HYPOTHETICAL TYPICAL FAT ALBUMINOID. (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 (C_{ 18}H_{ 35}O₂)₈.
In respect to their comparatively unoxidised condition fats compare favourably with starch and sugar, C₆H_{ 10}O₅ and C₆H_{ 12}O₆ respectively. It is evident that in starch the H_{ 10}O₅ = 5H₂O, and that in sugar H_{ 12}O₆ = 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.