Fats are natural oils, solid at the ordinary temperature, which occur in plants and animals. They are all compounds of glycerin with various fatty acids. The animal fats are to be regarded as ethers formed by replacing the removal atoms of hydrogen in the triatomic alcohol glycerin, , by the acid radicles of the acetic and oleic series. The most important of these fats are Palmitin, Stearin, and Olein.
In addition to these we have in milk, butter, &c. small quantities of the neutral fats of Butyric, Caproic, Caprylic, Capric, and Myristic acids.
They are oily bodies; some solid, others fluid, at the ordinary temperature. They are insoluble in water and in cold alcohol, soluble in hot alcohol, in ether, chloroform, &c. Under the action of caustic alkalies or superheated steam they are decomposed into their fatty acid and glycerin. The fat of different animals differs in its characters according to the relative proportion of these various fats which it contains. Thus, the fat of the dog, which contains a large proportion of olein, melts at a comparatively low temperature, while the fat of the sheep, on the other hand, which is particularly rich in stearin, has a much higher melting-point.
Whatever be the nature of the fat, it occurs in the animal body inside the living units or cells in the form of small globules. Fats appear to be an almost invariable constituent of all living protoplasm; but in certain tissues they accumulate in very large amounts. This is especially the case in the cells of loose fibrous tissue, where the deposition of fat is frequently so great as to form regular masses or lobules. Under the skin, in the abdomen, in the orbits, round the heart, and in various other places these masses are well marked.
The quantity of fat in the human body varies considerably at different periods of life. In the earlier stages of fetal existence we find scarcely any fat; in new-born children there is usually a considerable quantity of this substance deposited under the skin, and the organism continues rich in fat till the age of puberty, when a marked diminution of the substance occurs. It again increases about middle life, and then occasionally occurs in great excess; for example, three or four inches of fat are not unfrequently found under the skin of the abdomen in corpulent persons.
The fats of the body are derived not only from the fats of the food, but also from both carbohydrates and proteins. It has now been clearly shown that an animal takes up and stores in its body any fat which may be administered in the food, and that it may thus for the time being have the chemical nature of its fat entirely altered. After a time, however, the living protoplasm of the body seems to be able to convert the foreign fat so stored into the proper fat of the animal.
By feeding dogs on a flesh diet, and carefully analysing the excreta, Pettenkofer and Voil have shown that the flesh of the food is a source of fat in the body. That carbohydrates—such as sugar and starch—are also sources of fat is a matter of popular belief. Various researches have clearly demonstrated that this belief is correct. By feeding young growing pigs on a diet of known composition, and afterwards killing them and analysing their bodies, it was shown that the fat found was in such amounts that it could have been derived only from the carbohydrates of the food.
The physiological value of the fats is due partly to their physical and partly to their chemical characters. The uses of the fat deposited beneath the skin are, first, to protect the body from external shocks by a uniform diffusion of pressure through the whole adipose tissue; and, secondly, to keep up the heat of the body, by materially checking, through its very slight conducting power, the loss of free heat by radiation. This use of the fat is most clearly seen in some of the lower animals (the seal, whale, &c.), which are exposed to very low temperatures. Another physical use of fat is to promote the mobility of various organs. Hence, in cases of extreme emaciation, it always remains in the parts where motion is most essential, as the heart and the orbit of the eye.
The chief chemical use of fat is as a store of potential energy for the body (see FASTING).
On account of the large amount of carbon and the small amount of oxygen contained in the molecule, it is peculiarly valuable in this respect. Measuring the potential energy in gramme degrees, we find that 1 gramme of fat yields 9069 gramme degrees of heat; 1 gramme of proteins yields 4368; 1 gramme of carbohydrates yields 3912. Unfortunately the process of digestion and assimilation of fat is difficult, and for this reason, as well as on account of its relatively high price, only a moderate amount can be daily consumed (see OBESITY).