ABSTRACT
A little knowledge of lipid metabolism is a considerable help in understanding some of the more clinical aspects of the disorders of lipoprotein metabolism that are the main theme of this book.
Definitions are seldom easy, but the definition of a lipid is more than usually difficult. The question ‘What is a lipid, doctor?’, well-meant and coming from a patient attending a busy lipid clinic for the first time, can, if met unprepared, lead to a response that does little to inspire confidence in the expertise of the doctor. A lipid is, of course, a fat (also an oil or wax), and that explanation may well satisfy the patient. In more biochemical terms there is, however, no unifying structure, as in the case of proteins or sugars, to provide any satisfactory definition. Included among lipids are substances in which fatty acids are an essential component, such as triglycerides, glycerophospholipids, sphingophospholipids and waxes, but also substances as structurally diverse as cholesterol and other steroids, terpenes and prostaglandins. Thus, the only possible definition is that they are a heterogeneous group of substances that have in common their low solubility in water, but which are more readily soluble in a mixture of chloroform and methanol (2:1 v/v). They are also soluble in other non-polar (organic) solvents such as hydrocarbons, alcohols and ether. The difference between an oil and a fat is determined by the melting point.
