CHAPTER XXIII.

NUTRITION.

We can compare the incomings and outgoings of the economy, and should now be in a position to see what light can be thrown by this comparison upon the actual changes which take place in the textures of the body.

We have seen that the income is made up of substances belonging to the same groups of materials as are found in the body, viz., albumins, fats, carbohydrates, salts, and water, introduced by the alimentary canal, and oxygen, which is acquired by the respiratory apparatus; while the outgoings consist of urea from the kidneys, carbonic acid from the lungs, certain excrement from the intestine and other mucous passages, sweat, sebaceous secretion, epidermal scales, from the skin; together with a quantity of water from all these ways of exit. The milk, ova, and semen may be here omitted, being regarded as exceptional losses devoted to special objects.

In order that the body may be kept in its normal condition, it is necessary that the income should at least be equal to the outgoings of all kinds, and, except where growth is going on rapidly, an income equal to the expenditure ought not only to suffice, but ought to be the most satisfactory for the economy.

We know that animals can live for some considerable time without food, in which case a certain expenditure of material derived from the body itself is necessary to sustain life, and therefore the outgoings continue. We ought thus to be able to arrive in a very simple manner at the minimal expenditure necessary for the sustentation of the body. We shall find, however, that (1) an income equal to this minimal expenditure (that of starvation) does not at all suffice to keep up the body weight, and that (2) a considerable margin over and above this minimum is necessary in order to establish the nutritive equilibrium ; (3) further, that the proportion of material eliminated and stored

up in the body respectively varies as the income is increased; (4) and finally, that the quality of the food--i. e., the proportion of each group of food stuff present in the diet--has an important influence on the quantity required to establish the equilibrium, and that best suited to cause increase of weight or to fatten.

It will be convenient to consider the following different cases in succession.

1. No income, except oxygen, i. e., starvation.

2. An income only equal to the expenditure found during

starvation.

3. Establishment of perfect nutritive equilibrium. 4. Excessive consumption.

TISSUE CHANGES IN STARVATION.

As is well known, deprivation of oxygen-by cessation of the respiratory function-almost immediately puts an end to the tissue changes necessary for life, so that the oxygen income cannot be interfered with, or the experiment comes to an end. It has also been found that a small supply of water to drink makes the investigation of the various tissue changes more reliable, by facilitating them and prolonging life. We therefore speak of a total abstinence from solids as starvation.

When deprived of food, those tissues upon the activity of which life immediately depends must feed upon materials stored up in some tissues of less vital importance to the animal. The first questions to discuss are how much the body loses daily in weight during the time that it is thus feeding on itself, and how far the different individual tissues contribute to this loss.

The general loss of weight is directly estimated by weighing the animal, and the loss of the individual tissues is calculated by a careful analysis of the various excreta, by which the exact amount of nitrogen, carbonic acid, etc., is ascertained: the nitrogen corresponds to the loss of muscle; and the carbon (after excluding that portion which is the outcome of muscle change, which may be calculated from the nitrogen) corresponds to the fats oxidized.

Loss of Weight.-It has been found that a starving animal

loses weight rapidly at first, and subsequently more slowly. The cause of this difference is that the food last eaten continues to have influence during the first three or four days, and the materials eliminated are proportionately large in quantity. When the influence of the food taken prior to starvation has ceased, the daily amount of materials eliminated is much reduced, and remains nearly constant, decreasing slightly in proportion as the body weight diminishes slowly until the animal's death.

Adult animals generally live until they have lost about half of their normal body weight. Young animals die when they have lost about 20 per cent. of their weight.

Relative Loss in Various Tissues.-Roughly speaking, we may take the body of a man to be made up of the following proportions of the more important textures:

Seeing that the muscle tissue contributes such a large proportion to the body weight, we cannot be surprised that in starvation the greatest absolute loss occurs in this tissue, except in the case of excessively fat animals. Next comes adipose tissue, which almost entirely disappears, so that the relative loss is here greatest, but the absolute loss varies in proportion to the fatness of the animal at the beginning of the investigation. The spleen and liver lose more than half their weight, and the amount of blood is greatly reduced. The smallness of the loss that occurs in the great nervous centres is very striking. They seem to feed on the other tissues.

The following table gives the approximate percentage of loss which takes place in each individual tissue during starvation :

:

With regard to the portals by which the various materials make their escape, it has been found that practically all the nitrogen

passes off with the urine, and about nine-tenths of the carbon escapes by the lungs as CO2, the remaining one-tenth passing off by the intestine and kidneys. Three-fourths of the water is found in the urine, and one-fourth goes off from the skin and lungs.

The following table shows the items of the general loss, and the amount per cent. which passes out by the chief channels of exit :

As the loss of weight of an animal's body during starvation is at first rapid and then more gradual, so also the amount of material eliminated is found to diminish much more slowly after the first few days. This is well seen from the nitrogenous elimination. For the first four days the fall in the amount of urea excreted is very rapid, it then decreases slowly and almost constantly until the death of the animal. The subsequent fall is in proportion to the slow decrease in weight of the animal. This has led to the conclusion that the nitrogenous material eliminated during a full diet comes partly from used-up nitrogenous tissues, and partly from nitrogenous materials which have never really entered into the composition of the tissues, but are the surplus of nitrogenous food. Hence, two kinds of proteid are supposed to exist in the body, viz., (1) that forming part of the tissues, and (2) that circulating as a ready supply for the nutritive demands of the tissues.

AN INCOME EQUAL TO THE OUTPUT OF STARVATION.

In the second case mentioned, namely, where an amount of food is supplied which is just equal to the expenditure which was found to take place during starvation, one might suppose that the diet, though minimal, would yet suffice to preserve the nor

mal body weight. Practice, however, shows this to be far from being the case.

An animal fed on diet equal in quantity to the outgoings during starvation continues to lose weight, and the quantity of nitrogenous substance eliminated (urea) is in excess of the low standard found during complete abstinence from food. From this it would appear that even when supplied with an amount of nitrogenous material equal to that used by the tissues during starvation, an animal takes a further supply from its own textures, and eliminates some of the nitrogenous nutriment without using it. The body subsists on the scanty allowance of nutriment it borrows from the tissues during starvation only so long as there is absolutely no food income. When food is supplied, an increased expenditure is set up, the income is exceeded, and a deficit occurs in the nitrogen balance. Or, probably, some of the nitrogenous nutriment is rendered useless by the processes it undergoes in the intestine, even when the quantity is not sufficient to support the equilibrium (compare pp. 165, 166, 409, 410).

It follows, then, that feeding an animal on an amount of food stuffs exactly corresponding to the quantity of nutriment abstracted from its own textures during total abstinence is only a slower form of starvation.

With regard to nitrogenous substances, it has been proved that nearly three times as much as the amount eliminated during starvation is required to establish an equilibrium between the income and expenditure of those special substances, and that less than this leads to a distinct nitrogenous deficit.

NUTRITIVE EQUILIBRIUM.

The third case mentioned, viz., that in which the nutritive equilibrium is exactly maintained, so that the body weight remains unaltered, is the most important one for us to determine, since its final settlement would enable us to fix the most beneficial standard of diet. Unfortunately, this case is also the most difficult upon which to come to a satisfactory conclusion, for the following reasons :—

1. The elaborate nature of the conditions imposed during the