Cholesterol-lowering effects of guar gum: Changes in bile acid pools and intestinal reabsorption

Soluble fibers such as guar gum (GG) may exert cholesterol-lowering effects . It is generally accepted that bile acid (BA) reabsorption in portal blood is reduced, thus limiting the capacity of BA to down-regulate liver choles terol 7 alpha-hydroxylase, the rate-limiting enzyme of BA synthesis. In the present work, rats were adapted to fiber-free (FF) or 5% GG diets (supplem ented or not with 0.25% cholesterol), to investigate various aspects of ent erohepatic BA cycling. GG in the diet at a level of 5% elicited a significa nt lowering of plasma cholesterol during the absorptive period, in choleste rol-free (-13%) or 0.25% cholesterol (-20%) diet conditions. In rats adapte d to the GG diets, the small intestinal and cecal BA pools and the ileal ve in-artery difference for BA were markedly enhanced; reabsorption in the cec al vein was also enhanced in these rats. [C-14]Taurocholate absorption, det ermined in perfused ileal segments, was not significantly different in rats adapted to the FF or GG diet, suggesting that a greater flux of BA in the ileum might support a greater ileal BA reabsorption in rats adapted to the GG diet. In contrast, capacities for [C-14]cholate absorption from the cecu m at pH 6.5 were higher in rats adapted to the GG diet than to the FF diet. Acidification of the bulk medium in isolated cecum (from pH 7.1 down to pH 6.5 or 5.8) or addition of 100 mM volatile fatty acids was also found to s timulate cecal [C-14]cholate absorption. These factors could contribute to accelerated cecal BA absorption in rats fed the GG diet. The effects of GG on steroid fecal excretion thus appear to accompany a greater intestinal BA absorption and portal flux to the liver. These results suggest that some m echanisms invoked to explain cholesterol-lowering effect of fibers should b e reconsidered.