The last case we have to consider is that in which the supply of food material is in excess of the requirements of the economy. This is certainly the commonest case in man.
Much of the surplus food never really enters the system, but is conveyed away with the faeces.
In speaking of pancreatic digestion, reference has been made to the possible destiny of excess of nitrogenous food. In the intestine, some of it is decomposed into leucin and tyrosin, which are absorbed into the intestinal blood vessels. In the body these substances undergo further changes, which probably take place in the liver. As a result of the absorption of leucin, a larger quantity of urea appears in the urine, and hence the leucin formed in the intestine by prolonged pancreatic digestion is an important source of urea. This view is supported by the almost immediate increase in the quantity of urea eliminated when albuminous food is taken in large quantity.
From the fact that a considerable amount of fat may be stored up by an animal supplied with a liberal diet of lean meat, we must conclude that part at least of the surplus albumin goes to form fat. It has been suggested that, after sufficient albumin has been absorbed for the nutritive requirements of the nitrogenous tissues, the rest is split up into two parts, one of which is immediately prepared for elimination as urea by the liver, and the other undergoes changes, probably in the same organ, which result in its being converted into fat.
It would further seem probable, from the manner in which the urea excretion changes during starvation, that, as before mentioned, the absorbed albumin exists in the economy in two forms: one in which it has been actually assimilated by the nitrogenous tissues and forms part of them, and hence is called organ albumin; the other, which is merely in solution in the fluids of the body, being in stock, but not yet ultimately assimilated, and hence called circulating albumin. The latter passes away during the first few days of starvation, being probably broken up to form urea, and a material which serves the turn of non-nitrogenous food. The organ albumin appears to supply the urea after the circulating albumin has completely disappeared.
From the foregoing it will be gathered that we cannot say what are the exact destinies of the various food stuffs in the body. Proteids are not exclusively utilized in the re-integration of proteid tissues, as an excess gives rise to a deposit of fat. Carbohydrates are not employed simply to replace the carbohydrates constituting part of the tissues, but, as will be shown when speaking of muscle metabolism, they are intimately related to the chemical changes which take place during the activity of that tissue. If fats are chiefly devoted to the restitution of the fat of the body, they certainly are not the only kind of food from which fat can be made.
We may say, then, that all food stuffs are destined to feed the living protoplasm, whether it be in the form of gland cells, the cells of the connective tissues, or muscle plasma, so that all the food stuffs that are really assimilated contribute to the maintenance of protoplasm and subserve its various functions.
Besides nourishing itself and keeping itself up to a certain standard composition, protoplasm, or rather the various proto-plasmata, can make the different chemical materials we find in the body. Some produce fat, some animal starch (glycogen), and others manufacture the various substances we find in the secretions; while yet another group is devoted to setting free and utilizing the energy of the various chemical associations.
But all the food we eat is not assimilated; indeed, the destiny of the numerous ingredients of our complex dietaries is not easily traced. Of food stuffs proper, the following classification may be made, showing that even the same stuff may meet with a different fate under different circumstances: i. Stuffs which never enter the economy (faeces).
2. Materials absorbed and arriving at the blood are at once carried to certain portals of excretion (excess of salts).
3. Substances which are broken up in the intestine to facilitate their elimination (excess of proteid).
4. Substances absorbed and carried along by the fluids, but not really united to the tissues (circulating albumin).
5. Materials which after their absorption are really assimilated by the protoplasm of the tissues (a certain amount of all food stuffs).
6. Substances which, after their assimilation by the protoplasm, reappear in their original form and are stored up (fats). .
The question of the exact amounts and materials required to form the most economic and wholesome dietary is one of too great practical importance to receive adequate attention in this manual. As a rule, men, like other animals, partake of food largely in excess of their physiological requirements when they can get it. This may be seen by contrasting one's own daily food with the amount which has been found to be adequate in the case of individuals who have not the opportunity of regulating their own supplies of comestibles.
An adult man should be well nourished if he be supplied with the following daily diet: -
Albuminous foods, .....
. . 100 grms. or 3.5 ozs.
90 " " 3.1 "
Starch,......, . . .
. . 300 " " 10.7 "
30 " " 1.0 "
2000 " " 3 pints.
As a matter of fact, many persons do thrive on a much less quantity of proteid than that given in this table, but in their cases the fats and starches should be proportionately increased.
Such a dietary could be obtained from many comestibles alone, and hence the taste of the individual may be exercised in selecting his food without much departing from such a standard. Individual taste commonly selects foods with too much proteid - 1. e., an excess of nitrogen - while the cheapness of vegetable products dictates their use in greater abundance as food.
Compare Chap, v, p. 101, where the quantity of the different food stuffs in some of our common articles of diet is given.