More About Fats

In Chapter 1 we discussed fats in connection with blood cholesterol and the development of atherosclerosis. We observed that ordinary food fats are triglycerides, that is, combinations of three fatty acid molecules with one molecule of glycerol. The latter amounts to less than 10 per cent of the whole in most fats, and the differences between various fats depend entirely on the fatty acids involved.

Edible fats are divided into those that are more or less solid at room temperature, called simply "fats", and the "oils" that are liquid under the same conditions. They are all insoluble in water but dissolve in the "fat solvents" (ether, petrol, alcohol, chloroform), and they all yield about nine calories per gramme (4,100 per pound) when they are burned, either in the body or in an oil lamp.

The fatty acids differ from one another in the length of the chain of carbon atoms in them and in their "saturation". The fatty acids in foods range from butyric acid, with only four carbons in the chain, to chain lengths as long as 24 or more carbons, but very few food fats contain appreciable amounts of fatty acids with chains of more than 18 carbon atoms. In general, the longer the chain length the higher the melting point. There may be a tendency for the speed of digestion to decrease as the melting point goes up, but the chain length does not seem to have a great influence on the effect of the fat on the blood cholesterol.

The blood cholesterol, however, is greatly affected by the degree of saturation of the fatty acid, that is, the extent to which the carbons in the chain are loaded with hydrogen atoms. The saturated fatty acids raise the cholesterol in the blood, and the carbons in these fatty acids are fully saturated with attached hydrogens. Fatty acids with one double bond, called mono-enes, have one point of unsaturation, one place in the chain where two carbons are doubly joined, so that each of these carbons could take on one more hydrogen atom. Oleic acid, which makes up most of olive oil, is the main mono-ene, and it has little or no effect on blood cholesterol. Linoleic acid is the main example of a fatty acid with two double bonds, and this poly-ene, or polyunsaturated fatty acid, which is abundant in many fats of vegetable origin, tends to depress the blood cholesterol. Linolenic acid, prominent in linseed oil, has three double bonds, arachidonic acid has four, and the fatty acids in fish oils contain up to six or more double bonds. All of these poly-enes seem to act like linoleic acid in regard to cholesterol.

The more double bonds, the more liquid is the fat; conversely, the more saturated, the more solid is the fat. But if the chain length is short enough, even fully saturated fats can be liquid. This explains why coconut oil is liquid though it is almost completely saturated; a large proportion of the fatty acids in coconut oil have only 12 or fewer carbons in the chains. Similarly, fish oils contain chains of more than 20 carbons but these have many double bonds and the result is a liquid oil.

Hydrogenation

Double bonds are points of instability in the molecule so polyunsaturated fats tend to oxidize and become rancid more easily than the saturated fats. This is one reason why food processors like to hydrogenate their oils, treating them with hydrogen so as to force hydrogen atoms to fill the points of unsaturation. The result is a fat that keeps longer.

The other reason for hydrogenation is to convert a liquid oil into a solid fat. Solid fat can be used as a spread and hence can be offered as a butter substitute (margarine). Equally or even more important is the fact that hydrogenated vegetable fats can be made to resemble butter or lard in baking properties so as to be substituted in equal amounts in recipes. Since vegetable fats can be produced much more cheaply than lard or butter, the economic advantage of hydrogenation for cooking fat is obvious.

Experimentation with cookery, however, shows that liquid oils can be used practically as well as, and for some purposes better than, solid cooking fats by adjusting the recipes. Moreover, better oil refining methods, attention to marketing oils when they are fresh, and the use of refrigeration have combined to reduce the problem of oil rancidity, so some of the advantage of hydrogenation seems to be disappearing, much to the alarm of certain food processors.

Commercial hydrogenation does not usually saturate all of the double bonds. Some of the oleic acid is converted to a completely saturated fatty acid but the main change is usually the conversion of most of the linoleic acid to.a mono-ene, that is, only one of the two double bonds is saturated. But this does not mean merely changing linoleic acid to oleic acid. Some of the linoleic acid is so transformed, but a part of the linoleic acid, when it loses one of its double bonds, undergoes rearrangement and the result is a mono-ene of a type called a "trans" acid (the more natural type is a "cis" acid, the difference being in the way the molecule is joined at the double bond).

It has been suggested that trans acids, because they seldom occur naturally, are undesirable or even dangerous. These fears have little scientific foundation, and in experiments on man the cis and trans acids do not seem to differ in effect on blood cholesterol. It appears, therefore, that some arguments against hydrogenated fats have been unduly critical. The net result is that we do not believe hydrogenated margarines and cooking fats are uniquely bad or dangerous. They seem to be no worse than many natural fats of animal origin, though they may have less aesthetic virtue.

"Essential" Fatty Acids Experiments with growing rats have shown that a diet rigidly freed of all fats produces a variety of disorders that are cured with very small amounts of linoleic, linolenic, or arachidonic acids, which are called "essential" fatty acids for that reason. Many studies on man, including prolonged studies on infants, have failed to prove beyond a doubt that these fatty acids are essential for man. We incline to believe that one or another of these fatty acids is needed in the human diet but that the amounts are so minute and the time required to deplete any stores in the body is so long that no practicable human diet can be devised to disclose the need.

However, it is ludicrous to argue that any reasonable fat-restricted diet, even one far more rigorous than anything we would advocate, may provoke a deficiency of essential fatty acid. In many parts of the world we have observed populations who have lived for untold generations on diets seldom reaching the level of 15 per cent calories from all fats, yet nothing like signs of essential fatty acid deficiency appear. In any case, the diets we recommend actually contain more essential fatty acid than the usual British diet, far more linoleic acid than would be a curative dose for the baby rat that has been on a fat-free diet.