Our skin can turn 7-dehydrocholesterol into vitamin D with just a little help from sunshine. With the winter pall setting in, you're probably not getting much sun, but no biggie - you can also get vitamin D from your food. You may be wondering though - how much sunshine is enough?
Dminder (free, iOS/Android) is an app that figures that out for you. The calculation is not as simple as saying you should spend so-many minutes in the sun, because it's not just any sunlight that helps you make vitamin D, but specifically the same UVB rays that cause sunburns.
We only get a significant amount of UVB rays when conditions are right: the sun needs to be shining somewhat directly down on you. If you live near the equator, that includes the middle of every day. For the rest of us, we get our best vitamin D time in the middle of the day in the summer.
Your UVB needs also depend on factors like:
- Your altitude
- How dark your skin is, and how easily you tan
- How old you are
- How much clothing you're wearing
- And, of course, how much vitamin D you eat.
Most of us are probably fine, so if you have not been worried about your vitamin D status, no need to start. But if you're curious, you can enter a recent vitamin D test result (optional) and the amount of vitamin D you tend to get in your diet. (Dminder does not calculate your vitamin D intake from foods, but many food tracking apps can help you figure this out.)
Then, it uses your location and time of day to help you figure out how much D you can get from the sun. In my location (where it's currently summer), on a cloudy day, it will take me 20 minutes in jeans and a t-shirt to top up my stores of the vitamin. But I'll burn if I stay out for more than 10 minutes, so the app helpfully lets me know it's time to come in. Perhaps I can have a filet of salmon (425 IU in an 80-gram portion) instead.