It is estimated that as much as half the world, including more than 40% of Americans, have low levels of vitamin D (≤20 ng/mL). People with darker skin and people who seldom go unprotected under the sun are especially at risk.
Low levels of vitamin D have been associated with depression. The decrease in vitamin D production caused by reduced sunlight during the winter months may be a factor in seasonal affective disorder (SAD).
If your vitamin D levels are not low, supplementation isn’t likely to benefit your mood. If they are low, supplementation is more likely to help if you suffer from major depression.
If you suspect your vitamin D levels are low, you can have them assessed through a 25-hydroxyvitamin D blood test. Assessing your vitamin D levels twice in a year — in midsummer and midwinter, when there is the most and least sunlight — is an efficient way of estimating what your levels are around the year.