Many recent studies have supported the role of vitamin D in the prevention of various cancers, as well as many other health benefits. A 2008 European study discovered that these benefits are reduced with high levels of vitamin A. This study, published in the British Medical Journal, found that adequate vitamin D levels reduced the risk of colon cancer except for people with high vitamin A levels.
Previous studies found that vitamin A toxicity was associated with impaired calcium absorption and increased hip fractures; a higher rate of respiratory infections; and an increased mortality. Even a meal of beef liver has been shown to temporarily impair calcium absorption. It now appears that at least some of these results can be explained by vitamin A’s antagonistic relationship with vitamin D. It also may explain why some vitamin D cancer studies showed mixed results.
Dr. John Jacob Cannell, MD of the Vitamin D Council (www.vitamindcouncil.org) explains that retinol, a form of vitamin A, inhibits activity of vitamin D. Consequently, too much vitamin A is as bad as not enough vitamin D. A balanced ratio of these two vitamins is critical and further study is needed to determine what this ideal ratio is.
Cannell also points out that while vitamin A deficiency is common in developing countries, in the United States and Canada it is rare because vitamin A’s precursor beta carotene is abundant in colored vegetables like carrots. In fact, vitamin A toxicity is much more common in North America than its deficiency. Multi vitamins commonly have a low amount of vitamin D (200-400 IU) and excess vitamin A (5,000-10,000 IU). Cod liver oil and halibut liver oil are both extremely high in vitamin A and low in vitamin D. It now appears that these products should be avoided.
Beta-carotene is a precursor to vitamin A and does not affect vitamin D activity. Our body converts beta carotene to vitamin A as needed. Also, fish body oils (e.g. salmon and herring) commonly taken as omega 3 supplements do not contain either vitamin A or vitamin D so are not a concern here.
Bottom line: avoid cod liver oil and halibut liver oil supplements and choose a multi vitamin with beta-carotene as its vitamin A source. To ensure adequate vitamin D levels, sunbathe daily in the summer and take vitamin D3 up to 5,000 IU daily in the winter. Eat your veggies but take a pass on the liver.
Codfish and halibut (and children who hate liver) should be grateful for this study!
This article is intended for educational purposes only; for medical advice consult your licensed health practitioner.
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