What Role Does the Sun Play in Vitamin D Synthesis?

This article is part of the SciFri Science Club's Explain the Sun activity. Participate using the hashtag #ExplainTheSun.

Vitamin D is essential to healthy bones, because it helps you absorb calcium, which strengthens your skeleton. Without the vitamin, you could develop brittle bones, increasing the chance of getting osteoporosis when you’re older. Children with insufficient vitamin D can develop soft bones, putting them at risk for rickets, according to the National Institutes of Health (NIH). Inadequate amounts of vitamin D can also lead to a weakened immune system.

Adults need between 600 to 800 international units (the way the vitamin is measured) of vitamin D a day. You can obtain it from a few foods, including oily fish such as salmon and tuna, fortified milk, juice, and cereal. Taking supplements can provide sufficient amounts, too—although that’s not an ideal approach, according to some experts, who say that getting the vitamin naturally is best. Still, vitamin D is so important to health that, for example, it’s “the one supplement we provide to our astronauts,” says Scott M. Smith, manager for Nutritional Biochemistry at NASA’s Johnson Space Center. 

Unlike other organic compounds that can only be acquired through diet or supplements, however, you can also get the recommended amount of vitamin D with a little help from the sun.

There’s a common misconception about our star’s role, however. “The sun does not create vitamin D,” says Darrell S. Rigel, clinical professor of dermatology at New York University Medical Center. Rather, ultraviolet radiation “converts the vitamin D in your skin to its usable form.”

Here’s how it happens: Our skin naturally contains a precursor to vitamin D. When the sun’s ultraviolet rays touch the skin, they convert the precursor to a molecule called vitamin D₃, which then follows a metabolic pathway through the liver and finally to the kidneys, where it’s converted into a molecule called calcitriol. That’s the good stuff—the active form. (Vitamin D obtained from food or supplements must also follow that metabolic pathway to become active.) 

What about the adage that you must carve out 10 to 15 extra minutes in the sun daily to make enough active vitamin D?

Rigel says that most people soak up sufficent sunlight every day by simply going about their daily routines. Indeed, Americans get between 40 and 90 minutes of incidental exposure each day, on average, according to Rigel. “You have to really be a shut in to not get what you need,” he says.

Certain members of the population who don’t get much sun exposure—such as elderly nursing home residents who don’t spend a lot of time outside and people in cold places who stay indoors during wintertime—have a greater chance of vitamin D deficiency, especially if they don’t take supplements.

In particular, supplements seem to be crucial for people with a rare genetic disorder called xeroderma pigmentosum, or XP. A 1997 NIH study, which followed eight XP patients for six years, found that though these people covered every inch with layers of sunscreen and clothing before going outside, they maintained normal levels of vitamin D when they took supplements.

For those of us who can tolerate more sun, it’s still important to be conscious of your exposure and to protect your skin. Getting too much sun puts you at higher risk for skin cancer, of course, and too much vitamin D isn’t so great, either. Excess vitamin D can lead to “calcium in places it shouldn’t be,” Smith says, as well as kidney stones and other problems.

“It turns out,” adds Doris Day, a board-certified dermatologist in New York City, “having too much might be as bad as having not enough.”