Apart from iron and iodine deficiency, vitamin A deficiency is the most common deficiency worldwide. Although pronounced vitamin A deficiency is found primarily in developing countries, several studies have shown that the supply of vitamin A may also be inadequate in developed countries, albeit without visible clinical signs. Even before the incidence of visible consequences such as night blindness and general blindness, increased susceptibility to respiratory infections may be observed.
The increased susceptibility to infection is explained by the importance of vitamin A in maintaining intact mucous membranes in the respiratory tract. The mucous membranes of the mouth and throat represent the first physical barrier that carriers of infection need to overcome. An inadequate supply of vitamin A leads to immobility of the cilia (tiny hairs) of the mucous membranes of the nose and lungs, which cannot then function. Only mobile cilia are able to eject invading germs or other harmful substances from the airways before they make us ill. As the cilia become less mobile, more thick mucus forms, in turn becoming a breeding ground for germs. The intestinal mucosa also have an important barrier function: Here, too, vitamin A has a crucial role to play in protecting the organism against pathogens.
It is thought that vitamin A also has a direct effect on the immune system, where it influences the production and differentiation of white blood cells. This applies above all to the lymphocytes that produce antibodies, which have protective activity in the digestive tract. As a vitamin A precursor, beta-carotene plays an important role in preventing the development of autoimmune diseases.
In this context the interaction with vitamin D is important, because vitamins A and D act in close cooperation to form a strong team. Even if the supply of vitamin A is adequate, problems arise when intake of vitamin D is insufficient. If one of these two vitamins is not present in sufficient quantities, the other cannot exert its optimal effect.