing besides salts and extractives, and has a very slight potential indeed. It has a stimulating effect, however, both on digestion and on the nervous system. From the mistaken ideas generally held as to the nutritive properties of this substance, thousands of invalids are annually starved to death (see BEEF-TEA). Beef-tea made by infusing the beef in tepid water is more nutritious, especially if the beef be finely minced and eaten as well.

In addition to the stimulants that we have already considered are many substances known as condiments, such as mustard, pepper, pickles, sauces. These are of utility in gratifying the palate, and in addition they probably stimulate the secretory juices. Sufficient information has not as yet been obtained as to their action on gastric and pancreatic digestion. They certainly stimulate the flow of saliva, although the acid condiments will prevent the perfect action of the salivary ferment.

Common salt is a condiment, and at the same time it plays many other important parts in the animal organism. So necessary is it that both man and animals suffer great hardships if it be not supplied in sufficient quantity. It is necessary for the formation of the gastric juice; it is present in the blood and in all tissues of the body.

Inorganic salts, such as sulphates, phosphates, are required for the formation of the skeleton, and salts of iron for the colouring matter of the blood. Organic salts, such as citrates and tartrates, are also of importance. That food should be easily digested is a matter of great importance. The rapidity of digestion will depend upon the amount and quality of the digestive juices, the kind of food, and the condition in which the food is eaten. Rice, tripe, whipped eggs, sago, tapioca, barley, milk, raw eggs, lamb, parsnips, potatoes, hashes of chicken, fish, are all easily digested substances. Beef, mutton, pork, roast fowls, bread, veal, oysters, are digested more slowly. Inasmuch as perfect digestion can only be accomplished when the digestive juices have acted for some time on all parts of the food taken, it follows that fine subdivision of food is very essential. On this account liquid food rapidly disappears from the stomach, which retains for a longer time solid masses upon which the gastric juice acts with greater difficulty. When food is cooked it swells, its fibres and solid particles are separated one from another, and it is more readily permeated by the gastric and pancreatic juices. In addition, starch becomes gelatinous, and in that form is easily digested by the saliva and pancreatic juice. The diet of early infantile life differs from that of the adult, inasmuch as it should contain no starch. Milk, the natural food of the infant, is rich in fat, proteids, and a sugar called milk-sugar. There is in milk, however, an entire absence of starch, an article of food which the infant is destined to consume in such large quantities in later years. Moreover, during the first few months of its extra-uterine existence the child is unable to assimilate starch given with its food. On no account, therefore, must it be supplied with bread, potatoes, rice, or other vegetable food until the cutting of the teeth suggests a more solid diet. If the mother's milk be not in sufficient plenty, cow's milk diluted with one-third of water, with a pinch of sugar, may be given, or condensed milk diluted with twelve to twenty parts of water (see MILK). Condensed milk, owing probably to its very uniform composition, cannot be given alone for more than a few weeks together. It may, however, be given once or twice a day for months, and children thrive on it, when they in addition are supplied with good cow's, or still better, with their own mother's milk.

In conclusion, it may be well to consider the results which follow the neglect of the most obvious rules of dietetics. As the result of deficient food, one finds loss of muscular and nervous power, wasting of the tissues, and anaemia. If the deficiency be very great, feverish symptoms and great prostration result.

Many persons consume large quantities of food quite out of proportion to their size or activity. In this case, owing to a 'habit of digestion,' much of the food may pass through the digestive tract without being digested or assimilated. Under these circumstances the hearty eater is a wasteful eater, and is using for his own bodily needs only a fraction of the food he consumes. In addition, dyspepsia in various forms, constipation, and diarrhoea are apt to follow, indicating functional derangements of the digestive apparatus. There is often a tendency, especially in advanced years, to absorb more nourishment than is necessary for stoking the body and for replenishing ordinary tissue waste. The excess is stored up in the form of fat, which accumulates under the skin, chiefly under that of the abdomen. In addition, the muscles and internal organs are loaded with fat, the minute globules of which may be seen in the ultimate cells of which the tissues are composed. Hereditary tendency is well marked in cases of corpulency. In addition to corpulency, an excess of food is apt to engender various gastric troubles, engorgement of the liver, plethora, and an excess of effete extractions in the blood and urine. It is probable that humanity suffers more from an excess than from a deficiency of food. An excess of animal is more serious than an excess of vegetable food. The nitrogenous extractions, derived from the incomplete assimilation of meat, when present in large quantity, cause many symptoms, most of which are extremely obscure in their nature. These are provisionally spoken of as symptoms of gout. The ill effects which follow surfeit are more severe in those leading a sedentary and inactive life, bodily activity producing more efficient oxidation of the food taken. A healthy and abstemious man whose tastes have not been enslaved by the culinary art instinctively adapts his food to the requirements of his body. The cold of winter prompts the choice of substantial and energising food, while the heat of summer suggests a lighter dietary. After a country holiday, on returning to a sedentary life one at once reduces one's allowance of beef, or expects to pay the penalty that a disordered digestion is certain to exact. During the ages in which humanity has been evolving, there has been a constant adaptation of taste and desire to the needs of the economy. The natural gustatory inclinations as a rule are a good indication of the bodily wants. As a rule wholesome things have a pleasant taste, and the reverse also holds good. It is all-important, however, that the satisfaction of mere gustatory pleasure be not allowed to monopolise too much of the energy of any individual. Under these circumstances a surfeit is certain to result. There is a well-known law in physiology to the effect that greater and greater stimuli have to be applied in order to produce a series of equal sensations. It follows that the excesses of the glutton and drunkard are out of all proportion to the actual pleasures these excesses produce, the wise man drinking and eating only in moderation.

See the articles on COOKERY, FOOD, DIGESTION, INDIGESTION; also Pavy, Food and Dietetics (1874); Sir H. Thompson, Food and Feeding (1880); Sir W. Roberts, Lectures on Dietetics and Dyspepsia (1885); Fothergill, Manual of Dietetics (1886).