Vitamin E response to high dietary vitamin A in the chick

Chicks were fed diets containing three levels of tocopherol, and control or high levels of retinyl palmitate for at least 24 days. Glutathione peroxidase activity in erythrocytes, plasma and liver was enhanced in tocopherol-depleted chicks, and in chicks fed high dietary vitamin A. Hepatic malonaldehyde production increased in high vitamin A chicks. Superoxide dismutase activity was increased in erythrocytes and livers of chicks on low tocopherol diets and depressed by high dietary vitamin A. The effects of high dietary vitamin A were at least partially attenuated by addition of tocopherol to the diet. Chicks fed high vitamin A, low vitamin E diets demonstrated a sixfold increase in clearance of labeled tocopherol from plasma. Additional dietary tocopherol partially prevented the rapid clearance of tocopherol due to vitamin A. Absorption studies with a nonabsorbed reference substance (141Ce) revealed that a greater fraction of dietary tocopherol was oxidized prior to the digesta reaching the duodenum when high vitamin A levels were present in the diet although no differences in percentage tocopherol absorption were observed. Secretion into the intestine of glucuronides of tocopherol was enhanced almost twofold by feeding high vitamin A. It is concluded that the interactions between tocopherol and vitamin A include: enhanced oxidation of dietary tocopherol prior to the intestine, increased tocopherol turnover, due, in part, to increased conjugation to glucuronides, and selective changes in enzymes involved in protection of cells against oxidative damage.