Studies on the effect of dose size on the absorption of β-carotene by the rat in vivo

This study was carried out to choose between two hypotheses with respect to the regulation of beta-carotene (BC) conversion to retinol in the whole animal: uptake of BC into intestinal mucosa is limited by saturation Of an intestinal receptor; Or the conversion to retinol is limited by saturation of the conversion enzyme(s). Groups of rats were given five:different dose levels of labeled BC by stomach tube. Labeled and total BC and retinol were isolated from tissues and intestinal contents after 4 h. Results showed a positive: linear relationship between BC in: the intestinal wall and the dose administered, with no saturation level up to 1440-mu g administered. Per cent formation of newly formed retinol from newly absorbed (i. e., labeled) BC was 20-26% of the three-lower dose groups, 10% for the highest dose. No retinyl esters could be detected in the intestine. Most of the administered BC was in the intestinal contents, about 100-times more than in the:intestinal wall and mucosa. Newly formed retinol in plasma was about 10-times that in liver Small amounts of newly absorbed BC were found in liver, but no labeled retinyl esters. These results suggest that the absorption:Of BC is:very inefficient; that it does not Occur through an intestinal receptor; that the formation of retinol is regulated at the level of the conversion enzyme(s).