Differential Absorption of Endogenous and Exogenous Cholesterol in the Chick as Affected by Dietary Oil Level and Phytosterols

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Abstract
The absorption of total and endogenous cholesterol was studied in 20-day-old chicks which received for 1 week cholesterol-containing diets with 91YCl3 added to serve as a non-absorbable reference substance. Endogenous cholesterol was labeled by iv injection of (14C) cholesterol, and the ratios of total and labeled cholesterol to 91Y were determined in the contents of different segments of the small intestine. The data served to calculate the overall net absorption of dietary cholesterol, the percent cumulative net absorption of endogenous and total cholesterol between the duodenum and the lower ileum, and the changes in specific activity of cholesterol in the digesta along the small intestine. The experimental diets were designed to provide information on the effect of high and low levels of oil (15 and 5% olive oil) and of mixed phytosterols on the absorption of total and endogenous cholesterol. The changes in ratios of total and labeled cholesterol to 91Y, the data on the percent cumulative absorption and the drop in specific activity of luminal cholesterol along the upper small intestine, all lead to the conclusion that endogenous cholesterol is absorbed more rapidly and more completely than exogenous cholesterol. More direct evidence indicating incomplete mixing of these two sources of cholesterol in the lumen was provided by the observation that the specific activity of cholesterol in the aqueous phase was at least double that in the particulate fraction. The percent absorption of endogenous cholesterol was essentially constant in the various dietary treatments. The high-oil diet caused a slight but non-significant increase in the absorption of total cholesterol, whereas phytosterols significantly decreased the absorption of total, hence exogenous, cholesterol.