DISTINCT MECHANISMS OF PLASMA LDL LOWERING BY DIETARY FIBER IN THE GUINEA-PIG - SPECIFIC EFFECTS OF PECTIN, GUAR GUM, AND PSYLLIUM

Pectin (PE), guar gum (CG), and psyllium (PSY) lower plasma low densit y lipoprotein (LDL) cholesterol concentrations ill guinea pigs with di fferent orders of magnitude by inducing defined alterations in hepatic cholesterol homeostasis (Fernandez et al. 1994. Am. J. Clin. Nutr. 59 : 869-879; 61: 127-134 and 1995. J. Lipid Res. 36: 1128-1138). To furt her explore specific mechanisms responsible for the differences in pla sma and hepatic cholesterol lowering, the effects of these fibers were evaluated on cholesterol absorption, hepatic cholesterol 7 alpha-hydr oxylase activity, the rate-limiting enzyme of bile acid synthesis, and in vivo LDL transport to target specific primary and secondary mechan isms accounting for the observed responses. Fibers were fed with physi ological (0.04%), low cholesterol (LC), or pharmacological high choles terol (HC) (0.25%) levels to assess whether cholesterol intake influen ces plasma LDL lowering mechanisms. Intake of PE, GG, or PSY with LC o r HC diets lowered plasma and hepatic cholesterol concentrations (P < 0.001). PE and PSY up-regulated 7 alpha-hydroxylase activity 3-fold wi th LC and PE by 5-fold with HC diets. In contrast, GG intake had no ef fect on 7 alpha-hydroxylase activity. Cholesterol absorption was reduc ed 30% by PE intake while no differences were found between control an d PSY groups. GG reduced cholesterol absorption only with HC diets. In take of PE, GG, or PSY with HC diets resulted in faster plasma LDL fra ctional catabolic rates (FCR) (P < 0.01) with no effect on LDL apoB fl ux rates (FR) or pool size, suggesting that fiber reduced LDL choleste rol concentration without decreasing the number of LDL particles. In a ddition to reducing LDL apoB FR, PE and PSY increased LDL FCR with HC diets while cc effects were limited to lowering LDL apoB FR. These res ults indicate that the distinctive reductions in hepatic cholesterol i nduced by PE, GG, and PSY associated with plasma cholesterol lowering result from different mechanisms specific to each fiber and that the l evels of dietary cholesterol contribute to the different metabolic res ponses.