“In the winter months, when the sun’s rays are not intense enough to generate sufficient vitamin D production (this is the case for three to five months per year for locations in North American—you know it’s downtime for D when you can’t get a tan even with prolonged sun exposure), supplements and safe-designated artificial tanning implements can be useful or even essential if you are at high risk for deficiency.
If you have an indoor-dominant lifestyle or even a hint of aforementioned risk factors, you should regularly test your blood for vitamin D. (Make sure it’s for ’25-vitamin D’ or ‘serum 25(OH)D.’ You can order this test directly from an online provider like direct labs.com. or, if you are getting a routine checkup, insist they include vitamin D in your blood panels. (Unfortunately, it’s often not included.)”
—From The New Primal Blueprint 
For more on vitamin D sources, deficiency, testing and recommended levels, see The New Primal Blueprint or check out past articles  on MDA.