The vitamin A-zinc connection: a review

Area of Science:

• Biochemistry
• Nutritional Science
• Physiology

Background:

• Zinc deficiency is known to affect vitamin A levels in animals.
• The exact mechanisms and direct link between zinc status and vitamin A metabolism require further investigation.

Purpose of the Study:

• To elucidate the relationship between zinc deficiency and vitamin A metabolism.
• To differentiate between direct and indirect effects of zinc deficiency on vitamin A levels.
• To explore the role of zinc in visual function and enzyme activity related to vitamin A.

Main Methods:

• Review of animal studies examining plasma and liver vitamin A concentrations in zinc-deficient subjects.
• Analysis of retinol-binding protein (RBP) levels in relation to zinc status.
• Examination of the impact of zinc deficiency on retinal enzymes like retinene reductase.
• Consideration of limited human studies on zinc supplementation and vitamin A metabolism.

Main Results:

• Animal studies show reduced plasma vitamin A in zinc deficiency, primarily due to food restriction, not zinc deficiency per se.
• Liver vitamin A concentrations are generally unaffected, suggesting normal absorption and transport.
• Zinc deficiency appears to affect both plasma and liver RBP concentrations, potentially impairing RBP synthesis.
• Severe zinc deficiency may impact cellular vitamin A metabolism by affecting zinc-dependent enzymes like retinene reductase, potentially causing impaired dark adaptation in humans.

Conclusions:

• Reduced plasma vitamin A in zinc deficiency is mainly a nonspecific effect of food restriction.
• Zinc deficiency may impair vitamin A metabolism at the cellular level, particularly in severe cases, affecting enzymes critical for vision.
• Zinc supplementation may benefit vitamin A metabolism only in cases of prevalent zinc deficiency; further clinical studies are needed.