INTERACTIONS

Vitamin E increases the level/storing of vitamin A in various tissues.

Zinc is essential for 40-50 enzymes. Some of these are directly or indirectly involved in vitamin A's metabolism or function: retinaldehyde reductase oxidizing retinol in the eye, RAR og RXR (Zinc-finger proteins involved in binding to DNA), reduced RBP in plasma due to reduced secretion from liver in zinc deficiency.

Toxic substances like DDT, PCB, dioxins and ethanol reduce the vitamin A store in liver, probably due to the induction of catabolising enzymes, which also break down retinol.

Low level of plasma retinol can occur in PEM (protein energy malnutrition). Plasma retinol is necessary for synthesis of RBP.

During infection, plasma levels of retinol will be low due to reduced sythesis of RBP and TTR, which are negative acute-phase proteins.

Vitamin A - vitamin D antagonism

From epidemiological studies it is shown that the average dose of vitamin A that caused toxicity was higher when vitamin A was given with vitamin D, which could mean that vitamin A is less toxic when given with vitamin D.

Vitamin A and vitamin D could both decrease each other's activity (trans-activity) in biological studies.

These results raise some questions:

• Could vitamin D deficiency increase sensitivity for retinol poisoning?
• Could a high intake of retinol reinforce symptoms of vitamin D deficiency?

These questions are important to clarify, especially regarding the elder population where vitamin D deficiency is not uncommon and a risk factor for bone fractures.