It contains the same chemical elements as carbohydratescarbon, hydrogen, and oxygen. Relatively they contain more carbon and hydrogen and less oxygen, thereby having a greater oxygen combining power which makes them greater sources of heat and energy production. In the past, animals have been the principal source of supply, but in recent years large amounts are also prepared from the seeds of fruits and plants. Butter, butter substitutes, cream, vegetable oils of different kinds (olive, cottonseed, corn, peanut, etc.), lard, and lard substitutes are the most commonly used edible forms. The foods that supply protein contain considerable fats. Nuts are a rich source. Milk contains from 4 to 5 per cent; eggs have approximately equal quantities of fats and proteins; and meat, fish, and fowl average about 15 per cent. All cheese with the exception of cottage cheese is rich in fats. The avocado and olive contain as high a percentage as table cream.
Properties. There are many different kinds of fats; some are liquid and others solid. Similar to the proteins, fats are characterized by the different fatty acids which they contain. According to McCollum, all of these except linoleic acid may be synthesized in the body from sugar. Every fat due to its chemical composition, has an individual melting point which makes it a liquid, semi-solid, or solid substance at ordinary temperatures. Fats having the lowest melting point are the most easily and completely digested. Some of these are cream, egg yolk, and olive oil. Of the meat fats, pork has the lowest melting point, beef the next, and mutton having the highest, is not as completely digested and absorbed as the others.
Some of the fats, particularly cream, butter, and egg yolk, are rich in vitamin A which should be kept in mind when using butter substitutes. These substitutes have the same fuel value as butter and may be safely used in a low cost diet, but because of its vitamin content, but-ter fat should not be omitted regularly.
Functions. Fats have several functions in the body. Their energy value is more than twice that of carbohydrates along with which they are used as energy producing foods. The ratio of carbohydrates to fats used in different countries has been pointed out above and we believe that an ideal ratio for Americans is 3 or 4 parts of carbohydrates to one of fats. A diet providing such pro-portions is more. easily digested and utilized after digestion. At the same time if the carbohydrate foods have been selected properly, many additional food elements will have been supplied.
Fats are also used to build necessary fatty tissue. This serves as a protection to various organs in the body and as a support to hold them in position. It also serves as a protection to the nerves. The layer of fatty tissue under the skin prevents loss of heat because fat is a poor conductor. This store of fatty tissue forms a reserve sup-ply of fuel upon which the body can draw as needed, and while not in active use serves a valuable aesthetic effect in giving graceful form to body lines.
To meet the body requirement for fats we need a comparatively small amount. It has been estimated that from 25 to 35 per cent of the energy value of the diet should be derived from fats. In an average diet this would be approximately 70 to 120 grams.
Fats are Used Too Freely. The overconsumption of fat is probably equal to if not greater than the overconsumption of concentrated sweets. Fats from every avail-able source are being utilized. Salad oils are obtained from cottonseed, corn, and other products to supplement olive oil. The liberal use of commercial salad dressings made from oil is one of the chief reasons for the increased amount of fats in the diet. All kinds of lard substitutes have appeared on the market to meet the demand for extra fats for shortening, deep fat frying, and other forms of cooking. Dairies are striving to supply a milk with a higher fat content; and the demand for cream is in-creasing. The great variety of cheese and the attractive manner in which it is packed, together with many suggestions for its use have increased its consumption. The more general use of nuts, olives, and avocados has contributed to the fat content of the diet.
The most harmful effect of the overconsumption of fats is the increase of obesity. This is rather an alarming condition which will be discussed in a later chapter on overweight. Some digestive disturbances may be due to an excess of fats in the diet. A small amount of fats slows down the emptying time of the stomach and a large amount may prevent food from completely leaving the stomach until the next meal. Protein foods coated in fats are very slowly digested, so that the normal digestion of protein is often seriously impaired. Carbohydrate foods are made less digestible when cooked with fats. Fried foods, if used, should be very carefully prepared, for fats which have been decomposed by high temperatures in cooking prove very irritating in the digestive tract.
A certain amount of fat adds flavor to the diet, but the quantity that is being used today is unnecessary and harmful. The foods which supply fats are expensive and for economical reasons should be used sparingly. But of more value than mere financial economy is the prospect of better health when fats are used only in optimum amounts.