Absorption of Retinol and Retinyl Esters via Lymph and the Portal Vein in the Rat

3H-11,12-retinyl acetate (3H-RA) was administered orally at various dose levels to normal and vitamin A deficient male Wistar rats. While the greater proportion of the vitamin was absorbed as retinyl ester (RE), a small but measurable amount of retinol (ROH) consistently appeared in plasma and liver within 1 h after dosing. Comparison of the specific activities (S.A.) of plasma and liver ROH and RE indicated that ROH had been absorbed as such. Furthermore, labelled ROH was detected in the lymph of thoracic duct cannulated rats within 30 min after dosing. The S.A. of the recovered ROH was very similar to that of the administered 3H-RA. The presence of labelled ROH and RE in plasma of cannulated rats suggested simultaneous absorption via the portal vein. The ability of the portal vein to absorb these two forms of the vitamin was confirmed in thoracic duct ligated rats. Quantitative estimates of ROH and RE over the two routes of absorption could not be made from the present data.Thus a small amount of ingested vitamin A is consistently absorbed as ROH as a normal event during absorption. The possible physiological significance of these observations is discussed.