Cholesterol and plant sterols

Because the amount of dietary cholesterol is normally low, a substantial fraction of the cholesterol in the intestinal lumen comes from the biliary cholesterol secreted by the liver. ABCG5/8 also is expressed in the apical membrane of intestinal epithelial cells, where it helps control the amounts of cholesterol and plant-derived sterols absorbed apparently by pumping excess or unwanted absorbed sterols out of the epithelial cells back into the lumen (see Figure 18-11, step 5]). Partly as a result of this activity, only about 1 percent of dietary plant sterols, which are not metabolically useful to mammals, enter the bloodstream. Unabsorbed bile acids (normally <5 percent of the luminal bile acids) and unabsorbed cholesterol and plant sterols are eventually excreted in the feces. 

Several ABC proteins export bile components from he-patoc3rt es ABCB4 (phospholipids), ABCBll (bile salts), and ABCG5/8 (cholesterol and plant sterols). Na" -linked... 

NPCILI is a protein with 42% identity with NPCl (J.P. Davies, 2000). Several lines of evidence indicate that NPCILI plays a role in the intestinal absorption of cholesterol and plant sterols (S.W. Altmann, 2004) [22]. NPCILI is found on the plasma membrane of enterocytes in the proximal jejunum. NPCILI homozygous knockout mice absorb significantly less cholesterol than control mice and are insensitive to the effects of ezetimibe, a cholesterol absorption inhibitor that lowers plasma LDL cholesterol levels. 

Cholesterol, plant sterols, and bile acids Measurements of cholesterol and plant sterols in plasma and lymph are obtained by GC, but HPLC has also gained in importance as a method of sensitive... 

Miettinen TA, Vuoristo M, Nissinen M, Jarvinen HI, Gylling H (2000) Serum, biliary, and fecal cholesterol and plant sterols in colectomized patients before and during consumption of stanol ester margarine. Am J Clin Nutr 71(5) 1095-1102... 

Child, P and Kuksis, A (1983) Critical role of ring structure in the differential uptake of cholesterol and plant sterols by membrane preparations in vitro. J. Lipid Res., 24, 1196-1209. 

The cholesterol-lowering properties of phytosterols were first demonstrated in the early 1950s by Peterson (1951), who fed cholesterol and plant sterols to chicks and found that soy sterols inhibited the increase in cholesterol concentration otherwise induced by cholesterol feeding. Shortly afterwards, Poliak (1953) showed the same effect in humans, to whom he administered 5-10 g of crude sitosterol daily for up to 8 months. In similar studies in rabbits, he observed that sitosterol was poorly absorbed and that, if present in excess, it blocked the absorption of cholesterol and prevented atherosclerosis. Poor absorption and lack of any endogenous synthesis of sitosterol in humans were subsequently demonstrated by Salen et al. (1970). 

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