It not infrequently happens that an amount of food more than sufficient to supply the needs of the body is ingested. Among animals, which are subjected to the conditions of the change of season, it is very important that they store up the food excess of the summer time to tide them over the food shortage of the winter time. So Nature has devised an interesting method which consists in the deposit of food excess in the form of fat. Experiments have shown that these food reserves, or fat reserves, may be built up from any food source - that is, fat for deposit may be built up from ingested fats, from carbohydrates, and even from proteins.

Reserve fat built up from food fats requires only to be deposited in the form peculiar to the species concerned - that is, the fat deposited in the human subject must contain the stearin, pal-mitin, and olein in the proportions peculiar to human fat, though these neutral fats are ingested in the proportions very different, some coming from vegetable oils and some from milk and butter, and others from animal fats.

Reserve fat built up from carbohydrates represents an anabolic process in which the comparatively simple monosaccharid molecule is step by step built up into the rather complex fatty acid molecule, and these later combine into the neutral fats by uniting with glycerin. This anabolic process of building the fats out of monosaccharids is probably done in the liver, and it results in the liberation of free oxygen. It is not unlikely that this liberation of oxygen in the liver greatly facilitates the action of the oxidases in that organ, and may account for the fact that the great activity of oxidases, as in the habitual and repeated oxidation of alcohol, is associated with the formation of fat and its tendency to deposit in that organ, the fatty liver being a clinical phenomenon very widely and not infrequently observed.

Reserve fat built from proteins is probably the result of a cleavage of the protein molecules. These cleavages are, as a rule, easier to accomplish in the general metabolism than the building up. Just how these take place, however, in the formation of fats from proteins cannot be determined until we know the molecular construction of the protein molecule.