If cream instead of milk is used on the cereal or in the coffee, this with the butter on the bread, will add a considerable amount of another important food, fat. Fats form a large class of food stuffs which include the animal fats like cream, butter, suet, lard, cod liver oil and tallow, and vegetable fats like olive and cotton-seed oils, etc. Within the animal body ail fats are liquids, being held in little cells which make up the fatty tissue.

The digestion of fats is probably something like a process of soap making. With the intestinal fluids, the bile especially, the fats form an emulsion in which the globules are finely divided, and in some way are rendered capable of passing through the membranes into the circulatory system. The change, if any, does not destroy the properties of the fatty matters.

If we define cooking as the application of heat, then whatever we do to fats in the line of cooking is liable to hinder rather than help digestibility.

Fats may be heated to a temperature far above that of boiling water without showing any change; but there comes a point, different for each fat, where reactions take place, the products of which irritate the mucous membranes and therefore interfere with digestion. It is the volatile products of such decomposition which cause the familiar action upon the eyes and throat during the process of frying, and also, the telltale odors throughout the house. The indigestibility of fatty foods, or foods cooked in fat, is due to these harmful substances produced by too high temperature.

Many fats are solid at ordinary temperatures, while others are always liquids, but all fatty materials have a similar composition. When pure they contain only carbon, hydrogen, and oxygen. They differ from starch and sugar in the proportion of oxygen to the carbon and hydrogen, there being very little oxygen relatively in fats, hence more must be taken from the air for their combustion. If persons eat much fat they must have more fresh air to burn it. A person confined to the house needs to be careful what fats, and how much, are taken.

One pound of starch requires one and two-tenths pounds of oxygen, while one pound of suet requires about three pounds of oxygen for perfect combustion. This combustion of oxygen with the large amount of hydrogen, as well as with the carbon, results in a greater quantity of heat from fat, pound for pound, than can be obtained from starch or sugar. Experiments indicate that the fats yield more than twice as much heat as the carbohydrates; hence people in Arctic regions use large amounts of fat and everywhere the diet of winter may safely contain more fat than that of summer.

Composition of Fats

Heat from Fats

Both fats and carbohydrates are the sources of the energy or work done by the body as well as the heat to keep up the vital temperature and they must be increased in proportion as the mechanical work of the body increases. A man breaking stone needs more fat or starch than the student. If a quantity is taken at any one time greater than the body needs for immediate work, the surplus will be deposited as fat, and this will be drawn in case of a lack in the future supply of either; it is like a bank account.