Francis Bacon has laid it down as an axiom that experiment is the foundation of all real progress in knowledge. “Man,” he said, “as the minister and interpreter of nature, does and understands as much as his observations on the order of nature permit him, and neither knows nor is capable of more.”41 It would seem, then, as if there could be no subject on which man should be better informed than on the value of various articles of food, and the quantity in which each should be used. On most branches of experimental inquiry, a few men in each age—perhaps but for a few ages in succession—have pursued for a longer or shorter portion of their life, a system of experiment and observation. But on the subject of food or diet all men in all ages have been practical experimenters, and not for a few years only, but during their entire life. One would expect, then, that no questions could be more decisively settled than those which relate to the use or the abuse of food. Every one ought to know, it might be supposed, what kinds of food are good for the health, in what quantity each should be taken, what changes of diet tend to correct this or that kind of ill-health, and how long each change should be continued.

Unfortunately, as we know, this is far from being the case. We all eat many things which are bad for us, and omit to eat many things which would be good for us. We change our diet, too often, without any consideration, or from false considerations, of the wants of the body. When we have derived benefit from some change of diet, we are apt to continue the new diet after the necessity for it has passed away. As to quantity, also, we seldom follow well-judged rules. Some take less nutriment (or less of some particular form of nutriment) than is needed to supply the absolute requirements of the system; others persistently overload the system, despite all the warnings which their own experience and that of others should afford of the mischief likely to follow that course.

It is only of late years that systematic efforts have been made to throw light on the subject of the proper use of food, to distinguish between its various forms, and to analyze the special office of each form. I propose to exhibit, in a popular manner, some of the more important practical conclusions to which men of science have been led by their investigations into these questions.

The human body has been compared to a lamp in which a flame is burning. In some respects the comparison is a most apt one, as we shall see presently. But man does more than live; he works,—with his brain or with his muscles. And therefore the human frame may be more justly compared to a steam-engine than to the flame of a lamp. Of mere life, the latter illustration is sufficiently apt, but it leaves unillustrated man’s capacity for work; and since food is taken with two principal objects—the maintenance of life and the renewal of material used up in brain work and muscular work—we shall find that the comparison of man to a machine affords a far better illustration of our subject than the more common comparisons of the life of man to a burning flame, and of food to the fuel which serves to maintain combustion.

There is, however, one class of food, and, perhaps, on the whole, the most important, the operation of which is equally well illustrated by either comparison. The sort of food to which I refer may be termed heat-maintaining food. I distinguish it thus from food which serves other ends, but of course it is not to be understood that any article of diet serves solely the end of maintaining heat. Accordingly, we find that heat-maintaining substance exists in nearly all the ordinary articles of food. Of these there are two—sugar and fat—which may be looked on as special “heat-givers.” Starch, also, which appears in all vegetables, and thus comes to form a large proportion of our daily food, is a heat-giver. In fact, this substance only enters the system in the form of sugar, the saliva having the power of converting starch (which is insoluble in water) into sugar, and thus rendering it soluble and digestible.

Starch, as I have said, appears in all vegetables. But it is found more freely in some than in others. It constitutes nearly the whole substance of arrowroot, sago, and tapioca, and appears more or less freely in potatoes, rice, wheat, barley, and oats. In the process of vegetation it is converted into sugar; and thus it happens that vegetable diet—whether presenting starch in its natural form to be converted into sugar by the consumer, or containing sugar which has resulted from a process of change undergone by starch—is in general heat-maintaining. Sugar is used as a convenient means of maintaining the heat-supply; for in eating sugar we are saved the trouble of converting starch into sugar. A love for sweet things is the instinctive expression of the necessity for heat-maintaining food. We see this liking strongly developed in children, whose rapid growth is continually drawing upon their heat-supply. So far as adults are concerned, the taste for sweet food is found to prevail more in temperate than in tropical climes, as might be expected; but, contrary to what we might at first expect, we do not find any increase in the liking for sweet food in very cold climates. Another and a more effective way of securing the required heat-supply prevails in such countries.

As starch is converted into sugar, so by a further process sugar is converted into fat. It is by the conversion of sugar into fat that its heat-supplying power is made available. This conversion takes place in the vegetable as well as in the animal system, and thus fat appears in a variety of forms—as butter, suet, oil, and so forth. Now, precisely as sugar is a more convenient heat-supplier than starch, so fat exceeds sugar in its power of maintaining animal heat. It has been calculated that one pound of fat—whether in the form of suet, butter, or oil—will go as far towards the maintenance of animal heat as two pounds of sugar, or as two pounds and a half of starch. Thus it happens that in very cold countries there is developed a taste for such articles of food as contain most fat, or even for pure fat and its analogues—oil, butter, tallow, dripping, and other forms of grease.

I have spoken of starch, sugar, and fat as heat-forming articles of food; but I must note their influence in the development of muscles and nerves. Without a certain proportion of fat in the food a wasting of the tissues will always take place; for muscles and nerves cannot form without fat. And conversely, the best remedy for wasting diseases is to be found in the supply of some easily digestible form of fatty food. Well-fatted meat, and especially meat in which the fat is to be seen distributed through the flesh, may be taken under such circumstances. Butter and salad oil are then also proper articles of food. Cream is still better, and cream cheeses may be used with advantage. It is on account of its heat-supplying and fat-forming qualities that cod-liver oil has taken its place as one of the most valuable remedies for scrofulous and consumptive patients.

But it must be noted that the formation of fat is not the object with which heat-supplying food is taken. It is an indication of derangement of the system when heat-giving food is too readily converted into fat. And in so far as this process of conversion takes place beyond what is required for the formation of muscles and nerves, the body suffers in the loss of its just proportion of heat-supply. Of course, if too large an amount of heat-giving food is taken into the system, we may expect that the surplus will be deposited in the form of adipose tissue. The deposition of fat in such a case will be far less injurious to the system than an excessive heat-supply would be. But when only a just amount of heat-giving food is taken, and in place of fulfilling its just office this food is converted into adipose tissue, it becomes necessary to inquire into the cause of the mischief. Technically, the evil may be described as resulting from the deficient oxygenation of the heat-supplying food. This generally arises from defective circulation, and may often be cured by a very moderate but systematic increase in the amount of daily exercise, or by the use of the sponge-bath, or, lastly, by such changes in the dress—and especially in the articles of attire worn next to the skin—as tend to encourage a freer circulation of the blood. The tendency to accumulate fat may sometimes be traced to the use of over-warm coverings at night, and especially to the use of woollen night-clothes. By attending to considerations of this sort, more readily and safely than by an undue diminution of the amount of heat-supplying food, the tendency to obesity may frequently be corrected.

In warm weather we should diminish the supply of heat-giving food. In such weather the system does not require the same daily addition to its animal heat, and the excess is converted into fat. Experiments have shown that despite the increased rate at which perspiration proceeds during the summer months, men uniformly fed throughout the year increase in weight in summer and lose weight in winter.

So far as mere existence is concerned, heat-forming food may be looked upon as the real fuel on which the lamp of life is sustained. But man, considered as a working being, cannot exist without energy-forming food. All work, whether of the brain or of the limbs, involves the exhaustion of nervous and muscular matter; and unless the exhausted matter be renewed, the work must come to an end. The supply of heat-giving food may be compared to the supply of fuel for the fire of a steam-engine. By means of this supply the fire is kept alive; but if the fire have nothing to work upon, its energies are wasted or used to the injury of the machine itself. The supply of water, and its continual use (in the form of steam) in the propulsion of the engine, are the processes corresponding to the continual exhaustion and renewal of the muscles and nerves of the human frame. And the comparison may be carried yet further. We see that in the case of the engine the amount of smoke, or rather of carbonic acid, thrown out by the blast-pipe is a measure of the vital energy (so to speak) within the engine; but the amount of work done by the engine is measured rather by the quantity of steam which is thrown out, because the elastic force of every particle of steam has been exerted in the propulsion of the engine before being thrown out through the blast-pipe. In a manner precisely corresponding to this, the amount of carbonic acid gas exhaled by a man is a measure of the rate at which mere existence is proceeding; but the amount of work, mental or muscular, which the man achieves, is measured by the amount of used-up brain-material and muscle-material which is daily thrown off by the body. I shall presently show in what way this amount is estimated.

In the composition of the muscles there is a material called fibrine, and in the composition of the nerves there is a material called albumen. These are the substances42 which are exhausted during mental and bodily labour, and which have to be renewed if we are to continue working with our head or with our hands. Nay more, life itself involves work; the heart, the lungs, the liver, each internal organ of the body, performs its share of work, just as a certain proportion of the power of a steam-engine is expended in merely moving the machinery which sets it in action. If the waste of material involved in this form of work is not compensated by a continual and sufficient supply of fibrine and albumen the result will be a gradual lowering of all the powers of the system, until some one or other gives way,—the heart ceases to beat, or the stomach to digest, or the liver to secrete bile,—and so death ensues.

The fibrine and albumen in the animal frame are derived exclusively from vegetables. For although we seem to derive a portion of the supply from animal food, yet the fibrine and albumen thus supplied have been derived in the beginning from the vegetable kingdom. “It is the peculiar property of the plant,” says Dr. Lankester, “to be able, in the minute cells of which it is composed, to convert the carbonic acid and ammonia which it gets from the atmosphere into fibrine and albumen, and by easy chemical processes we can separate these substances from our vegetable food. Wheat, barley, oats, rye, rice, all contain fibrine, and some of them also albumen. Potatoes, cabbage, and asparagus contain albumen. It is a well-ascertained fact that those substances which contain most of these ‘nutritious secretions,’ as they have been called, support life the longest.” They change little during the process of digestion, entering the blood in a pure state, and being directly employed to renew the nervous and muscular matter which has been used up during work, either mental or muscular. Thus the supply of these substances is continually being drawn upon. The carbon, which forms their principal constituent, is converted into carbonic acid; and the nitrogen, which forms about a sixth part of their substance, re-appears in the nitrogen of urea, a substance which forms the principal solid constituent of the matter daily thrown from the system through the action of the kidneys. Thus the amount of urea which daily passes from the body affords a measure of the work done during the day. “This is not,” says Dr. Lankester, “the mere dream of the theorist; it has been practically demonstrated that increased stress upon the nervous system, viz., brain work, emotion, or excitement from disease, increases the quantity of urea and the demand for nitrogenous food. In the same manner the amount of urea is the representative of the amount of muscular work done.”

It has been calculated that the average amount of urea daily formed in the body of a healthy man is about 470 grains. To supply this daily consumption of nitrogenous matter, it is necessary that about four ounces of flesh-forming substance should be consumed daily. It is important, therefore, to inquire how this substance may be obtained. The requisite quantity of albuminous and fibrinous matter “is contained,” says Dr. Lankester, “in a pound of beef; in two pounds of eggs; in two quarts of milk; in a pound of peas; in five pounds of rice; in sixteen pounds of potatoes; in two pounds of Indian meal; in a pound and a half of oatmeal; and in a pound and three-quarters of flour.” A consideration of this list will show the importance of attending to the quality as well as the quantity of our food. A man of ordinary appetite might satisfy his hunger on potatoes or on rice, without by any means supplying his system with a sufficient amount of flesh-forming food. On the other hand, if a man were to live on bread and beef alone, he would load his system with an amount of nitrogenous food, although not taking what could be considered an excessive amount of daily nourishment. We see, also, how it is possible to continually vary the form in which we take the required supply of nitrogenous food, without varying the amount of that supply from day to day.

The supply itself should of course also vary from day to day as the amount of daily work may vary. What would be ample for a person performing a moderate amount of work would be insufficient for one who underwent daily great bodily or mental exertions, and would be too much for one who was taking holiday. It would appear, from the researches of Dr. Haughton, that the amount of urea daily formed in the body of a healthy man of average weight varies from 400 to 630 grains. Of this weight it appears that 300 grains results from the action of the internal organs. It would seem, therefore, that the amount of flesh-forming food indicated in the preceding paragraph may be diminished in the proportion of 47 to 40 in the case of a person taking the minimum of exercise—that is, avoiding all movements save those absolutely necessary for comfort or convenience. On the other hand, that amount must be increased in the proportion of 74 to 63 in the case of a person (of average weight) working up to his full powers. It will be seen at once, therefore, that a hardworking man, whether labourer or thinker, must make good flesh-forming food constitute a considerable portion of his diet; otherwise he would require to take an amount of food which would seriously interfere with his comfort and the due action of his digestive organs. For instance, if he lived on rice alone, he would require to ingest nearly seven pounds of food daily; if on potatoes, he would require upwards of twenty-one pounds; whereas one pound and a third of meat would suffice to supply the same amount of flesh-forming food.

Men who have to work, quickly find out what they require in the way of food. The Irishman who, while doing little work, will live contentedly on potatoes, asks for better flesh-forming food when engaged in heavy labour. In fact, the employer of the working man, so far from feeling aggrieved when his men require an improvement in their diet, either as respects quality or quantity, ought to look on the want as evidence that they are really working hard in his service, and also that they have a capacity for continuous work. The man who lives on less than the average share of flesh-forming food is doing less than an average amount of work; the man who is unable to eat an average quantity of flesh-forming food, is unable to do an average amount of work. “‘On what principle do you discharge your men?’ I once said,” relates Dr. Lankester, “to a railway contractor. ‘Oh,’ he said, ‘it’s according to their appetites.’ ‘But,’ I said, ‘how do you judge of that?’ ‘Why,’ he said, ‘I send a clerk round when they are getting their dinners, and those who can’t eat he marks with a bit of chalk, and we send them about their business.’” At a lecture delivered at the Royal Museum of Physics and Natural History at Florence, by Professor Mantegazza, a few years since, the Professor dwelt on the insufficient food which Italians are in the habit of taking, as among the most important causes of the weakness of the nation. “Italians,” he said, “you should follow as closely as you can the example of the English in your eating and in your drinking, in the choice of flesh-meat (in tossing off bumpers of your rich wines),43 in the quality of your coffee, your tea, and your tobacco. I give you this advice, dear countrymen, not only as a medical man, but also as a patriot. It is quite evident, from the way millions of you perform the process which you call eating and drinking, that you have not the most elementary notions of the laws of physiology. You imagine that you are living. You are barely prolonging existence on maccaroni and water-melons. You neither know how to eat nor how to drink. You have no muscular energy; and, therefore, you have no continuous mental energy. The weakness of the individual, multiplied many millions of times, results in the collective weakness of the nation. Hence results insufficient work, and thence insufficient production. Thus the returns of the tax-collector and the custom-house officer are scanty, and the national exchequer suffers accordingly.” Nor is all this, strange as it may sound, the mere gossip of the lecture-room. “The question of good feeding,” says Dr. Lankester, “is one of national importance. It is vain to expect either brain or muscles to do efficient work when they are not provided with the proper material. Neither intellectual nor physical work can be done without good food.”

We have now considered the two principal forms of food, the heat-forming—sometimes called the amylaceous—constituents, and the flesh-forming or nitrogenous constituents. But there are other substances which, although forming a smaller proportion of the daily food, are yet scarcely less important. Returning to our comparison of the human system to a steam-engine—we have seen how the heat-forming and flesh-forming constituents of food correspond to the supply of fuel and water; but an engine would quickly fall into a useless state if the wear and tear of the material of which it is constructed were not attended to and repaired. Now, in the human frame there are materials which are continually being used up, and which require to be continually restored, if the system is to continue free from disease. These materials are the mineral constituents of the system. Amongst them we must include water, which composes a much larger portion of our bodies than might be supposed. Seven-ninths of our weight consists simply of water. Every day there is a loss of about one-thirtieth part of this constituent of our system. The daily repair of this important waste of material is not effected by imbibing a corresponding supply of water. A large proportion of the weight of water daily lost is renewed in the solid food. Many vegetables consist principally of water. This is notably the case with potatoes. Where the water supplied to a district is bad, so that little water is consumed by the inhabitants—at least, without the addition of some other substance—it becomes important to notice the varying proportion of water present in different articles of food. As an instance of this, I may call attention to a remarkable circumstance observed during the failure of the potato crops in Ireland. Notwithstanding the great losses which the people sustained at that time, it was noticed that the amount of tea imported into Ireland exhibited a remarkable increase. This seemed at first sight a somewhat perplexing phenomenon. The explanation was recognized in the circumstance that the potato—a watery vegetable, as we have said—no longer formed the chief portion of the people’s diet. Thus the deficiency in the supply of water had to be made up by the use of a larger quantity of fluid food; and as simple water was not palatable to the people, they drank tea in much larger quantities than they had been in the habit of taking before the famine. But we have to consider the other mineral constituents of the system.

If I were to run through the list of all the minerals which exist within the body, I should weary the patience of the reader, and perhaps not add very much to the clearness of his ideas respecting the constitution of the human frame. Let it suffice to state generally that, according to the calculations of physiologists, a human body weighing 154 pounds contains about 17½ pounds of mineral matter; and that the most important mineral compounds existing within the body are those which contain lime, soda, and potash. Without pretending to any strictly scientific accuracy in the classification, we may say that the lime is principally found in the bones, the soda in the blood, the potash in the muscles; and according as one or other of these important constituents is wanting in our food, so will the corresponding portions of the frame be found to suffer.

We have a familiar illustration of the effects of unduly diminishing the supply of the mineral constituents of the body in the ravages which scurvy has worked amongst the crews of ships which have remained for a long period ill-supplied with fresh vegetables. Here it is chiefly the want of potash in the food which causes the mischief. An interesting instance of the rapid—almost startling—effects of food containing potash, in the cure of men stricken by scurvy, is related by Dana. The crew of a ship which had been several months at sea, but was now nearing the land, were prostrated by the ravages of scurvy. Nearly all seemed hopelessly ill. One young lad was apparently dying, the livid spots which were spreading over his limbs seeming to betoken his rapidly approaching end. At this moment a ship appeared in view which had but lately left the land, and was laden with fresh vegetables. Before long large quantities of the life-bearing food had been transferred to the decks of the other ship. The instincts of life taught the poor scurvy-stricken wretches to choose the vegetable which of all others was best suited to supply the want under which their frames were wasting. They also were led by the same truthful instincts to prefer the raw to cooked vegetables. Thus the sick were to be seen eating raw onions with a greater relish than the gourmand shows for the most appetising viands. But the poor lad who was the worse of the sufferers had already lost the power of eating; and it was without a hope of saving his life that some of his companions squeezed the juice of onions between his lips, already quivering with the tremor of approaching death. He swallowed a few drops, and presently asked for more. Shortly he began to revive, and to the amazement of all those who had seen the state of prostration to which he had been reduced, he regained in a few days his usual health and strength.

The elements which we require in order to supply the daily waste of the mineral constituents of the body are contained in greater or less quantities in nearly all the articles which man uses for food. But it may readily happen that, by adopting an ill-regulated diet, a man may not take a sufficient quantity of these important elements. It must also be noticed that articles of food, both animal and vegetable, may be deprived of a large proportion of their mineral elements by boiling; and if, as often happens, the water in which the food has been boiled is thrown away, injurious effects can scarcely fail to result from the free use of food which has lost so important a portion of its constituent elements. Accordingly, when persons partake much of boiled meat, they should either consume the broth with the meat, or use it as soup on the alternate days. Vegetables steamed in small quantities of water (this water being taken with them), also afford a valuable addition to boiled meat. In fact, experience seems to have suggested the advantage of mixing carrots, parsnips, turnips, and greens with boiled meat; but unfortunately the addition is not always made in a proper manner. If the vegetables are boiled separately in large quantities of water, and served up after this water has been thrown away, more harm than good is done by the addition; since the appetite is satisfied with comparatively useless food, instead of being left free to choose, as it might otherwise do, such forms of food as would best supply the requirements of the system. Salads and uncooked fruits, for instance, contain saline ingredients in large proportion, and could be used advantageously after a meal of boiled meat. Potatoes are likewise a valuable article of food on account of the mineral elements contained in them. And there can be no doubt that the value of potatoes as an article of food is largely increased when they are cooked in their skins, after the Irish fashion.

Lastly, we must consider those articles of food which promote the natural vital changes, but do not themselves come to form part of the frame, or, at least, not in any large proportion of their bulk. Such are tea, coffee, and cocoa: alchoholic drinks; narcotics; and lastly, spices and condiments. We may compare the use of these articles of food to that of oil in lubricating various parts of a steam-engine. For, as the oil neither forms part of the heat-supply nor of the force-supply of the steam-engine, nor is used to replace the worn material of its structure, yet serves to render the movements of the machine more equable and effective, so the forms of food we are considering are neither heat-producing nor flesh-forming, nor do they serve to replace, to any great extent, the mineral constituents of the body, yet they produce a sense of refreshment accompanied with renewed vigour. It is difficult to determine in what precise way these effects are produced, but no doubt can exist as to the fact that they are really attributable to the forms of food to which we have assigned them.

Tea, coffee, and cocoa owe their influence on the nervous system to the presence of a substance which has received the various names of theine, caffeine, and theobromine. It is identical in composition with piperine, the most important ingredient in pepper. It may be separated in the form of delicate white, silky crystals, which have a bitter taste. In its concentrated form this substance is poisonous, and to this circumstance must be ascribed the ill effects which follow from the too free use of strong tea or coffee. However, the instances of bad effects resulting from the use of “the cup which cheers but not inebriates” are few and far between, while the benefits derived from it are recognized by all. It has, indeed, been stated that no nation which has begun to make use of tea, coffee, or cocoa, has ever given up the practice; and no stronger evidence can be required of the value of those articles of food.

Of alcoholic liquors it is impossible to speak so favourably. They are made use of, indeed, almost as extensively as tea or coffee; they have been made the theme of the poet, and hailed as the emblems of all that is genial and convivial. Yet there can be little doubt that, when a balance is struck between the good and evil which have resulted to man from their use, the latter is found largely to preponderate. The consideration of these evils belongs, however, rather to the moralist than to the physiologist. I have here simply to consider alcoholic liquors as articles of food. There can be little doubt that, when used with caution and judgment, they afford in certain cases an important adjunct to those articles which are directly applied to the reparation of bodily waste. Without absolutely nourishing the frame, they ultimately lead to this end by encouraging the digestive processes which result in the assimilation of nutritive articles of food. But the quantity of alcohol necessary to effect this is far less than is usually taken even by persons who are termed temperate. It is also certain that hundreds make use of alcoholic liquors who have no necessity for them, and who would be better without them. Those who require them most are men who lead a studious sedentary life; and it is such men, also, who suffer most from excess in the use of alcoholic liquors.

It remains that I should make a few remarks on mistakes respecting the quantity of food.

Some persons fall into the habit of taking an excessive quantity of food, not from greediness, but from the idea that a large amount of food is necessary for the maintenance of their strength. They thus overtax the digestive organs, and not only fail of their purpose, but weaken instead of strengthening the system. Especially serious is the mistake often made by persons in delicate health of swallowing—no other word can be used, for the digestive organs altogether refuse to respond to the action of the mouth—large quantities of some concentrated form of food, such as even the strongest stomach could not deal with in that form. I knew a person who, though suffering from weakness such as should have suggested the blandest and simplest forms of food, adopted as a suitable breakfast mutton-chops and bottled stout, arguing, when remonstrated with, that he required more support than persons in stronger health. He was simply requiring his weak digestive organs to accomplish work which would have taxed the digestive energies of the most stalwart labourer working daily in the open air for many hours.

On the other hand, a too abstemious diet is as erroneous in principle as a diet in excess of the natural requirements of the system. A diet which is simply too abstemious is perhaps less dangerous than persistent abstinence from the use of certain necessary forms of food. Nature generally prevents us from injuring ourselves by unwisely diminishing the quantity of food we take; but unfortunately she is not always equally decided in her admonitions respecting the quality of our food. A man may be injuring his health through a deficiency in the amount either of the heat-forming or of the flesh-forming food which he consumes, and yet know nothing of the origin of the mischief. It may also be noted that systematic abstinence, either as respects quantity or quality of food, is much more dangerous than an occasional fast. Indeed, it is not generally injurious either to abstain for several days from particular articles or forms of food, or to remain, for several hours beyond the usual interval between meals, without food of any sort. On the contrary, benefit often arises from each practice. The Emperor Aurelian used to attribute the good health he enjoyed to his habit of abstaining for a whole day, once a month, from food of all sorts; and many have found the Lenten rules of abstinence beneficial. As a rule, however, change of diet is a safer measure than periodical fasting or abstinence from either heat-producing or flesh-forming food. It must be noticed, in conclusion, that young persons ought not, without medical advice, to fast or abstain for any length of time from the more important forms of food, as serious mischief to the digestive organs frequently follows from either course.