Animal as well as plant fats are being used as part of diet as well as for a variety of other human needs since prehistoric times. Animal fat, particularly the fat obtained from milk as ‘ghee’, has traditionally been a very important part of human diet. The vegetable oils obtained from a variety of oil-yielding plants have also been major sources of fats. A diet without fat, apart from being less appealing, shall also result in impaired body functions and a number of health problems. Fats are important in the nutrition because they provide fatty acids that are needed for a variety of structural, functional and metabolic roles in the body. However, with increasing health problems associated with dietary fats, awareness of different aspects of chemical nature of fats, their relation with body metabolism and possibilities of modifying dietary fats to better suit the human body has become quite important.
Chemically the fat is defined as triacylglycerol that is solid at normal room temperatures. The term oil is used for triacylglycerol that is liquid at room temperatures. Simple fats and oils obtained from either animal or plant sources are esters of glycerol with a variety of fatty acids. Therefore, fats and oils yield glycerol and a mixture of fatty acids on hydrolysis. This mixture may contain a variety of saturated, monounsaturated and polyunsaturated fatty acids.
Nutritionally the fatty acids yielded by fats are classified into non-essential and essential fatty acids. Non-essential fatty acids can be synthesized in the animal body itself and include palmitic acid (C16, saturated), stearic acid (C18) and oleic acid (C18, monounsaturated). Essential fatty acids are not synthesized in the animal bodies. And have to be obtained from dietary intake of suitable fats. These include linoleic (C18, diunsaturated) and linolenic (C18, triunsaturated) acid. Linoleic acids. (9, 12–octadecadienoic acid) is considered n-6 or ω6 fatty acid while linolenic acid (9, 12, 15–octadecatrienoic acid) is first member of the n-3 or ω3 series of fatty acids . In the linoleic acid molecule, the last double bond is at the distance of six carbon atoms from the methyl end. This feature is important for the function of some enzymes. Most crucial fact of nature is that animals do not synthesize linoleic acid and linolenic acid while plants synthesize these two fatty acids. Therefore, animals including human beings have to depend on dietary intake of plant fats for obtaining these two essential fatty acids. For normal function of the body, the ratio of n-6/n-3 fatty acids in the diet should be between 5:1 and 10:1.
DIETARY FAT AND HEALTH
The most important role of dietary fats is to provide energy to the body. In addition, fats reduce the bulk of diet due to their very high (about 9 calories/g) caloric density. A tablespoon contains about 14 g of fat that can provide about 125 calories to the body regardless of its degree of saturation. Fats are also deposited in various tissues and around various organs in the body and help in protecting the internal organs from shocks and mechanical trauma. Fat layers (adipose tissue), particularly below the skin act as insulator and help in maintaining the body temperature at constant levels. Thus, fats protect internal organs from damage by extremes of heat and cold. Fats also aid in absorption of fat-soluble nutrients such as vitamins A, D, E, K and carotenoids in the body. Thus, fats act as carriers of these absorbed nutrients and help in their transportation within the body. In view of the importance of fats for human body, the American Dietic Association and International Food Information Council Foundation have given their recommendations of the dietary fat for those over the age of two years.
Animal fats, including those derived from milk, formed major portion of human dietary fat intake for a long duration of human history. Use of vegetable oils as cooking medium had started in the tropical and sub-tropical regions quite early in the human history. However, psychosocial bias towards animal fats, particularly those derived from milk continues to prevail throughout the world. Even in India, despite adoption of vegetarianism as predominant cultural ethos regarding diet, this bias towards milk fats still persists. Various modern studies on the role of different fats and fatty acids in the body metabolism related to various aspects of human health have brought remarkable change in this perspective. In recent times, the nutritional properties of lipids, particularly the metabolic effects associated with consumption of triglycerides having specific fatty acid compositions and positional distributions have been more thoroughly understood.
Saturated fatty acids
Many studies of oriental migrants who have adapted to western lifestyles have established that the intake of saturated fatty acids like lauric acid, myristic acid, palmitic acid etc. increases the LDL cholesterol level and aggregation of platelets. These acids also activate the process of blood coagulation leading to thrombosis. Japanese migrants settled in United States who adapted to westerns diet containing high amounts of saturated fatty acids exhibited markedly increased risk ailments like myocardial infraction or colon cancer as compared to their brethren in the home country (Fig. 2a). Intake of saturated fats in the diet is directly related to coronary risk (Fig.2b). Replacement of saturated fatty acids by unsaturated fatty acids without any decrease in dietary intake of total fat results in a favourable lipoprotein profile. Changes in the composition of dietary fats may have effects on atherosclerosis. This may be because
saturated fatty acids, especially those having 12-16 carbon atoms, raise the cholesterol-level. However, saturated fatty acid stearic acid (C18:0) has little or no effect on plasma cholesterol concentration.
Trans-fatty acids
Plastic fats (e.g. ‘vanaspati ghee’) are manufactured by the hydrogenation of oils using nickel as catalyst. Main disadvantage of hydrogenation is that trans-fatty acids are formed in the product, which cause severe health problems. A very important health problem caused by these is thrombosis that leads to coronary heart disease. High intake of trans-fatty acids may also increase the risk of breast cancer in post-menopausal women. Trans-geometrical isomers of 18-carbon essential fatty acids (e.g. linoleic acid and linolenic acid) identified quite early have been detected in several commonly used foodstuffs, ranging from ‘vanaspati ghees’ and low-calorie spreads to infant feed formulae. The trans-fatty acids are packed more closely than their cis- isomers producing harder fats. These harder fats have higher melting points than softer fats obtained from the corresponding cis-fatty acids. The trans- isomer is regarded as an intermediate between an original cis- unsaturated fatty acid and a completely solid fatty acid. The most abundant trans- isomers of C18: 1 (Octadecaenoic acid) resulting from industrial hydrogenation are those with unsaturation at positions 9, 10 and 11.
The presence of trans- isomers of essential fatty acids in the dietary fat is of particular concern because essential fatty acids become chain-elongated into higher metabolites in the body. These latter formed metabolites of trans- isomers can modify various physiological functions such as platelet aggregation and eicosanoid metabolism. The rat pups whose mothers were fed with trans-n-3 fatty acids exhibited changes in the electroretinograms as compared to the progeny of mothers fed with corresponding cis- isomers. There are indications that the trans-fatty acids and saturated fatty acids having similar shapes may act similarly with respect to their ability to raise serum and LDL cholesterol levels. Trans-9, trans-12-octadecadienoic acid has greatest potential of interfering with metabolism of essential fatty acids and eicosanoids at the enzyme level causing symptoms of essential fatty acid deficiency.
In view of increasing evidence of the harmful effects of trans-fatty acids, fat modification technologies other than hydrogenation are being investigated for obtaining solid fats from oils, particularly vegetable oils.
Essential fatty acids and body metabolism
Risks due to dietary intake of saturated and trans- fatty acids have been well publicized through numerous medical studies and health education programs. However, it has only been realized recently that the consumption of certain types of fats and oils is necessary because they contain compounds that are essential for growth, maintenance of health and prevention of diseases in infants as well adults. The role of essential fatty acids is particularly important in the body. The ratio of n-6/n-3 acids in the diet should be between 5:1 and 10:1.
These acids maintain normal and healthy skin, promote body growth and allow normal reproductive function. Specific enzyme systems act upon the essential acids and produce other long-chain fatty acids known as eicosanoids (also known as omega-6and omega-3) in the body (Fig. 3).
These eicosanoids become precursors for very important metabolites that are short-lived, potent, locally acting hormone-like substances called prostaglandins and similar other compounds (e.g. prostacyclins, thromboxanes and leucotrienes). Tocotrienols (Fig. 1c) are also similar important metabolites derived from the essential fatty acids. All these substances are essential for maintaining a variety of biological functions in the body such as immune functions, smooth muscle contraction, myocardial contractility, blood viscosity, platelet aggregation, clotting, pain relief, male fertility, pregnancy and normal delivery in females etc. Medical researchers have also shown that linoleic acid (9, 12-octadecadienoic acid) found only in vegetable oils is antiatherogenic i.e. it can reduce the risk of heart disease. Linoleic acid has been shown to moderately reduce serum and LDL cholesterol levels. Linoleic acid (18:2 n-6, 9) and other polyunsaturated fatty acids (PUFAs) prevent the build up of cholesterol in the blood.
The awareness regarding the type of dietary fat and associated health consequences has rapidly increased worldwide in the past few decades. As a result, the trend in fat and oil consumption in recent years has markedly shifted from animal fats to vegetable fats. The use of fats modified chemically through appropriate fat modification technologies is also increasing worldwide. These considerations have rapidly led to the development of novel fat modification technologies for the improvement of the nutritional properties of fats and oils.
See also
http://goodfats.pamrotella.com/
