fats

fats or, more technically, lipids, together with carbohydrates and proteins, are one of the three staples of the diet. As the seventeenth-century nursery rhyme relates, ‘Jack Sprat he ate no fat/His wife she ate no lean’. This does not mean, however, that Jack Sprat could not get fat, because excess carbohydrates and proteins can be broken down in the body and the fragments synthesized into fat. But had he eaten no fat whatsoever he would have been deprived of certain essential fatty acids, and of the fat-soluble vitamins. Fats serve many purposes: they not only provide thermal insulation and a source of energy, with stores in reserve, but also are involved in important functions, providing for example the materials for components of cell membranes, of the myelin sheaths that electrically insulate nerve fibres.

Chemically, fats are esters — that is, formed by combination of an alcohol with an acid. In this case the alcohol is glycerol, which has three alcohol groups, allowing it to combine with three acidic groups. The acids are fatty acids: long chains of carbon atoms linked to a carboxyl (acidic) group. Three fatty acids therefore combine with one glycerol molecule to form a triglyceride or neutral fat molecule.

Triglycerides are the main fats in the diet. After breakdown and absorption in the alimentary system, they are resynthesized and stored in special cells in which fat globules coalesce to form a large droplet, almost filling the cell. Aggregations of fat cells form adipose tissue (as in the white fat of uncooked meat). White fat can be deposited in a variety of tissues, especially beneath the skin and in muscles — but a great deal of human endeavour is today devoted to getting rid of adipose tissue collected around the waist, even to the extent of having liposuction. Fat acts as a rich fuel reservoir that can be utilized during starvation. Oxidation of fat yields about twice the energy that can be derived from equal amounts of carbohydrate or protein. However, white fat is not easily got rid of physiologically, since it is poorly perfused with blood. Hence hormones that mobilize fat do not reach the target in high concentration, nor is there a high flow rate to carry the energy-giving molecules in the blood to the tissues, such as muscles, where they can be burned.

There is a second type of fat deposit, namely brown fat, found at the base of the neck and between the shoulder blades. This tissue is specialized for thermogenesis, i.e. the rapid mobilization of fat to generate heat. The cells contain many small lipid droplets, a pigment, and many mitochondria, the latter essential for breaking down the lipids into simple two carbon fragments from which energy can be rapidly generated. Furthermore the tissue is well perfused with blood and has a rich innervation by nerves liberating noradrenaline, one of the hormones that can rapidly mobilize the breakdown of fats. Brown fat is important for heat production in infants, and is retained variably into later life. Those who retain the most brown fat into adulthood find it easier to avoid putting on weight. Hibernating animals lay down large amounts of brown fat to see them through the dormant period.

Many different fatty acids are found among the lipids. They vary in the number of carbon atoms in the chain, which in some cases are branched, and also in the number of double bonds they contain, if any. Those with double bonds are known as unsaturated, and those without as saturated. For example palmitic acid and palmitoleic acid both have 16 carbon atoms, but the latter has one double bond (monounsaturated). Oleic, linoleic, and arachidonic acid have respectively 16, 18, and 20 carbon atoms in the chain and 1, 2, and 4 double bonds. Oleic and linoleic acids are essential fatty acids, that is they cannot be synthesized in the body and are therefore essential dietary constituents. Readers will be familiar with the term ‘rich in polyunsaturates’ applied to many supermarket products like margarines, sunflower oils, etc. There is evidence that diets rich in polyunsaturated fats are less likely to cause atherosclerosis than ones rich in animal fats, with their predominance of saturated fats.

Fatty acids are essential constituents of phospholipids and glycolipids. In phospholipids the glycerol moeity is combined with only two fatty acids, the remaining alcohol group being combined with a phosphate group linked to an alcohol (e.g. to serine, choline, or inositol, to give the phospholipids phosphatidylserine, phosphatidylcholine, and phophatidylinositol). These compounds are ‘amphipathic’, that is, they have a polar head group, which is compatible with an aqueous environment, and a non-polar tail, which is not. Such molecules can form films (as does oil spread on the surface of water) in which the hydrophobic tails interact with each other, projecting out of the aqueous surface, while the polar head groups remain in the water. Now imagine such films are brought together so that the hydrophobic tails of one film are opposed to those of the second. This is a very close approximation to the structure of all cell membranes, where the polar head of one lipid layer contacts the extracellular environment, while the head groups of the other layer contact the aqueous environment within the cell. These structures, so-called lipid bilayers, form flexible membranes, which are very impermeable to the movement of substances across them, whilst particular permeability properties are provided by the inclusion of protein molecules in the membrane. Some membrane fatty acids can be mobilized as autacoids (released to affect other cells) and as intracellular messengers. For example, arachidonic acid gives rise to prostaglandins in cell membranes, and phosphatidylinositol is the source of the important ‘second messenger’ inositol triphosphate, implementing an internal response to a chemical message from outside the cell.

Alan W. Cuthbert

See also body weight; cell membrane; metabolism; obesity.