Proteid foods are pre-eminently important, as they construct and keep in repair the tissues of the body. They are not used solely for this purpose. A large share of the energy of the body is derived from the metabolism of proteids. The amount required for these purposes will be discussed on page [32]. Meanwhile, it may be said that it is not found to be compatible with efficient health simply to supply an amount of proteid food which will suffice to replace the wear and tear of the tissues, leaving fats and carbohydrates to supply the energy of the body. Deficiency of proteid food always leads to ill-health; and it would appear that in all cases proteid food determines, to a large extent, the metabolism of non-nitrogenous food, and so is favourable to all vital action. The action of nitrogenous food in thus increasing metabolism may make it, when in relative excess, a tissue waster. Banting’s cure for corpulence is founded on this principle, lean meat alone being taken, all starchy and saccharine foods being carefully avoided.
By metabolism is meant the changes undergone by food before it reaches the state in which it is finally eliminated from the body. It is commonly spoken of as oxidation, but this word less exactly represents the facts. The complexity of the changes undergone by food in the body may be better appreciated by a glance at the following schematic statement, which only gives an approximation to the truth:-
| HYPOTHETICAL ALBUMINOID. | TYPICAL FAT (STEARIN). | STARCH. | GRAPE- SUGAR. |
| C₇₂H₁₁₂N₁₈SO₂₂. | C₃H₅(C₁₈H₃₅O₂)₃. | x(C₆H₁₀O₅). | (C₆H₁₂O₆). |
| ─────────────────────┬──────────────────── | |||
| Various intermediate products, which are finally broken down into and eliminated as | |||
| ┌──────────┬────────────┐ | ||
| Urea, CH₄N₂O. | Carbonic acid, CO₂. | Water, H₂O.> |
Hydrocarbons, or fats, consist of three elements, carbon, hydrogen, and oxygen, the amount of oxygen present not being sufficient to oxidise completely either the hydrogen or the carbon. Thus the molecule of stearin, which may be taken as a typical fat, has the formula C₃H₅ (C18H35O₂)₈.
In respect to their comparatively unoxidised condition fats compare favourably with starch and sugar, C₆H10O₅ and C₆H12O₆ respectively. It is evident that in starch the H10O₅ = 5H₂O, and that in sugar H12O₆ = 6H₂O, so that in both cases only carbon remains uncombined with oxygen. Dried fats produce by their oxidation 2¼ times as much heat as a corresponding amount of sugar or starch; but the relative advantage of fat is not quite so great as would appear from this comparison, inasmuch as metabolism within the body is not identical with oxidation.
The fat obtained from food is not simply deposited in the body as such, to form a store of combustible matter, and to fill up the interstices between the different tissues. If this were so, the kind of fat deposited would vary with the food, which is not the case. The fat of the body is probably not formed directly from fatty food, but as the result of the metabolism of nitrogenous foods when this metabolism is incomplete. In the formation of milk this can be distinctly proved: the fat cells are formed from the protoplasm of the cells of the mammary gland.
Possibly carbohydrate food may be a source of fat, as well as nitrogenous and fatty food. This appears to be the case in the Strasburg goose, which is kept penned up in a warm room, and fed entirely on barley-meal, in order to produce an enormous fatty liver for the delicacy termed pâté de foie gras. But it may be that the large accumulation of fat in the liver is due to the warmth and inaction of the goose diminishing metabolism, and producing a fatty degeneration of the nitrogenous material of the liver.
Fats and carbohydrates, unlike proteids, do not excite metabolism in the system, and so, if in excess of the requirements of the system, can be stored up with comparative ease. Quiet and warmth, diminishing metabolism, conduce to the accumulation of fat in animals being fed for the market; and the same applies to human beings.
Carbohydrates or amyloids include the various starchy and saccharine foods. They are inferior to fats in nutritive power, but, being very digestible, are in much greater favour. In the process of digestion, starch is converted into grape sugar, and starch and sugar are practically equal in nutritive power.