The effects of vitamin A, beta-carotene and canthaxanthin on vitamin A metabolism and immune responses in the chick.

Chicks were fed diets containing, 0, 0.85 and 350 mg/kg vitamin A and 1 g/kg beta-carotene or canthaxanthin from hatching. Carotene increased and canthaxanthin depressed plasma and hepatic vitamin A concentrations. High vitamin A levels decreased the concentration of both carotenoids. Neither carotene nor canthaxanthin affected intestinal carotene cleavage in vitro. T-lymphocyte proliferative responses were decreased at low vitamin A intakes and enhanced at the high vitamin A intake. Carotene and canthaxanthin fed with 0.85 mg/kg vitamin A had no effect on immune response and with 350 mg/kg vitamin A prevented the enhancement of the proliferative response. It is concluded that immune response in the chick is modulated by vitamin A; carotene and canthaxanthin effects are probably due to influences on vitamin A metabolism.