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Saturday, February 7, 2009

The ABC's of Vitamins - Vitamin A Carotenoids

Last week I discussed preformed vitamin A and this week I'll be talking about provitamin A, also known as carotenoids. Carotenoids (beta-carotene, lycopene, xanthophylls) are compounds found in plants and photosynthetic microorganisms that work as light-harvesting pigments with chlorophyll. Animals cannot produce carotenoids and must get them from their diets.

A molecule of retinol must be present in the structure of a carotenoid in order for it to possess provitamin A activity. The most common and well known of the carotenoids is beta-carotene which has 2 retinol molecules and therefore the highest provitamin A activity. Only about 50 of 500 carotenoids are considered provitamin A.

Fruits and vegetables are where we get our carotenoids. Yellow, orange and red are where we expect to find them, but they are also present in dark green leafy vegetables too. As with preformed vitamin A, carotenoids are available as supplements, primarily as beta-carotene. Crystalline beta-carotene supplements are absorbed at about 50% efficiency; while a raw carrot is only absorbed at about 1% efficiency. Luckly, cooking vegetables increases the efficiency of carotenoid absorption (sorry all you raw food fans!).

Carotenoids that are not used immediately by the body are stored in the fatty tissues, liver and organs much like preformed vitamin A. However, since the carotenoids are not yet in the biologically active form, they are perfectly safe and do not induce toxicity. Carotenoids consumed in large quantities can cause hypercarotenosis (a yellowing of the skin) which is reversible and doesn't have any long-term toxic effects. No USRDI has been established for carotenoids, but 10 -30 mg/day seems to be the target.

Much of what you hear about beta-carotene involves its role as an antioxidant. Carotenoids have the ability to trap peroxyl free radicals and to deactivate singlet oxygen molecules. This appears to be due to their extensive conjugated double bond systems and helps us by reducing some of the mechanisms associated with some cancers. In addition to this, carotenoids help enhance our immune system. See, your mom was right all along - you have to eat your fruits & vegetables. They may just help to save your life!

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