Hyperlipidaemia of diabetes.

There is general agreement as to the effects of diabetes on the concentrations of certain plasma lipoprotein lipids. Subdivision of diabetic subjects into several clearly defined subgroups has revealed that the detailed patterns of lipoprotein and lipid changes are dependent upon several factors, perhaps the most important of which is the degree of glycaemic control. Several aspects remain controversial, the most outstanding being whether or not plasma LDL cholesterol levels are elevated. It is possible that this is the case in certain clearly defined subgroups but not in others. In assembling the results of recent research for this review, two important aspects have emerged which require clarification. The first is the question of whether or not insulin directly stimulates hepatic VLDL secretion. The possibility that insulin might regulate production of lipogenic substrate by the gut may have an important bearing on this problem. The exact means by which insulin co-ordinates the metabolic activities of these organs in such a way as to ensure plasma lipid balance is not yet known and further research in this area may help to resolve some outstanding problems associated with diabetic hyperlipidaemia. Second, changes in the relative lipid composition of certain lipoprotein fractions in diabetic subjects has provided indirect evidence that increased lipoprotein 'remnant' concentrations may contribute to the abnormalities observed in some groups of diabetic subjects. This interesting possibility has been supported by metabolic studies, mainly in experimental animals. If this proves to be correct, then it remains to be determined whether the remnants involved are of hepatic or intestinal origin and whether the metabolic defect is related to abnormal production, clearance, or both. Recent work on the effects of changes in the apoprotein and lipid content on the metabolism of other lipoproteins in diabetes may have a useful bearing on studies of this type. In this respect, the bulk of the evidence seems to suggest that these factors, rather than changes in lipoprotein receptor activity per se, are important in determining the clearance of atherogenic lipoproteins such as LDL in diabetes.