Differential absorption of exogenous and endogenous cholesterol in man.

Cholesterol absorption was measured in six patients by a triple-lumen intubation technique: 1) to determine whether isotopic exchange between radiolabeled lumenal cholesterol and unlabeled mucosal cholesterol occurs during cholesterol absorption measurements, and 2) to differentiate the rates of absorption of endogenous cholesterol ([1,2-3H]cholesterol marker given intravenously 6 weeks prior to the tests), exogenous cholesterol ([4-14C]cholesterol marker infused into the proximal duodenum), and total cholesterol mass (sitosterol marker incorporated in the infusion mixture). Measurements of endogenous cholesterol absorption during infusion of a cholesterol-free formula produced identical results whether calculated by the [1,2-3H]cholesterol marker (46.2 +/- 18.2%) or sitosterol marker (44.3 +/- 16.9%) in eight studies. When exogenous cholesterol was administered in liquid formula, its absorption was significantly lower than that of endogenous cholesterol in six out of nine experiments (endogenous = 46.4 +/- 15.4%; exogenous = 33.6 +/- 8.3%; total = 40.9 +/- 10.0%). When exogenous cholesterol was dissolved and administered in triglycerol monooleate, its absorption was higher than that of endogenous cholesterol in four of seven experiments (endogenous = 29.6 +/- 14.5%; exogenous = 33.4 +/- 9.1%; total = 29.9 +/- 12.0%). These comparisons indicate that differential absorption of endogenous and exogenous cholesterol can occur over a 1- or 2-meter segment of the upper small intestine, and that the rate of cholesterol absorption is critically dependent on the physicochemical state of the intralumenal contents. Our results indicate that currently available methods for measurement of cholesterol absorption (most of which are based on the use of radioisotopic sterols for differentiating exogenous from endogenous cholesterol) reliably quantitate the absorption of dietary cholesterol, and that the results of such tests are not significantly confounded by in vivo isotopic exchange.