Those UVB rays aren't ALL bad
In recent decades, health experts have warned that exposure to the ultraviolet B rays in sunshine can kill us by way of skin cancer, and as a result, more and more people have taken heed and avoid the sun completely, or load up on sunscreen when they're outside. This is a good idea if you don't want skin cancer (which nobody does), but it also means your body doesn't get to do the cool thing it was designed to do, which is convert those UVB rays into vitamin D.
The general consensus is that some sun exposure is OK (experts say about half the time it takes your skin to turn pink and begin to burn is a goal you should shoot for), but supplementation is probably necessary to obtain optimal levels, especially if you're at high risk for skin cancer.
How do I know if this is something I should worry about?
A lot of factors play a role here, including your race, your habits, and your age. In other words, if your skin is dark, you lurk indoors, you shun the sun, you're a vampire, or you're growing older (as we all are), then your vitamin D levels may not be where they should be. Since only about 30% of people fall within adequate ranges, it's more likely than not that you could stand a bit more.
There really is no general consensus on optimal levels (obtained through a blood test), which makes diagnosing a true deficiency challenging. The Vitamin D Council says that anything under 30 ng/ml should be considered deficient, but the National Endocrine Society says anything under 20 ng/ml is cause for concern, although those who test between 20 and 30 ng/ml might want to consider supplementation.
Bottom line? If you feel like garbage, have your doctor run a blood test to see where you fall. If your doctor suggests supplementation, do it -- and don't worry, even the 5,000-IU pills are tiny.
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Monica Beyer is a health writer who is vitamin D deficient and knows more about it than she ever thought possible. Follow her: @monicabeyer.