Nearly 100 years ago Vitamin A was the first successfully isolated Vitamin. Pure Vitamin A (Retinol) is a fat-soluble Vitamin and can only be found in animal foods. Our body can absorb it quite efficiently and once absorbed, retinol uses a protein “taxi” (retinol-binding protein or ‘RBA’ for short) for its transport in our blood.
Amazingly, most of the Vitamin A we need is not supplied by pure (preformed) Vitamin A but from carotenoids instead. Our body converts these ‘provitamin A carotenoids’ into retinol. However, we can absorb preformed Vitamin A two to four times more efficiently.
We have a “back-up” of Vitamin A in our liver, and in our lung and kidney tissues. Our liver stores the biggest part, approximately 90%, which is comparable to an adult’s two years’ supply of Vitamin A. Despite that, we still need Vitamin A regularly because it has a shorter “expiry date”.
Vitamin E can be Vitamin A’s “little helper” but if taken in large amounts, it may have the opposite effect. Zinc is also important to activate and transport stored retinol. Positive interactions with iron have also been reported.
Vitamin A is a weak antioxidant compared to carotenoids. Yet, it works deep inside the core of our cells and, although best known for its role in our vision, plays many important roles in our health. Vitamin A:
Vitamin A can support the treatment of many problems and might therefore already be included in some medications. Some possible uses for Vitamin A include the treatment of:
Despite the fact that our body stores Vitamin A, a deficiency can still occur. Problems with our digestion reduce Vitamin A absorption; a deficiency of zinc lowers Vitamin A activity; and medicines (like laxatives, antibiotics, or a simple contraceptive) as well as certain lifestyle habits (like smoking or excessive alcohol intake) diminish our Vitamin A stores. Additional amounts of Vitamin A may also be required during pregnancy or illness.
If we are running low on Vitamin A, we might experience some of the following signs:
The World Health Organisation (WHO) Global Database on Vitamin A deficiency shows that Vitamin A deficiency (VAD) is still a concern in many countries. Because VAD is rather uncommon in our industrialized world, supplementation has therefore been considered unnecessary or should at least been limited to less than 5,000 IU per day.
Unlike carotenoids, too much preformed Vitamin A can be toxic. Hypervitaminosis A is relatively rare but does occur, even though it is more likely caused due to excessive use of supplements. Signs of a Vitamin A overdose can be:
Critical adverse effects of Vitamin A toxicity include an increased risk of birth defects and liver abnormalities. Recent studies suggest that a long-term intake of too much preformed Vitamin A might reduce bone mineral density (BMI) and therefore increase the risk of osteoporosis and hip fracture. However, the available data is conflicting and further research needs to be done.
What food is best for your eyesight? Seafood... well, there is some truth in it. Good sources of preformed Vitamin A certainly are fish liver oil, especially cod liver oil, and oily fish such as halibut, herring, pilchards, sardines, or tuna. According to the 2000 National Health and Nutrition Examination Survey (NHANES), good sources of retinol are milk, margarine, eggs, beef liver, and fortified breakfast cereals.
Retinol can only be found in animal foods. It is quite resistant to heat but can easily oxidise and should therefore be protected from air and light.
The current Dietary Reference Intake (DRI) established by the Food and Nutrition Board of the Institute of Medicine states for a healthy adult male 900mcg (3,000 IU) and for a healthy adult female 700mcg (2,333 IU). An upper level (UL) intake of 3000mcg (10,000 IU) is considered to be safe for healthy adult males and females.