The major effect of the fibrates on triglycerides is to promote triglyceride-rich lipoprotein catabolism through increased lipoprotein lipase activity. Fibrates also enhance lipolysis of plasma triglycerides by a means different from that of caloric restriction. Their effect on very low-density lipoprotein metabolism also differs from that of nicotinic acid. The effect of fibrate therapy upon low-density lipoprotein-cholesterol concentrations depends upon the patients' overall lipoprotein status. The responsible mechanisms are not understood. In hypertriglyceridemic patients, fibrates often reverse abnormal changes in low-density lipoprotein composition; low-density lipoprotein heterogeneity is reduced and small dense low-density lipoproteins are eliminated, apparently secondary to reduced levels of triglyceride-rich lipoproteins. Kinetic studies indicate that fibrates do not enhance low-density lipoprotein formation rates, thus contradicting the idea that fibrate therapy causes increased low-density lipoprotein cholesterol levels via increased conversion of very low-density lipoprotein to low-density lipoprotein. Though enhanced low-density lipoprotein catabolism in hypertriglyceridemia could occur via several mechanisms, the responsible factors are largely reversed by fibrate therapy. In non-hypertriglyceridemic patients, fibrates may actually enhance the fractional clearance of low-density lipoprotein and thus reduce low-density lipoprotein levels. Fibrate therapy reverses the typical high-density lipoprotein pattern of hypertriglyceridemic patients, producing more high-density lipoprotein2a and less high-density lipoprotein2b. Such treatment also increases high-density lipoprotein cholesterol levels in patients without definite hypertriglyceridemia. Synthesis rates of apolipoproteins AI and All may be affected by fibrates. The fibrates' major effects on sterol metabolism are interference with cholesterol and bile acid synthesis and increased cholesterol secretion into bile. Although bile saturation increases in most patients, in only a relatively small percentage do gallstones actually develop; super-saturated bile is not sufficient to induce gallstone formation in most patients. Available data strongly imply that fibrates mobilize cholesterol out of tissue pools, perhaps by altering tissue cell membranes to allow cholesterol release from the cell surfaces.
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