β-Carotene absorption and cleavage in rats is affected by the vitamin A concentration of the diet

Abstract

The purpose of this study was to examine whether intestinal β-carotene cleavage activity, measured with the dioxygenase assay, is affected by vitamin A intake and whether this in vitro activity is a determinant of β- carotene cleavage in vivo, measured in lymph-cannulated rats. 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 β-carotene/kg in the last 6 wk. Intestinal dioxygenase activity was 90% higher (P < 0.05) in the animals fed the unsupplemented low vitamin A diet than in the animals fed the unsupplemented high vitamin A diet, whereas in β-carotene-supplemented rats intestinal dioxygenase activity was significantly lower than in unsupplemented rats. The molar ratio between retinyl esters and β-carotene in lymph collected over 8 h after a single intestinal dose of β-carotene (250 μg) to β-carotene-unsupplemented rats fed the three levels of vitamin A was correlated with intestinal dioxygenase activity (r = 0.66, P = 0.003). Dioxygenase activity in the liver was not affected by the vitamin A concentration of the diet but was 70% higher in the β-carotene-supplemented rats. Based on the difference in liver vitamin A contents between β- carotene-supplemented and unsupplemented rats we estimated β-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. Intestinal β-carotene cleavage activity is higher in vitamin A-deficient rats than in rats with a high intake of either vitamin A or β-carotene. The intestinal dioxygenase activity as measured in vitro is an adequate indicator of in vivo β-carotene cleavage activity.