Bile salts, sterols, sterol esters, glycerides and fatty acids in micellar and oil phases of intestinal contents during fat digestion in man.

The relationship between intestinal bile acids and the micellar solubilization of lipids was studied by determining micellar bile acids and distribution of cholesterol and 0-sitosterol, their esters, and of hydrolysis products of triglycerides (labeled with H-tristearate), free fatty acids, monoglycerides and diglycerides between the micellar and oil phases of the intestinal contents for 1% h after the administration of a fatty meal to normal human subjects. The dilution of radioactivity indicated that non-labeled lipids were secreted during fat digestion into the gut lumen particularly into the micellar free fatty acids fraction. About one third of the total intestinal lipids was solubilized by the micellar phase, which consisted primarily of free fatty acids and monoglycerides while triglycerides and diglycerides were concentrated in the oil phase. Half of the total intestinal cholesterol and only one third of the ^-sitosterol were in the micellar phase, primarily in the free form. Esterified sterols were enriched in the oil phase which also contained, compared to cholesterol, a relatively high amount of free ß-sitosterol, suggesting that low micellar solubilization of this sterol is a factor in its low absorption. Calculations indicated that unabsorbable jS-sitosterol could be used as an internal marker for lipid absorption. The levels of total micellar lipids and micellar ßsitosterol were not correlated with the micellar bile acid concentration, while the concentration of intestinal cholesterol showed a correlation with the bile acids or the molar sum of bile acids, monoglycerides and free fatty acids. The level of free, esterified and total micellar sitosterol was correlated with free fatty acids, total lipids and the molar sum of micellar bile acids, monoglycerides and free fatty acids, indicating that the digestion products of triglycerides actually contribute to micellar solubilization of sterols.