The past few years have been a period of great and fruitful activity for the biological chemist. When one surveys the results that have been amassed, one feels that the requisites of diet are so numerous as to make failure in nutrition altogether more probable than success; it seems scarcely possible that all the known demands shall be satisfied. But as a corrective upon this impression, we have before us the fact that the human race, without scientific guidance, has maintained through the ages a fair measure of health and power.
In view of this one may incline to think that scientific discoveries in the field of dietetics are academic rather than practical. A judicial mind will hold to a middle course. There need he no discouragement because the requirements are so many; on the other hand, it is not the part of wisdom to discount all that has come from the chemist’s patient labor.
Even though mankind has survived, we can concrive that human standards may he bettered. If this is true of the race, it is much more conspicuously true of ill-nourished individuals. We must consider in what respects the conditions of modern life have modified habits of diet, and whether, in any instances, the changes have been for the worse.
The ease is paralleled by that of the man who begins to consider the possibilities of disease. There are many maladies to be thought of that the retention of health seems like keeping one’s balance on a tight-rope. The preservation of health is really a tight- rope performance, there is a factor which gives constant support, the remarkable stability of health in the normal subject – his constitution which is like a gyroscope in its steadying effect. Here, too, it is rational not to ignore the dangers, but at the same time to recognize our relative security.
Adequacy of diet is a matter both of quantity and composition. The two phases of the question can be considered to greater advantage, if we first briefly recall the purposes which food subserves. These may be said to be three growth, repair, and operation. It might be thought that repair would prove to be typical growth, offset by disintegration, but we have recent evidence that growth has features distinguishing it quite clearly from the processes of maintenance.
In the child, a moderate part of the diet is incorporated into the increasing mass of the tissues. Later, when these are no longer on the increase, a rather small, but perfectly definite, fraction of the food taken is still devoted to compensation for their wear and tear. But first and last, the major part of the foot! serves for the operation of the mechanism, and can be correctly described as fuel.
The body may be likened to a house which had to be built from certain materials, maintained by other supplies, but which is operated, day by day, at the co.st of vast quantities of coal. It is clear that, in the long run, the coal may greatly outweigh the machinery and the structure which houses it.
Our analogy is faulty, of course, particularly in that the power-house is not operated until it has been completed, while the human organism active from early embryonic life, burning fuel, and setting the energy free while its construction goes steadily on. There is another report, however, in which our comparison is entirely justified. For purposes of construction it is necessary to have precisely the right materials; among fuels there is a greater possibility of sub- stitution.
Just as the plan of,the builder calls for wood in one place, steel in another, and glass in a third, so the development of the human frame requires a larger number of distinct and specific supplies than we realized a short time ago.
Plainly, the diet must furnish material adapted, first to make, and always to maintain, the sum of all the tissues. In other words, it must bear a certain likeness to the body it is to nourish. But it need not be rigidly similar, for the cells have a capacity to transform certain compounds into others. Thus it is possible to make haemoglobin, the valuable red pigment of the blood, from vegetable substances^ which seem quite remote from it in their chemical nature.
A hundred years ago physiologists thought that animals had to obtain all their necessary constituents, ready-formed, from the plant world. This is by no means the case; many syntheses are carried out by animal tissues yet the constructive power has its limits, and sometimes are unexpectedly manifested.
There are two standards by which, in the past, diets have been appraised, one of these is the fuel value. It is clear that this one criterion which must be satisfied, though it does not by itself show that a ration is suilicient; it is merely a measure of quantity, and not an indication of suitable composition. The unit of fuel-value is the calorie, which is primarily a standard quantity of heat.
The heat value of an average diet may be set down as twenty-five hundred calories per day. This is for an individual doing hut little physical work. The allowance for such subjects may be scaled down to two thousand but must be increased for those whose labor is heavy. Farmers, the world over, seem to require about thirty-five hundred calories. A maximum in the vicinity of seven thousand has been recorded for the Maine lumberman.
It is interesting to note that an alcohol lamp, burning a pint of its proper fuel in twenty-four hours, is as large a source of heat and, potentially, of other energy as an average man.
The second standard by which diets have been judged is their protein content. Proteins are the compounds in our foods which most nearly resemble the leading constituents in muscles, glands, and living tissues generally. There is no doubt of their peculiar importance for growth and for the upkeep of the organs. But it has proved difficult to fix upon the ideal amount for these services.
Most people, choosing their food with no thought of its nature and guided only by appetite, take from two to three ounces (fifty-six to eighty-four grains) daily. It has com-monly been held that a selection so widely concurred in cannot he far wrong.
We may assume that our remote ancestors had a restricted choice of food, that they were frequently on short commons, and that they were seldom tempted to eat merely to enjoy varied flavors. Appetites adapted to their lot would lead to over-consumption if inherited by descendants having access to food of many kinds. This argument may he applied either to the question of total quantity or to that of the protein allowance.
If proteins were of no use but for construction like the bricks, tiles, and glass brought to our hypothetical power-house, there would evidently be no value in an excess over the current requirement. They are actually more adaptable than this; they are more like lumber which can be turned to account as a fuel supply if not needed for building material.
A strong suggestion is conveyed that such use of proteins is somewhat extravagant, and this is probably just. The objections to the consumption of much protein food may be concisely stated.
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