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Tuesday, August 19, 2008

Vitamin D, the Sun, and your Heart

Vitamin D comes from the sun. Higher levels of Vitamin D go with lower blood pressure, better insulin and glucose regulation, a healthier immune system, stronger bones, and less chance of colon, breast, and prostate cancers. The US Food and Nutrition Board has recommended that 200IU (5 mcg) of vitamin D is adequate for young to middle-aged adults, but recent research indicates that might not be enough.

It's hard to get enough vitamin D from food. Fortified milk has less than 100IU per serving. Vitamins for adults usually contain 400IU. But, if you are not getting enough sunlight, a vitamin pill may not give you enough vitamin D...

The most important source for vitamin D is the sun. We can make all the vitamin D we need (up to about 20,000IU in less than an hour) if we get enough UVB rays on enough of our bare skin. A light-skinned person living near Boston can typically get enough vitamin D by going outside in shorts and a short-sleeved shirt, without using sunscreen, 3 times every week from mid-March to mid-October for 5 to 10 minutes between 11AM and 2PM. And be able to store enough to last through the winter, when there is not enough UVB light for making vitamin D. Further north, the "vitamin D winter" lasts longer. Closer to the equator, a person needs less time in the sun. Sunscreen blocks UVB rays -- that's its job. Even an SPF8 sunscreen cuts production of vitamin D by 95%. Darker-skinned people may need 5 to 10 times longer in the sun, depending on the amount of melanin (the dark pigment) in their skin. Older people may not be able to make vitamin D in their skin as quickly. People with liver disease may not be able to produce enough of the provitamin-D3 that the skin uses to make vitamin D. Obese people may need more vitamin D because so much of it gets stored in body fat. There are a lot of variables.

Exposing your arms and legs to the sun for 20 minutes at the right place and time could get you a dose of 20,000IU of vitamin D -- much higher than we get from food. This means food is not the most important source of vitamin D. Which makes it hard to calculate an RDA for vitamin D from food. But, if people spend more time inside and more of their outdoor time using sunscreen, they'll have to get it from foods and supplements, or risk their health.

So get a little noonday sun every day for as long as summer last. In the Northern hemisphere, Vitamin D summer lasts until October or November, depending on how far north you live. It has already already begun in the northern Australia, and continues all year long in the tropics.

How much vitamin D is too much? Vitamin D toxicity has never been observed in people getting their vitamin D from the sun. Most people are unlikely to have any problem from as much as 10,000 IU/day from supplements. But some medical conditions (lymphoma, sarcoidosis, tuberculosis, and primary hyperparathyroidism) can cause a bad reaction to vitamin D pills.


MizFit said...

interesting post---and something with the end of summer (sigh) we all might think about supplementing.


Family Nutritionist said...

Unless you've been stocking up on sunshine. Why not start now? Maybe 10 minutes a day, before you start your sun protection regimen.