Fats (diet), naturally occurring organic compounds, chemically, esters of three molecules of fatty acid with glycerol; they are known as triacylglycerols or triglycerides. Fats and oils are oily, greasy, or waxy substances, lighter than, and insoluble in, water. The distinction between fats and oils is that fats are solid at room temperature, whereas oils are liquid, only solidifying at lower temperatures. Waxes are esters of fatty acids with higher alcohols, and are hard solids at ordinary temperatures.
Fats are important in the diet as a source of energy, yielding 9 kcal (37 kJ) per gram (0.035 oz). In developed countries 40 per cent or more of the total energy intake may come from fat. This is higher than is considered desirable for health—high intakes of fat are associated with obesity, gall bladder and heart disease, and some forms of cancer. Nutritional guidelines therefore recommend that fat should provide no more than 30 per cent of energy intake. In less developed countries, fats may provide less than 15 per cent of energy—at this level of intake it is difficult to eat enough food to meet energy requirements. Fat is also important for the absorption of the fat-soluble vitamins A, D, E, and K, as well as β-carotene. Much of the flavour of foods is contained in the fat.
In addition to triacylglycerols, foods contain small amounts of phospholipids. These are also esters of fatty acids with glycerol, but contain only two fatty acids per molecule; the third ester to the glycerol is to phosphate and a water-miscible group. They are important constituents of the membranes of all cells. They will emulsify oils in water, and some (for example, lecithin) are commonly used for this purpose as food additives.
The different fatty acids that make up dietary fats and oils contain between 8 and 24 carbon atoms (always an even number). They can be classified as saturated or unsaturated. This describes the chemistry of the carbon chain of the fatty acid: in saturated fatty acids all the carbons carry their full quota of hydrogen atoms, and there are only single bonds between the carbon atoms. In unsaturated fatty acids two or more carbon atoms carry only a single hydrogen atom each, and there are one or more double bonds between carbon atoms. Fatty acids with only one double bond are called monounsaturated, those with two or more double bonds are polyunsaturated. The temperature at which a fat melts is determined by both the length of the fatty acid chains and also the degree of unsaturation. Fats containing longer chain fatty acids melt at a higher temperature, while those with more unsaturated fatty acids melt at lower temperatures. Hard animal fats contain relatively more saturated fatty acids than do softer fats and oils from vegetable sources.
Polyunsaturated fatty acids are dietary essentials in small amounts, since they are the precursors of prostaglandins and other locally acting hormone-like compounds in the body. There are two groups of these compounds, derived from different polyunsaturated fatty acids. The difference between the two parent fatty acids is the position of the first carbon-carbon double bond counting from the methyl end of the fatty acid. It may be at the 3rd or 6th carbon in, and they are known as the n-3 (or ω-3, omega-3) and n-6 (or ω-6, omega-6) fatty acids.
The type of fat in a person’s diet, as well as the total amount, is an important consideration with regard to health. Saturated fats tend to increase the concentration of cholesterol in the bloodstream, and are considered undesirable, since they contribute to the development of atherosclerosis and heart disease. It is unlikely that for most people the amount of cholesterol in the diet is an important factor in determining the blood concentration, since there is control over the synthesis of cholesterol in the liver; when more is taken in the diet, less is synthesized in the body. However, the cholesterol content of foods is a useful guide to the type of fat, since cholesterol is only found in the mainly saturated fats of animal origin; there is none in vegetable oils.
Unsaturated fats (those containing mainly unsaturated fatty acids), by contrast, tend to reduce the concentration of cholesterol in the bloodstream and hence reduce the risk of atherosclerosis and heart disease. Furthermore, the long-chain ω-3 polyunsaturated fatty acids (found especially in fish oils) have beneficial effects in reducing the tendency of the blood to clot undesirably, and hence reduce the risk of thrombosis. The reduction in total fat intake that is considered desirable on health grounds (from 40 per cent of energy to 30 per cent) should be entirely at the expense of saturated fats, coming down from the present average of 17 per cent of energy to only 10 per cent. Monounsaturated fats should provide about 12 per cent of energy and polyunsaturated about 6 per cent, as at present.
In naturally occurring oils, almost all of the unsaturated fatty acids are in the cis-configuration, in which the carbon chain continues on the same side of the molecule either side of each double bond. The process of hardening oils for manufacture of margarine and vegetable shortenings involves saturating some of the double bonds (thus raising the melting point so that the oil is solid at room temperature). A proportion of the double bonds that are not saturated in the process undergo a change to the trans-configuration (in which the carbon chain continues on opposite sides of the double bond). Trans-fatty acids do not have the beneficial effects of cis-fatty acids, and there is some evidence that high intakes of trans-fatty acids are associated with higher incidence of heart disease. It is therefore recommended that intakes of trans-fatty acids should not rise above the current average of about 2 per cent of energy intake.
In order to help people to reduce their intake of fat, a number of low-fat spreads have been developed to replace some or all of the butter and margarine (which contain 80-82 per cent fat) in the diet. Dairy spreads are blended cream and vegetable oil, containing 72-75 percent fat, while reduced fat spreads are mainly vegetable oils, containing 60-70 per cent fat. Both of these can be used for cooking in the same way as butter or margarine. Low-fat (37-40 per cent) and very low-fat (20-25 per cent) spreads contain dairy fat and vegetable oils, and are not suitable for cooking. Extremely low fat spreads (5 per cent fat) are made using a fat substitute such as Simplesse, a modified protein, or Olestra (both are registered trademarks), a sucrose polyester that is not digested.
See also Artery; Food Labelling; Nutrition, Human; Oil Seeds and Sources.