Studies on the Intestinal Absorption of Radioactive β-Carotene and Vitamin A in Man: Conversion of β-carotene into vitamin A

Abstract

β-carotene-15, 15^1-14C, β-carotene-15, 15^1-3H, vitamin A alcohol-15^-14C, and vitamin A-15-³H acetate were fed to eight patients in whom polyethylene cannulae had been inserted in the thoracic duct in the neck. The lymph lipids were extracted and chromatographed on columns and on thin-layer plates of alumina. Only 8.7-16.8 per cent of the fed labelled β-carotene is normally absorbed via the lymphatic pathway and mainly recovered as retinyl esters. Unchanged labelled β-carotene comprised only 1.7 to 27.9 per cent of the radioactivity after ingestion of labelled β-carotene.

The recovery of the radioactivity in the lymph lipids after feeding labelled vitamin A alcohol or acetate was in the range of 6.7 to 66.9 per cent. Absorption of radioactivity into the lymph followed the appearance of the chylomicron triglycerides.

Labelled retinyl esters, mainly retinyl palmitate, represented 75 to 90 per cent of the absorbed radioactivity after feeding labelled vitamin A, and 60 to 75 per cent after feeding labelled β-carotene. It is concluded that the reaction mechanism of vitamin A biosynthesis in the human intestinal mucosa consists of central cleavage of β-carotene into two molecules of vitamin A alcohol.

In one patient fed β-carotene-15, 15^1_14C, 46.9 per cent of the fed radioactivity was recovered as β-carotene, suggesting the presence of an enzymatic block in the conversion mechanism.

Key Words:

• Carotene
• vitamin A
• gastrointestinal system
• carbon isotopes
• hydrogen isotopes