Vitamin A is taken up via the food we eat. Animal-derived products including liver, milk and eggs contain vitamin A as retinol, whereas plant-derived foods including fruits, vegetables and nuts provide carotene, a precursor of vitamin A.
Vitamin A supports health and wellbeing by regulating different cellular processes throughout the body:
Vitamin A preserves the eyesight: Vitamin A protects against age-related macular degeneration, night blindness, keratitis and dryness of the eyes.
Vitamin A strengthens the immune system: Vitamin A prevents infections such as cold and flu, and fights inflammations.
Vitamin A supports the protective function of the skin by keeping it healthy: Vitamin A is a key component for building and maintaining epithelial tissue and mucosa.
Vitamin A prevents cellular ageing: Vitamin A regulates cell division and differentiation on several levels. Vitamin A also contributes to the apoptosis (cell death) of cancer cells.
Vitamin A is absorbed from food. Malnutrition therefore is a frequently observed cause for Vitamin A deficiency, particularly in developing countries.
Vitamin A deficiency can also be caused by insufficient absorption and utilisation of fats via the mucous membranes. This can be associated with disorders including gluten intolerance, leaky gut syndrome, irritable bowel syndrome, or cystic fibrosis.
The concentration of Vitamin A is indicated in retinol equivalents (RAE) as a common denominator of the different forms of vitamin A. The recommended daily intake (RDA) of vitamin A is according to the DGE (German Nutrition Society) determined by age:
* 1 µg RAE corresponds to 3,333 IU
Vitamin A is an essential vitamin for basic cellular functions, and is therefore required daily. People with higher vitamin A requirements, for example in the presence of mucosal disease, or during pregnancy and lactation associated with malnutrition, need to increase their daily uptake of vitamin A. 
Light particles hitting the retina are triggering signals to the brain, where these are converted to form a picture. Rhodopsin is a visual pigment component of the rods within the retina. These are responsible for light-dark vision.
Vitamin A is an essential building block to make rhodopsin. A vitamin A deficiency causes a shortage of rhodopsin, thereby reducing the eye's capacity to adjust to different shades of brightness. Eventually this insufficiency advances to night blindness.
During the course of age-related macular degeneration, the vision in the middle of the retina, called the macula, deteriorates. The macula is responsible for focussing in the center of the field of vision. In the advanced stage of disease, objects are becoming blurred and can only be perceived at the periphery of the field of vision. Macular degeneration can further progress to blindness. Vitamin A can slow down progression of macular degeneration.
The skin continuously undergoes renewal, in order to respond to environmental impact and to heal. The skin requires vitamin A to optimally support functions of the uppermost layer of the skin. Vitamin A works internally as well as applied externally. Vitamin A is crucial for healthy, protective skin. Vitamin A helps to fight acne and can contribute to the prevention of skin cancer.
Vitamin A is key to an effective immune response to fight invading germs. Vitamin A orchestrates different groups of cells of the immune system, boosting the immune response. Vitamin A plays a major role in the local immune response of the mucosa.
In children, vitamin A's contribution to the immune system can dampen the effect of potentially lethal infections.
Vitamin A is also considered an antioxidant, neutralising free radicals in the body. The antioxidant effect of vitamin A also reduces cellular over-reactivity. This specific function of vitamin A can reduce cellular responses during inflammation, and can protect against developing allergies caused by chronic inflammation. 
Vitamin A appears to be involved in the renewal and differentiation of virtually every cell in the body. Vitamin A is critical for activation and cell division of intestinal epithelial cells, as well as of bone and cartilage tissue. Vitamin A is also involved in the specification of sperm cells. However, vitamin A does not necessarily exert activating functions - it also contributes to the apoptosis of cancer cells.  
Due to its role in cell division and tissue development, vitamin A is critically involved during pregnancy and lactation. Vitamin A deficiencies during this time can have fatal consequences for both mother and child. Caution is advised when administering vitamin A however, as over-treatment can also be potentially harmful for the unborn child. 
The vitamin A intake during pregnancy and lactation should therefore always be discussed with a physician.
Beta carotene is a pro-vitamin that needs to be converted into the bioactive component before use. This conversion takes place in the body after ingestion.
Vitamin A, also called retinol, is contained in animal derived foods including liver or dairy products. Beta carotene belongs to the biochemical family of carotenes. These are found in plants.
Vitamin A derived from animal products is therefore directly bioavailable, while plant-derived vitamin A needs to be converted before use. Both forms of vitamin A fulfil the same cellular functions though.
Vitamin A is found in all animal products. Products including liver, salmon, eel, egg yolk, dairy products and butter are especially high in vitamin A.
Products containing high amounts of beta carotene include green leafy vegetables such as broccoli, spinach, peas, cabbage, as well as orange and yellow fruits and vegetables including sweetcorn, sweet potatoes, carrots, pumpkin, blood grapefruits, oranges, melons, cherries, and papaya. 
Uptake of excessive amounts of vitamin A can have side effects. This is of particular importance during pregnancy, when both vitamin A deficiency and excess can have detrimental effects on the child.
Excessively high amounts of vitamin A can also lead to intoxication. It is noteworthy, however, that intoxication can only occur when taking vitamin A, not by ingestion of the plant-derived provitamin beta carotene. 
Some medications can affect the absorption of vitamin A. The over the counter weight loss supplement orlistat reduces the absorption of fat in the intestine. It thereby also blocks the uptake of vitamin A, potentially contributing to a vitamin A deficiency.
Several prescription medications for the treatment of skin diseases such as psoriasis are based on synthetically produced retinoids. In combination with vitamin A supplements, these can contribute to hypervitaminosis (vitamin A intoxication).
People with liver and kidney dysfunction should take vitamin A supplements only after consulting the doctor.
Careful study of the package leaflet is therefore strongly recommended during concomitant use of vitamin A supplements and medications.
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