The bioavailability of alpha- and beta-carotene is affected by gut microflora in the rat

The present study examined whether the intestinal microflora could affect the bioavailability and vitamin A activity of dietary alpha- and beta-carotene in the rat. In the first set of experiments, we used conventional, germ-free (axenic), and human-flora-associated (heteroxenic) rats. In a second series, conventional rats were treated with either an antibiotic mixture or a potent inhibitor of gastric secretion (Omeprazole). All animals were first depleted of vitamin A over 4 weeks and then were fed on a sterilized diet supplemented with 14 mg beta-carotene and 3 mg alpha-carotene/kg for 2 weeks. In both experiments, a reduction in the intestinal microflora resulted in an increased storage of beta-carotene, alpha-carotene and vitamin A in the liver. Neither the nature of the metabolism of the intestinal microflora (aerobic or anaerobic) nor treatment with omeprazole, to modify intestinal pH, induced a significant effect on the measured variables. When incubated with 15 mumol beta-carotene/l for 72 h, neither the anaerobic nor the aerobic sub-fractions obtained from rat or human faeces contributed to beta-carotene degradation or to vitamin A synthesis. These findings suggest that reduction in gut microflora results in a better utilization of alpha- and beta-carotene by rats, although bacteria do not have a direct effect on the bioavailability of these pigments.