This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Fats and oils contain but three elements - namely, carbon, oxygen, and hydrogen. In the starches and sugars the proportion of oxygen and hydrogen is such as to form water, H20, when their molecules are split up; but in the group of fats oxygen is not present in sufficient quantity to form water with all the hydrogen atoms, and in their combustion with oxygen considerable heat is evolved. In some fats, like butter, very little oxygen is present, and carbon and hydrogen compose the bulk of the substance. The amount of fat which from time to time is stored in the body is regulated to a greater degree than any other substance by muscular exercise, which, if active, always tends to prevent its accumulation. The storage of fat is favoured by sleep as well as inactivity. ( See Obesity and Leanness).
About one fifth of the entire body weight is composed of fat, but only about a quarter of an ounce is contained in the blood. Before death results from starvation 90 per cent of the body fat is consumed. The chief sources of this fat in the human body are undoubtedly starches and sugars, but it is probable that under certain conditions it may also be derived from fatty food.
Fat is required to promote the earlier stages of growth and development of the organism, and there are also many forms of disease and degenerative changes which are accompanied by increased accumulation or production of fat in and between the tissues and cells. It is as impossible to live in perfect health without fatty food, as it is to live long upon fat alone, for it soon disorders digestion.
The chief uses of fatty food are:
1. To furnish energy for the development of heat.
2. To supply force.
3. To serve as covering and protection in the body.
4. To make more plastic various structures of the body and give rotundity to the form.
5. To spare the tissues from disintegration, for, although their combustion in the body results largely in the production of heat, they also take part to some extent in tissue formation.
6. To serve for storage of energy.
The various forms of energy manifested in the different nitrogenous tissues - as muscular action, secretion, nerve force, etc. - are more or less intimately dependent upon fat combustion. It was originally believed that the force of the body was supplied by the oxidation of nitrogenous materials. Fat eaten with the food was supposed to be deposited again as fat in the tissues of the body without material change, but of recent years this theory has been very largely recast, and the primary value of fatty food consists undoubtedly in its contribution to force production and its power of saving other tissues, especially the albuminous, from destruction by oxidation, whereas its secondary use is in connection with tissue formation. Fats do, however, enter into the composition of many different tissues, even those of the nervous system.
The fats and oils which are employed as food all serve essentially the same purpose, and may therefore be grouped together as a distinct class. There is a general resemblance in their physical properties, although they differ considerably in the melting point. The several food fats and oils are of various chemical composition, but after being absorbed they are recognised mainly in one or two simple forms, chiefly as stearin and olein.
The use of animal oils, such as lanolin, and of the petroleum products, like purified vaseline, has very largely superseded the external application of other fatty substances - lard, etc. - to the skin for the purpose of lubrication.
It is not possible to get very much nourishment into the body by osmosis through the integument, but some improvement seems to follow the rubbing of fats and oils, such as cacao butter, olive oil, or cod-liver oil through the skin of marasmic children and other patients. (See Marasmus.) One or two teaspoonfuls of the oil may be rubbed in twice a day on the thighs, abdomen, and chest. The statement that the application of fats to the surface of the body by inunction reduces the body temperature is not substantiated by experience.
The treatment of biliary calculi by the administration of large doses of olive oil - two or three ounces at a time - given upon an empty stomach, has been suggested, apparently with the idea that it might have some local lubricating action. Cases have been reported in which gallstones have been said to appear in the feces as a result of this treatment, but it has been shown that the oil itself may become mixed with inspissated intestinal mucus and form small, hardened masses, which have been mistaken for gallstones. There is no foundation for the belief that oil is of any value for cholelithiasis, nor is it possible that it should enter the bile ducts to "lubricate" them.
Fats and oils are useful preservatives of many foods by preventing access of air, drying, and decomposition. A layer of oil floating on top of a flask of wine is capable of preserving its delicacy of flavour for a long time (Chambers). Oil preserves fish, like sardines, and layers of lard are used to protect jars of potted meats, pdte-de-fois-gras, etc. Oils and butter protect eggs from decomposition.