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caption: Little does this cat know, UVB rays from sunlight are a good source of Vitamin D during the summer, but not during the fall and winter.
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Little does this cat know, UVB rays from sunlight are a good source of Vitamin D during the summer, but not during the fall and winter.

It's that time of year to pay attention to your vitamin D intake

Blame the sun — even when it's sunny.

Now that it’s fall, your body’s not making vitamin D like it used to. The angle of the sun is the problem.  

Your body is a vitamin D factory. In the summertime, ultraviolet rays zap molecules in the skin and create vitamin D. (To be specific, we’re talking about UVB rays and cholesterol molecules). But in the winter, the sun passes through the sky at a lower angle. 

“Essentially, it’s traveling through more of the atmosphere,” said Adrian Gombart, a vitamin D expert at Oregon State University. The atmosphere filters out the UV rays you need to make vitamin D. “There’s not enough UVB rays making it through the atmosphere basically to synthesize vitamin D in the skin."

So your internal factory, takes a break for the winter, even when you do see the sun.

“And, you’d be lucky to see the sun anyway,” Gombart said, and laughed.

Instead, you’ll have to get more of this essential nutrient from your food or from supplements. And that means you’re probably not getting enough, said UW family medicine professor Dr. Lucille Marchand.

“It’s harder to get enough food sources of vitamin D. Many milk products are fortified with vitamin D, but it’s still a low amount,” she said.

The list of foods high in the vitamin includes fortified orange juice, cod liver oil, margarine, salmon, tuna and mushrooms.

The National Academy of Sciences recommends most adults get 600 units of vitamin D per day. Dr. Marchand recommends patients take an additional 800 to 1,000 IU of vitamin D per day from September through May. She personally takes 2,000 IU because of a history of osteoporosis.

Every body is different, though, and factors such as sex, race, age, and body weight can play a role in how your body creates and holds onto vitamin D.

Vitamin D is essential for strong muscles and bones.

A blood test is the only definitive way to see how much of the vitamin is in your body.

“It’s fairly hard to just tell by feeling if you have adequate levels or not,” Gombart said. “Unless you’re severely deficient, and then you’d start to see signs of that. You might have muscle pain, bone pain.”

If you’re feeling draggy or down, it might not be vitamin D deficiency as much as lack of sunlight, Marchand said. “It’s hard to tease out all the different factors that might be at play.”

Still, it’s important to eat a balanced diet and get enough exercise and outdoor time.