The intestinal absorption and metabolism of vitamin A and beta-carotene in man.

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Abstract
[beta]-Carotene-15,15[image]-H3, vitamin A alcohol-15-C14, and vitamin A-15-H3 acetate were fed to patients in whom polyethylene cannulae had been inserted in the thoracic duct in the neck. Serial samples of lymph were collected, and the lipid was extracted and chromatographed on columns and on thin layer plates of al?mina. Absorption of radioactivity into the lymph mainly occurred between 3 and 10 hr. During this time washed chylomicrons contained 70-80% of the absorbed radioactivity. Labeled vitamin A esters predominated in all lymph samples, representing 80-90% of the absorbed radioactivity after the feeding of labeled preformed vitamin A, and 60-70% after [beta]-carotene. The fatty acid composition of the vitamin A esters in lymph bore no resemblance to the composition of the diet and was remarkably constant, regardless of the fatty acid composition of the diet, regardless of whether the vitamin A esters were derived from dietary vitamin A or from [beta]-carotene, and regardless of whether chylomicron or nonchylomicron lipid was analyzed. Vitamin A palmitate predominated in all samples, and saturated esters (vitamin A palmitate plus stearate, in an average ratio of 2.4 ) consistently comprised 75-85% of the labeled esters. Small amounts of vitamin A oleate and linole-ate were also found in all samples. Unchanged labeled [beta]-carotene comprised only 20-30% of the absorbed radioactivity, after ingestion of [beta]-carotene-H3. The human intestine possesses only an extremely limited ability to absorb unchanged dietary [beta]-carotene into the lymph. The fatty acid composition of the lymph vitamin A esters was similar to but not identical with that of the [image][image] position of lymph lecithin.