BETA-CAROTENE ABSORPTION AND CLEAVAGE IN RATS IS AFFECTED BY THE VITAMIN-A CONCENTRATION OF THE DIET

The purpose of this study was to examine whether intestinal beta-carot ene cleavage activity,measured with the dioxygenase assay, is affected by vitamin A intake and whether this in vitro activity is a determina nt of beta-carotene cleavage in vivo, measured in lymph-cannulated rat s. Six groups of 10-20 rats were fed a diet with a low, normal or high retinyl palmitate concentration (120 RE, 1200 RE and 12,000 RE per kg , respectively) for 14 to 18 wk, either supplemented or not with 50 mg beta-carotene/kg in the last 6 wk. Intestinal dioxygenase activity wa s 90% higher (P < 0.05) in the animals fed the unsupplemented low vita min A diet than in the animals fed the unsupplemented high vitamin A d iet, whereas in beta-carotene-supplemented rats intestinal dioxygenase activity was significantly lower than in unsupplemented rats. The mol ar ratio between retinyl esters and beta-carotene in lymph collected o ver 8 h after a single intestinal dose of beta-carotene (250 mu g) to P-carotene-unsupplemented rats fed the three levels of vitamin A was c orrelated with intestinal dioxygenase activity (r = 0.66, P = 0.003). Dioxygenase activity in the liver was not affected by the vitamin A co ncentration of the diet but was 70% higher in the beta-carotene-supple mented rats. Based on the difference in liver vitamin A contents betwe en beta-carotene-supplemented and unsupplemented rats we estimated bet a-carotene conversion factors of 9:1 for the rats fed the high vitamin A diet and 4:1 for the rats fed the normal and low vitamin A diets. I ntestinal beta-carotene cleavage activity is higher in vitamin A-defic ient rats than in rats with a high intake of either vitamin A or beta- carotene. The intestinal dioxygenase activity as measured in vitro is an adequate indicator of in vivo beta-carotene cleavage activity.