Monday, March 22, 2010

Vitamin A

Vitamin A is an important fat-soluble vitamin. As a fat-soluble vitamin, it is more likely to be toxic than water-soluble ones. Its role is particularly important for normal and healthy vision. Deficiencies in the United States are not common. However, up to half a million children become blind each year as a consequence of its deficiency. This vitamin is a great multi-tasker and plays a wide variety of roles. It is an essential constituent to visual pigments necessary for black and white and night vision. It is also important in gene transcription, making DNA and RNA.

It is also needed in normal embryonic development and reproduction. Both its deficiency and toxicity can affect fetal development. As early as 1937 research suggested that vitamin A deficiency and early pregnancy increased the risk of fetus deaths. This is linked with cell differentiation and the inability of these cells to become what they are genetically predetermined to be.

It is also important in the growth and development of bones. It is needed for the formation of blood components. It is necessary for the maintenance of normal skin cells. Some research shows that it also has a role in reducing the risk of heart disease, but that’s not clear enough yet. It has antioxidant activity. Antioxidant means that it protects cell membranes from oxidation (damage).

What about the bioavailability of this vitamin? There are three different forms of vitamin A. Retinol is an active type and it is found predominantly in animal foods, things like liver, whole milk, butter, eggs, cheese and some fortified foods. Because it is a pre-formed vitamin it is much more likely to be toxic. Retinol and its cousins retinal and retinoic acid are the most biologically available forms of this vitamin.

Protein is required to transport this fatty vitamin in blood. Retinol and its cousins need what is called retinol-binding protein to be store in the liver. About 90% of vitamin A is actually stored in the liver.

Carotenoids, such as beta-carotene, are considered inactive forms of vitamin A. This means they can be converted to vitamin A in the body. We have about 600 carotenoids in food, but about 50 to 60 are found in the human diet, and fewer than this have vitamin A activity. Carotenoids, because they are precursors, are relatively non-toxic. The brakes can be put on if the body has enough retinol.

The absorption of carotenoids depends greatly on the amount of fat that is ingested with it. Fat in the diet increases its absorption.

Carotenoids are found in plant sources, such as green vegetables and carrots. They have an important role in the prevention of macular degeneration, an eye disease we get with advancing age; and the prevention of prostate cancer.

We now find many supplements of beta-carotene in the market, but they might not be a good idea. In a recent study called the ATBC they looked at 30000 people between the ages of 50 and 69, and those taking beta-carotene supplements died with increased frequency from lung cancer, heart disease and stroke. Current research indicates that beta-carotene supplements can cause oxidative DNA damage in lung cells. This might be the cause of increased mortality. Keep in mind there is a difference between the beta-carotene you get from your food and that from supplements.

Additionally, beta-carotene as a supplement is associated with heart disease risk and a possible increase in gastrointestinal cancer.

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