Vitamin A absorption and metabolism in the chick: response to high dietary intake and to tocopherol.

Absorption of 14C-labelled retinol was determined along the gastrointestinal tract of chicks, using 141Cerium as a non-absorbed reference substance, as influenced by high dietary vitamin A and the addition of tocopherol to the diet. The site of major vitamin A absorption was the proximal small intestine and high intake of vitamin A reduced slightly the percentage over-all absorption. Labelled glucuronides were secreted into the duodenum and were 50-60% reabsorbed in the small intestines. High-vitamin-A intakes enhanced the duodenal flow of retinyl glucuronides. In vitro hepatic retinol esterification and glucuronide production were enhanced in chicks given high dietary vitamin A. Plasma transport and clearance of retinol were enhanced by feeding high levels of vitamin A and further enhanced when tocopherol at high concentrations was present in the diet. Dietary tocopherol had no effect on absorption, increased hepatic vitamin A stores and in vitro retinol esterification, and decreased retinyl glucuronide flows through the duodenum.