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

Abstract

1. 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.2. 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.3. Labelled glucuronides were secreted into the duodenum and were 5&60% reabsorbed in the small intestines. High-vitamin-A intakes enhanced the duodenal flow of retinyl glucuronides.4. In vitro hepatic retinol esterification and glucuronide production were enhanced in chicks given high dietary vitamin A.5. 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.6. 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.

Serum retinol is influenced by social factors and antioxidant nutrients among adolescent girls in urban Bangladesh

“…The serum a-tocopherol (vitamin E) concentration has been positively associated w i t h the level of serum retinol (vitamin A) (Rahim et al, 1993). Thus the observation that plasma a-tocopherol concentration shows a positive correlation with the ponderal index (Herbeth et al, 1988) may be direct, but as vitamin E exerts a sparing effect on vitamin A by reducing its turnover, and increasing liver storage (Sklan, 1983), indirect effects can not be ruled out. Vitamin C plays a critical role as a tissue antioxidant in its own right, but is also effective in reducing vitamin E radicals, thereby regenerating vitamin E in a functional form to act as lipid antioxidant (Packer et al, 1979).…”

Effects of retinoids on immune responses in birds

“…Once sufficient intake of vitamin A is achieved no change in growth or laying is observed over a 10-fold increase in vitamin A intake. Intakes of vitamin A at levels approximately 50 times greater then requirements give rise to increases in circulating retinyl esters together with signs of toxicity including membranolytic changes and skin lesions (Kriz and Holman, 1969;Sklan, 1983;Bondi and Sklan, 1984).…”

“…This approach was based on previous observations that a high intake of vitamin A increased tocopherol turnover, and high tocopherol levels ameliorated the effects of toxic levels of vitamin A (Sklan and Donoghue, 1982;Sklan, 1983). The findings indicate that increased tocopherol nutriture partially protected antibody production in high intakes of dietary vitamin A (Friedman, Bartov, Heller and Sklan, unpublished observations).…”

“…This is probably due to their different chemical and biological properties, such as sensitivity to oxidation, bioavailability and efficiencies of intestinal absorption and metabolic conversion (Sklan, 1983; Blomhoff e f al., 1991). RA or 13-cis-RA seem to be marginally the most potent.…”

Vitamins E and A

“…In the intestinal mucosa a copper-containing dioxygenase (Goodman et aL, 1967;Sklan, 1983a) catalyses cleavage of the carotene molecule at C-15, yielding retinal. A zinc-containing retinene reductase, or alcohol dehydrogenase, then reduces the retinal to retinol.…”