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. We331 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.332 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.333 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 in334 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 to335 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 compensated336 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 demand337 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. It338 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............