Fats, also known as lipids, are important components of living tissues, and are used by the body for making cell membranes and for storing energy. Fats come in a variety of different biochemical types, which may be obtained from the diet or can be synthesised within the body. Many cells of the body can convert certain types of fat into others, but by preference, fats will be obtained from the diet, if available. The fatty acids that cannot be synthesised by the body and therefore must be obtained from the diet are called essential fatty acids. To understand the different types of fat (see Figure 8), and how the body can make use of them, you need to know about the building blocks from which they are made.
Many fats are formed by three fatty acids linked together by a molecule of glycerol. Each of the fatty acids has a long tail (acyl groups), so this compound is called a triacylglycerol (often called a triglyceride). The length of the acyl groups can vary, as can their chemical type (see Figure 8(a)). Monounsaturated fats have a particular chemical bond which produces a kink in the acyl groups (see Figure 8(c)). Polyunsaturated fats have a number of kinks, which vary in number and position (see Figure 8(c)). The lipids present in cell membranes are usually diacylglycerols, in which one of the fatty acids is substituted with a completely different type of molecule – generically called a ‘head group’ (see Figure 8(d)). Cholesterol is not a fat, but its structure is functionally similar to the fatty acids, so it can sit in cell membranes in a similar way to them. The liver can synthesise cholesterol, but in practice most is derived from the diet (see Figure 8(e)).
Fatty acids can be classified as saturated or unsaturated, depending on their chemical structure. Generally, complex fats that include unsaturated fatty acids are more fluid than those containing saturated fatty acids, because their tails do not pack together so neatly. However, in one group of unsaturated fatty acids, trans fatty acids, the kink(s) which are present in more abundant unsaturated fats are minimal. So, like saturated fats, the compounds formed from them also have lower fluidity. The precise position(s) of the special chemical bond(s) which produce the kinks in an unsaturated fatty acid is indicated by its biochemical nomenclature, e.g. omega-3 fatty acids. The pattern of kinks is also fundamental to the kinds of more complicated molecules that can be built from fatty acids.
Fatty acids and cholesterol are transported around the body in the blood or the lymph (clear fluid that bathes tissues), in association with specific types of proteins that link them together and prevent them from sticking to other molecules. Some are just attached to a protein in the blood called albumin, but larger and more complex combinations of lipids and protein are called lipoproteins. Different types of lipoprotein are classified according to their density, which reflects their lipid and protein composition. They may be high-density lipoproteins (HDL), low-density lipoproteins (LDL) or very low-density lipoproteins (VLDL). Fat is less dense than protein, so, as an example, LDL has a higher proportion of cholesterol and saturated fatty acids than HDL. Dietary fats are transported in the blood to the liver as the very large lipoproteins called chylomicrons. Those that reach the liver are processed and may be converted to other fatty acids before the blood carries them on to other tissues. The liver also acts as a way-station (store) for fats from the tissues that are to be used as a source of energy or for synthesising cell membranes.