The low UV index of winter sun makes it difficult, if not impossible, for us to make vitamin D, increasing the odds of colds, flu, and feeling blue.
To tell if the sun is strong enough to to make any vitamin D at all, look at your shadow, suggests the Linus Pauling Institute. If your shadow is the same height as you, or shorter, you can make some vitamin D. But if it’s longer you’re out of luck.
In both cases, the institute recommends getting 2,000 IU of vitamin D daily, from food and supplements, to maintain optimal levels.
Before supplementing, however, it’s best to get your vitamin D levels checked with a simple blood test that’s usually covered by health insurance.
betternutrition January 2018