Vitamin A // Retinol
Vitamin A (retinol) has several functions in the body. The most well known is its role in vision. Retinylesters are transported to the retina, located at the back of the eye, where they are processed to 11-cis-retinal. 11-cis-retinal is transported to the rod (photoreceptor) cells where it binds to a protein called opsin to form the visual pigment ’rhodopsin’. Rod cells with rhodopsin can detect very small amounts of light, making them important for night vision. Absorption of a photon of light catalyzes a transformation of 11-cis-retinal back to all-trans-retinal and results in its release from the protein. This triggers a cascade of events, leading to the generation of an electro-chemical signal to the optic nerve, which is processed and interpreted by the brain (1). A lack of retinol available to the retina results in impaired dark adaptation, known as ’night blindness’.
Vitamin A in the retinoic acid form plays an important role in regulation of gene expression. Once retinol has been taken up by a cell it can be oxidized to retinal, which can be oxidized to retinoic acid. Retinoic acid is a very potent molecule that binds to different nuclear receptors to initiate or inhibit gene expression (2). Through the regulation of the expression of specific genes, retinoic acid plays a major role in cellular differentiation; the specialization of cells for highly targeted physiological roles.
Vitamin A is required for normal functioning of the immune system (3). Retinol and its metabolites are necessary to maintain the integrity and function of skin and mucosal cells (lining airways, digestive and urinary tract), acting as a barrier and forming the body's first line of defence against infection (4). Vitamin A plays a central role in the development and differentiation of white blood cells such as lymphocytes, which play critical roles in immune response.
Vitamin A is essential for embryonic development (3). During fetal development, it functions in limb development and formation of the heart, eyes, and ears (5). Additionally, retinoic acid has been found to regulate expression of the gene for growth hormones. Both vitamin A excess and deficiency are known to cause birth defects.
Vitamin A is required for normal development of precursor cells (stem cells) into red blood cells. Additionally, vitamin A appears to facilitate the mobilization of iron from storage sites to the developing red blood cell for incorporation into hemoglobin; the oxygen carrier in red blood cells (6, 7).
The vitamin A metabolism is thought to interfere with zinc and iron in several ways. Zinc deficiency results in decreased retinol transport (2), decreased retinol release in the liver (1), and a decrease in the conversion of retinol into retinal (2). Vitamin A supplementation has beneficial effects on iron deficiency (anemia), and improves the nutritional status of iron among children and pregnant women. The combination of supplemental vitamin A and iron seems to reduce anemia more effectively than either supplemental iron or vitamin A alone (8).
The European Food Safety Authority (EFSA), which provides scientific advice to assist policy makers, has confirmed that clear health benefits have been established for the dietary intake of vitamin A in contributing to:
- normal cell differentiation;
- a normal development and function of the immune system;
- the maintenance of normal skin and mucous membranes;
- the maintenance of normal vision;
- normal iron metabolism.
Authored by Dr Peter Engel in 2010, reviewed by Dr. Adrian Wyss on 31.08.2017.