A model for cholesterol absorption: isotope vs. mass; single dose vs. constant infusion.

A study is presented to evaluate the relative merits of isotope and cholesterol mass measurements for cholesterol absorption. In this study, cholesterol absorption is simulated as a sequence of 10 two-pool segments in which a concentration gradient of cholesterol mass and/or label exists between the site of exogenous cholesterol entry and that of fecal loss. The model is governed by first order rate constants both for label and for mass. The appearance of labeled cholesterol in lymph and feces provides a reliable measure of the cholesterol mass increment in lymph due to exogenous cholesterol absorption. Net cholesterol absorption, calculated from constant infusion experiments, differs numerically from this mass increment. A dual isotope fecal ratio method agrees with other labeling techniques, but gives reliable information only when feces are collected for a sufficiently long time.