While the fats are carbonaceous foods, the nutrient material of fats is quite different from that of starches and sugars, as explained above. The fats contain a large proportion of carbon and a proportion of oxygen far too small to oxidize the hydrogen of the compound. For that reason, as explained above, the oxidation of fats yields a far larger quantity of heat than does the oxidation of the same weight of sugar or starch.

Fats are comparatively easily assimilated. They are digested in the small intestine so they pass through the mouth and stomach with no digestive change. However, to facilitate their ready passage out of the stomach, all fatty tissues such as fat meat should be very thoroughly masticated. Within the stomach these fatty tissues are broken up by the digestion of the connective tissue, thus releasing the fat globules which become more or less emulsified within the stomach contents and pass through the small intestine, where they are subjected to the action of the mild but copious carbonates of the intestinal juices, and to the ferment action of lipase (steapsin). The combined action of these digestive agents changes the fat into an emulsion and eventually saponifies it, thus releasing the glycerin of the fat molecule and reducing the fat to soluble components, soap and glycerin.

Thus dissolved, the products are absorbed, changed back into fat and carried by the circulation out to the working tissues, where they are assimilated and for the most part used by the tissues as sources of energy. However, a portion may be deposited in the body in the form of fat, and held in reserve for future use.

The chemical subdivision of fats into palmitin, stearin, and olein, as given above, has little dietetic value; a more practical classification would be into animal and vegetable fats. The animal fats comprise butter, which is practically pure milk fat; and cream, which contains a considerable admixture of protein, and of sugar, analysis being water 66 per cent, protein 2.7 per cent, sugar 2.8 per cent, salts 1.8 per cent, and fat 26.7 per cent. While there is considerable uniformity in the composition of butter, being ninety-eight per cent fat, there is considerable variation in the combination of cream, the variation arising from the quality of the milk, and the method of gathering. If rich Jersey milk is chilled as soon as drawn and allowed to stand twenty-four hours in open crocks, the cream which rises has almost the consistence of butter, and it may be cut with a spoon or spread with a knife; but cream which is separated from milk of only moderate richness by a centrifugal separator has only slightly greater consistency than the whole milk produced by a Jersey cow. It is, however, deeper in color and conforms approximately to the above analysis in composition.

Meat Fat

The fat of beef, mutton, and pork is an important source of diet fat. As a rule, the fat is so closely associated with the lean that it is taken more or less incidentally along with the proteins of the lean meat. However, its quantity can be in a general way determined by inspection and more accurately determined by actual separation of the fat and weighing it. Meat fats should be very thoroughly cooked in the presence of moisture, if they are to be easily digestible. This is readily accomplished in the usual method of cooking meats in which there is a large admixture of fat. When thoroughly cooked the connective tissue of the fat is readily digestible and is thoroughly broken up in the stomach, thus releasing the fat in the globules. When fat is thus thoroughly cooked it is very easy of mastication and digestion. Uncooked or rare fat is much more difficult both of mastication and of digestion. Certain meat fats, particularly side pork and bacon, as usually found in the market, possess so small a proportion of protein that that foodstuff can be practically ignored and such meats classified not among the protein foods, but among the fats.

The usual method of preparing bacon is to fry it in a pan, or bake it in the oven in thin slices. In either case it should be done brown but not cooked to the point of frying out most of the fat and making it brittle. When thus prepared and thoroughly masticated along with bread or potatoes, bacon becomes a most wholesome and easily assimilated source of fat.

Cod liver oil, as the name suggests, is derived from the liver of the cod and differs from the common food fats in possessing an admixture of bile salts and certain other materials which give it an odor and flavor not pleasing to most people. However, this fat is more easily absorbed and assimilated than any of the other fats. It has, therefore, been resorted to very extensively as a nutrient in cases of wasting diseases and malnutrition, as in tuberculosis, etc.

The pure oil is perhaps better to adminster than any of the numerous emulsions exploited. If unpalatable, it may be made less so by adding to it a few drops of lemon juice, a pinch of salt, some strong coffee, or some maple syrup. It is often given in flexible capsules.

Vegetable Fats

The most common of the vegetable fats is olive oil, which is derived from ripe olives by reducing them to a pulp from which the oil is pressed out. Olive oil is nearly pure olein, and because of the sweet taste which it possesses when very fresh, has been called "sweet oil." It is a most wholesome form of fat and is used very largely in salads and salad dressings. It may be substituted for cod liver oil whenever the latter is indicated. Cottonseed oil, derived, as the name would suggest, from cotton seed, does not possess the sweet taste typical of fresh pure olive oil. It is, however, largely used as a substitute for olive oil, and when perfectly fresh is not objectionable.

Nut oil, for example peanut oil, is derived from the nuts by pressure. These oils are wholesome and may be substituted for olive oil in the dietary. They are not, however, in general use.