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The second member of the organic food group, and one which is almost as widely distributed throughout animal and vegetable life as the carbohydrates, is found in the fats. This foodstuff, while composed of the same chemical elements that go to make up the carbohydrates, contains these elements in different proportions; that is, fats contain less oxygen and more hydrogen than carbohydrates.

Typical Fats.—The fats (as already shown in the Table on page 5) are derived from both animal and plant life, but, like the carbohydrates, do not always occur in the same form. Those of animal origin include:

Adipose Tissue of man and animals, tallow of mutton, suet, and oleo oil of beef, lard of pork.

Phosphorized Fats, which include lecithin and lecithans, occur abundantly in the brain and nerve tissues and to a less extent in the cells and tissues of man, animals, and plants of which it seems an essential part. Egg yolk is the most abundant source of phosphorized fat in food material, but milk likewise furnishes an appreciable amount.

Cholesterol (fat-like substances).—“The fatty secretions of the sebaceous glands of man and of the higher animals which furnish the natural oil for hair, wool and feathers,” (Starling), lanoline, which is a purified wool fat, consist chiefly of cholesterol. According to Mathews, cholesterol is an essential constituent of the blood, and is found in the brain and in nearly all living tissues. It is likewise believed to be the “mother substance” from which bile acids are derived.

Fat Soluble “A.”—The vitamine factor which occurs dissolved in certain fats, namely, milk (whole), butter, egg yolk, the organs of animals, and codfish liver.

Definition of Fat.—The fats are all glycerides; that is, they are substances made up of combinations of fatty acids and glycerine, which constitute a definite group of chemical compounds, certain members of which are liquid in form, while others are solid, or semi-solid. The liquid fats are known as fatty oils. The fatty acids in which we are chiefly concerned in this study are: Butyric, Stearic, Oleic, and Palmitic. Most of the common fats owe their form and flavor to the type and amount of the various fatty acids of which they are composed. For example, butter is made up of ten fatty acids; but its soft, solid form is due to the olein and palmitin (glycerides of oleic and palmitic acids) which it contains; and its characteristic flavor, as well as its name, to its butyric acid content (about 5 to 6%). It is evident that the degree of softness or hardness of a fat may be determined chiefly by the amount of oleic acid in its composition. Most of the common oils with which we are familiar in food are composed chiefly of olein. Stearin (the glyceride of stearic acid) is the hardest of the fatty acids, while palmitin, although classed with the solid fats, is not so hard as stearin. Lard and butter are higher in olein and palmitin and are consequently semi-solid, while suet and tallow, consisting chiefly of stearin, are much harder than the other food fats.

Characteristics of Fats.—The fats are all insoluble in water, and only partially so in cold alcohol, but they dissolve readily in ether. As a rule, the fat occurring in the animal body is more or less characteristic of the species. For example, animals that live on land have a harder fat than those living in the water; warm blooded animals, harder fats than cold blooded ones (fish); and carnivorous animals, harder fats than herbivorous species.

Fats are lighter than water, hence will float in it. An emulsion is a suspension of fat in a fluid, and the fat in this case must be very finely divided and mixed with some other material which will prevent a coalescence of the fat globules. In milk, which is one of the best natural emulsions, the additional substance is protein.

Effect of Heat upon Fat.—When fats are brought to a high temperature, the glycerine which they contain decomposes with the production of a substance known as acrolein, which has an irritating effect upon the mucous membranes. It is possible that the over-heated fatty acids add their quota to the production of irritating fumes. As a rule, it is inadvisable to use frying as a method of preparing food for the sick or for children. Doubtless, if every cook understood the exact degree of heat to apply in frying, and knew just how moist to have the food mixture which she intended to cook in this manner, better results would be obtained; but since the average cook knows little about the scientific application of heat to fat or the changes brought about thereby, it is safer to make use of other methods of food preparation under the circumstances.

Functions of Fat.—This foodstuff undoubtedly serves as the most compact form of fuel available to the body for the production of energy. Weight for weight, fat furnishes twice as much heat as the carbohydrates, and in bulk the difference is even more striking; for example (about) two tablespoonfuls of sugar are required to produce 100 calories, whereas one scant tablespoonful of olive oil will produce a like number of heat units. As a source of supply for reserve energy in the body, fat is most valuable. This reserve fuel is stored in the form of adipose tissues underlying the skin and surrounding the vital organs, lending contour to the form and protecting the organs from jars and shocks. Distributed throughout the body, fat may be found as (a) cholesterol (in the cells of the muscles, organs, and nerve tissues), which acts as a protection against the destruction of the red blood cells; (b) phosphorized fat (lecithin), the universal distribution of which, according to Starling, seems to indicate that it plays an important part in the metabolic process of the cells, serving as a source of phosphorus which is required for the building up of the complex nucleoproteins of the cell nuclei.