Six Things to Know About Vitamin D

We miss you, sun. Please come back soon. Photo via Shutterstock.

If you live north of Atlanta (ahem, Boston), it’s impossible to produce vitamin D from the sun in the winter, because the sun ray’s never get high enough in the sky for the ultraviolet B rays (UVB) to penetrate the atmosphere. This can be a problem for most of us that aren’t lucky enough to have winter homes near the Equator.

Vitamin D keeps bones strong, wards off cancer, and protects against tuberculosis, diabetes, colds, and the flu. A recent study also found an increased risk of heart attack in people with low vitamin D levels.

Vitamin D is made when sunlight hits the skin. Our skin absorbs the invisible UVB rays and a reaction takes place that enables skin cells to manufacture vitamin D. You can also get the vitamin from food, but mainly because its been added to food; very few foods are natural sources of vitamin D. There is also the option of taking a supplement.

According to Harvard Health Publications, there are six important factors that influence a person’s vitamin D levels:

1. Where you live. The further away from the Equator you live, the less vitamin D–producing UVB light reaches the earth’s surface during the winter. Residents of Boston, for example, make little if any of the vitamin from November through February. Short days and clothing that covers legs and arms also limit UVB exposure.

2. Air quality. Carbon particles in the air from the burning of fossil fuels, wood, and other materials scatter and absorb UVB rays, diminishing vitamin D production. In contrast, ozone absorbs UVB radiation, so pollution-caused holes in the ozone layer could end up enhancing vitamin D levels.

3. Use of sunscreen. Sunscreen prevents sunburn by blocking UVB light. Theoretically, that means sunscreen use lowers vitamin D levels. But as a practical matter, very few people put on enough sunscreen to block all UVB light, or they use sunscreen irregularly, so sunscreen’s effects on vitamin D might not be that important. An Australian study that’s often cited showed no difference in vitamin D between adults randomly assigned to use sunscreen one summer and those assigned a placebo cream.

4. Skin color. Melanin is the substance in skin that makes it dark. It “competes” for UVB with the substance in the skin that kick-starts the body’s vitamin D production. As a result, dark-skinned people tend to require more UVB exposure than light-skinned people to generate the same amount of vitamin D.

5. Weight. Body fat sops up vitamin D, so it’s been proposed that it might provide a vitamin D rainy-day fund: a source of the vitamin when intake is low or production is reduced. But studies have also shown that being obese is correlated with low vitamin D levels and that being overweight may affect the bioavailability of vitamin D.

6. Age. Compared with younger people, older people have lower levels of the substance in the skin that UVB light converts into the vitamin D precursor. There’s also experimental evidence that older people are less efficient vitamin D producers than younger people.

If you are fair-skinned, you only need 10 minutes of sunlight (in shorts and a tank top with no sunscreen) in summer to get enough radiation to produce the needed units of vitamin D. Dark-skinned individuals and the elderly produce less vitamin D, so their time in the sun is recommended to be a little longer. But bring the SPF if you will be out in the sun longer than the recommended times needed for vitamin D production. Skin cancer is still deadly.

Just the thought of laying out in the sun, even for 10 minutes, might be enough to get us through the rest of winter.