Facts about Fats
The lay public has been fed a heavy diet of facts about fats by well-informed and well-meaning specialists during the last three decades. The advice was so consistent for more than two decades that everyone seemed to know everything on the subject. In a nutshell, it seemed settled for good that cholesterol is bad and so are saturated fats, and that unsaturated facts are good. But new facts discovered during the last decade have smeared this clear black and white picture so badly that nobody seems to be sure of anything. All talk about fats has been primarily in relation to coronary heart disease. In coronary heart disease the arteries get partly blocked. As a result, the function of the heart is impaired. The material which blocks coronary arteries is rich in fats, especially in a type of fat called cholesterol. Cholesterol is a normal constituent of the body and it also circulates in the blood. However, the concentration of cholesterol in the blood of those having coronary heart disease is, on an average, higher than in those not having the disease. Further, diet can influence the concentration of cholesterol circulating in the blood. Therefore it stands to reason that consuming a diet which lowers the concentration of cholesterol in the blood will reduce the possibility of cholesterol getting deposited in the coronary arteries. However, things are rarely as simple as they seem. First, it is not certain whether cholesterol deposits in the coronary artery are the cause of the disease or only an effect of a primary abnormality or injury in the arteries of the affected individual. Second, cholesterol circulating in the blood has several fractions, only one of which (LDL) is particularly vicious, and one fraction (HDL) even seems to protect against heart disease. Third, diet is only one of the factors which affect the concentration of cholesterol circulating in the blood. And, finally, extreme diets which reduce the concentration of circulating cholesterol, often have other effects which are not entirely desirable. With all these reservations and complications in mind, let us take a close look at dietary fats.

Dietary Fats
The fats in our diet mostly belong to the class known as triglycerides. The name derives from the fact that each molecule of triglycerides is a combination of glycerol and three fatty acids. The glycerol part is constant; it is variations in the fatty acids which make one fat different from another. Fats are called saturated or unsaturated depending on the type of fatty acids they have. The next question is how a saturated fatty acid differs from an unsaturated one. All fatty acids are made up of carbon, hydrogen and oxygen. Each carbon atom has a fixed capacity to combine with other atoms. This capacity is called valency, and the valency of carbon is 4. Valency is somewhat like a vacancy - it may not be always full.
Fatty acids in which all their carbon atoms have all their valencies occupied are called saturated. Hence they have no special ‘urge’ to combine with other substances. They can be kept for long periods of time without undergoing any chemical change. In contrast, in unsaturated fatty acids at least a few vacancies are available. Therefore, on storage, unsaturated fatty acids combine with oxygen of the air. The combination leads to the formation of a different class of substances. When these substances are formed we commonly say that the fat has become rancid. Different fatty acids in the same category are also not exactly alike. For example, saturated fatty acids in butter are not the same as those in coconut oil.

It is good to be familiar with at least two saturated fatty acids:

• palmitic acid
• stearic acid.

Similarly, unsaturated fatty acids in corn oil are not the same as in fish oil. The differences are not only chemical but also physiological. In view of recent work, it is specially important to recognise three categories of unsaturated fatty acids:

• monounsaturates
• n-3 polyunsaturates
• n-6 polyunsaturates.

Monounsaturates are fatty acids which have only one vacancy per molecule while polyunsaturates have more than one. The nomenclature n-3 and n-6 is based on the position of the first vacancy in the fatty acid molecule. As prototypes of different unsaturated fatty acids, it is well to be familiar with oleic acid as the major monosaturate, linoleic acid as the major n-6 polyunsaturate and linolenic acid as the major n-3 polyunsaturate.

Cholesterol in Focus
Although almost all the fat in our diet is in the form of triglycerides, it is important to talk about one more fat which may be present in small quantities in our diet. The reference is to the much-maligned cholesterol. Cholesterol is, chemically, entirely distinct from triglycerides. It is a normal constituent of the body but is not an essential item of the diet because the body can manufacture it. In fact, the body does manufacture it regularly. Further, the production of cholesterol in the body is finely regulated. If there is no cholesterol in the diet, the body manufactures more of it; if there is too much cholesterol in the diet, the body may stop making it altogether. However, the regulation is not perfect; it is possible to raise the concentration of cholesterol in the blood by eating too much cholesterol. Therefore when a brand of vegetable oil claims that it is a zero-cholesterol oil, their claim is true but not unique. No vegetable oil contains any cholesterol. All vegetable oils are 100 per cent triglyceride. The way one vegetable oil differs from another is in its influence on the concentration of cholesterol in blood. No vegetable oil, or any diet, can bring down the concentration of cholesterol in blood to zero; it is neither possible nor desirable. All that the oils and some other constituents of diet can do is to alter; i.e. raise or lower, the concentration of cholesterol in the blood by a few per cent.
The chemical nature of fats in different dietary facts and oils follows a pattern. Fats of animal origin are generally saturated while fats of plant origin are generally unsaturated. Important exceptions are fish oils, which are unsaturated, and coconut and palm oils, which are saturated. Among vegetable oils, there is a gradation, however. Safflower oil (kardi), sunflower oil, corn oil and sesame oil (til) are the most unsaturated. Groundnut oil, mustard oil and soyabean oil are only moderately unsaturated. Hydrogenation of vegetable oils means addition of hydrogen to the oils. Hydrogen fills up the ‘vacancies’ in the unsaturated fatty acids. Therefore hydrogenation makes vegetable oils saturated. Palm oil is quite saturated but has a considerable proportion of monoun-saturates. The polyunsaturates of vegetable oils are mostly of the n-6 variety.
Fish oils have a considerable amount of n-3 polyunsaturates. In Indian diets, the main sources of n-3 polyunsaturates are mustard oil, rapeseed oil and soyabean oil.

Recent Trends Towards Unsaturated Fats
It might be said that polyunsa-turates, both of the n-3 and n-6 varieties, lower blood cholesterol. The effect of monounsaturates on blood cholesterol is controversial but the usual practice is to treat them as neutral in this respect, i.e. to consider as it they neither raise nor lower blood cholesterol. The saturated fats are generally said to raise blood cholesterol. But this effect belongs to palmitic acid rather than stearic acid. It was on the basis of these facts that vegetable fats were advocated in preference to animal facts. Those at risk of getting heart disease were specially advised extreme diets in which almost the entire fat came from highly unsaturated vegetable fats such as corn oil and sunflower oil. Health conscious people took to these facts as a preventive measure against heart disease in a big way. For the first time in human history so many people took diet so high in n-6 polyunsaturates. It must be admitted that such lopsided fat consumption did no major demonstrable damage even when continued for decades. But it is doubtful if the practice did as much good as was anticipated. And there is now a growing suspicion that such an unbalanced fat consumption may inflict slow and subtle but serious damage on the body.

Newer Understanding on Fats
The newer thoughts have arisen from the realisation that fats do a lot more in the body than just affect blood cholesterol. Facts are a component of cell membranes. The type of fats incorporated in cell membranes are somewhat flexible and depend on the type of fats in the diet. If the diet has a very high unsaturated fat content, the body incorporates more of unsaturated fats in membranes. As on the kitchen shelf, in the body also, unsaturated fats are more susceptible to chemical change. A particularly vicious form of change in the body is peroxidation. Peroxidation of fats in the body damages cell membranes. Cells with damaged membrane are more prone to various insults and injuries. Cells of the immune system with damaged membranes are likely to be less effective for defence. There is some evidence that peroxidation damage may also make the cells more prone to turn cancerous. Besides the generalised effect on cell membranes, there are other ways in which excessive consumption of unsaturated fats may be harmful. The ‘bad’ cholesterol, i.e. LDL, also incorporates fats. A recent work has shown that LDL fat undergoes peroxidation. Peroxidati-vely modified LDL cholesterol is thought to get deposited in the walls of blood vessels leading to coronary heart disease. There are several apparently harmful effects of consuming excess of n-6 polyunsaturates at the expense of n-3 polyunsaturates. First, the brain manufactures a substance - docosahexaenoic acid (DHA) - from an n-3 polyunsaturate, linolenic acid. On a high n-6 diet, the brain uses an n-6 substitute for DHA in its cell membranes. The brain which uses the n-6 substitute is more susceptible to damage by alcohol. Second, a high n-3 diet induces the body to manufacture thromboxane A3 and leucotriene B5.
In contrast, a high n-6 induces the formation of thromboxane A2 and leucotriene B4. Thromboxane A3 is less likely to clump blood platelets and leucotriene B5 is less likely to injure tissues than the corresponding products induced by n-6 polyunsaturates. Clumping of platelets and injury to vessel wall are important factors in the causation of coronary heart disease, for the prevention of which a diet high in n-6 polyunsaturates was initially advocated. Finally, it has been realised that all saturated fats are not equally potent in raising blood cholesterol. Stearic acid and some other saturated fatty acids have no appreciable effect at all on blood cholesterol.

Restoring the Balance
In short, an extreme diet in which almost the entire dietary fat comes from n-6 polyunsaturates such as corn oil seems to be undesirable. The ratio of n-6 to n-3 fats in traditional diets was about one to one. But the lopsided diets consumed during the last few decades altered the ratio to as much as 25 to 1. The body adjusted apparently well to such liberties, but gradually the possible ill-effects of the drastic change are being realised. Therefore it is now proposed to restore the n-6 to n-3 ratio to at least 10 to 1. Further, since all saturated fats are not harmful, some saturated fat may also be added to the diet. And, since monounsaturates are relatively neutral in their effects, it may be a good idea to temper the diet with some of these as well. Thus what is now advised is actually a mixture of saturated, polyunsaturated and monounsaturated fats in roughly equal proportion. Moreover, the polyunsaturates would provide a sensible mixture of n-6 and n-3 fats. As in all else in life, moderation seems to be the golden rule.

Importance of Fats
A radical question one may pose at this juncture is whether fats are essential in our diet at all.

Strictly speaking, visible fats, i.e. butter, ghee or oils are not essential. These are products of technology which came much later than the advent of man on the planet.

But some invisible fat, which is present in grains and leafy vegetables, is essential, which ensured our survival before we learnt how to extract fat from seeds or milk. However, visible fats have also been around long enough to have proven their relative harmlessness. A moderate quantity of these man-made products in reasonable variety adds to the appeal of a diet without making it undesirable in any important respect. Taking all these facts into consideration, the Indian Council of Medical Research (ICMR) has recommended about 20 g visible fat consumption per day, or about 7 kg per head per year. For those accustomed to rich diets, the upper limit has been placed at about 18 kg per head per year. Since the amount of butter, ghee and oils consumed by a family per year can be conveniently found out, it is easy to know whether fat consumption is within the upper limit. Since saturated fats, monounsaturates and polyunsa-turates (n-6 and n-3) are now considered equally important, visible fat may be consumed in the form of a reasonable mixture of butter/palmolein and mustard/soyabean oil. Adding some groundnut oil will not be harmful, but it is preferable to retain some mustard/soyabean oil as a source of n-3 PUFA. The reason why a deficiency of n-3 PUFA has not been felt as acutely as one might imagine is that about one-third or more of the invisible fat obtained from grains and leafy vegetables is in the form of n-3 PUFA.
It is often said that there is no complex subject which does not become more complex as a result of research. Research on fats conducted during the last four decades illustrates the basic truth of this cynical statement as few other areas of research possibly can.

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