Vitamin A metabolism —
Zinc in the scientific context

What does vitamin A metabolism stand for?

Vitamin A metabolism describes the totality of the biochemical processes by which the human body takes up, converts, transports and finally uses vitamin A in the target tissues. Vitamin A — also known as retinol — is a fat-soluble vitamin of importance for a great many physiological processes.

In food, vitamin A occurs in two main forms: as preformed retinol in animal foods and as provitamin A carotenoids (in particular beta-carotene) in plant sources. Both forms have to be converted metabolically in the body before they can fulfil their biological functions.

The European Food Safety Authority (EFSA) has scientifically examined the relationship between certain minerals and vitamin A metabolism. On this basis the EU Commission has published authorised health claims.

Vitamin A in the human organism

Vitamin A belongs to the group of fat-soluble vitamins and is stored in the body mainly in the liver. It is involved in numerous physiological processes, among them:

• Vision: Retinal is a component of the visual pigment rhodopsin in the rod cells of the retina. It enables the perception of light stimuli at low brightness.
• Cell differentiation: Retinoic acid, another vitamin A derivative, regulates gene expression and thereby influences the maturation and specialisation of cells.
• Skin function: Vitamin A is involved in the normal renewal of skin cells and plays a role in maintaining the integrity of mucous membranes.
• Immune system: Various of the body’s defence mechanisms depend on an adequate supply of vitamin A.

For all these processes to run, vitamin A first has to be metabolised correctly. Vitamin A metabolism comprises uptake in the gut, transport through the blood, storage in the liver and conversion into the biologically active forms retinal and retinoic acid.

How zinc influences vitamin A metabolism

Zinc acts as a cofactor at several points of vitamin A metabolism. The scientific basis for this has been reviewed by EFSA:

“Zinc contributes to normal vitamin A metabolism” — pursuant to Regulation (EU) No 432/2012

Specifically, zinc is involved in the following steps: it acts as a component of alcohol dehydrogenase, which converts retinol into retinal. In addition, zinc is needed for the synthesis of retinol-binding protein (RBP) in the liver, which enables the transport of vitamin A in the blood. Without this transport protein, stored vitamin A cannot leave the liver and cannot reach the target tissues.

Natural sources of zinc in the diet

Zinc is an essential trace element that has to be supplied through food. Under EU Regulation 1169/2011 the daily reference value for zinc is 10 mg. Among the most zinc-rich foods are oysters, beef, pumpkin seeds, lentils and oats. On a predominantly plant-based diet, bioavailability can be reduced by phytic acid — soaking and sprouting legumes can counteract this effect.

Vitamin A sources and reference values

Under EU rules the daily reference value for vitamin A is 800 µg retinol equivalent. Animal sources such as liver, eggs and dairy products supply preformed retinol. Plant foods such as carrots, sweet potatoes and spinach contain beta-carotene, which is converted to retinol in the body — a process in which zinc-mediated enzymes are likewise involved.