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Sunday, February 1, 2009

The ABC's of Vitamins - Vitamin A

I've spent a fair bit of time discussing the macronutrients (protein, fat, carbs), so I thought it was time to start on the micronutrients. Because I am a scientist and like things orderly, I'm going to go alphabetically rather than oil vs. water soluble. That means today's vitamin is A.

Vitamin A isn't a single thing, rather it is a generic term for compounds (other than carotenoids) having the biologic activity of retinol. The retinoids include: retinol, retinaldehyde and retinoic acid. These compounds are referred to as preformed vitamin A because they do not need to undergo metabolic conversion to be biologically active.

Vitamin A is essential to several of our life processes like metabolism, skeletal and soft tissue growth, formation and maintenance of epithelial tissues (skin & mucous membranes), reproduction and vision (although retinoic acid cannot support these last 2 because they need retinol or retinaldehyde).

Dietary sources of preformed vitamin A are animal tissues and milk - the animals convert carotenoids into vitamin A which we then consume as meat, milk, cheese, butter, etc. We can also get our vitamin A from supplements. Vitamin A is measured in IU (international units) and 1 IU is equal to the biologic activity of 0.3 micrograms of retinol. The USRDI for vitamin A is 3000 IU (900 micrograms) with an upper limit (the maximum daily intake that can be safely consumed) of 10000 IU (3000 micrograms). Vitamin A deficiencies are not very common, at least not in developed nations, but if you don't eat dairy, fruits or vegetables you could get into trouble. Early symptoms of deficiency include night blindness, dry skin and susceptibility to infection. If left untreated, it can result in permanent blindness. Vitamin A can become toxic since it is stored in the fatty tissues, liver and organs, but only if you are taking massive amounts of supplements or eating nothing but liver.

Preformed vitamin A absorption occurs in the small intestine and about 5-20% of the vitamin A ingested is not absorbed and is eliminated from the body. Everyone has a different absorption ratio, but typically vitamin A is absorbed at only 20-50% efficiency. In order to use and metabolize vitamin A, it needs some friends. Since vitamin A is an oil soluble vitamin, it stands to reason that it is better absorbed when consumed with fat. But vitamin A also needs protein; the lack of protein will result in a lack of vitamin A metabolism due to a decrease in enzyme levels. Zinc is also needed; a lack of it causes low plasma vitamin A levels even when vitamin A supplementation is given. And vitamin E is necessary; a lack of vitamin E causes a lack of vitamin A to be stored in the liver.

Next time I'll talk about provitamin A, also known as carotenoids, so eat well to be well!

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