Sunny Dispositions

In the video above, Barbara Gilchrest, a MED professor and former chair of dermatology, and Michael Holick, a School of Medicine professor of medicine, physiology, and biophysics, discuss the risks and benefits of the sun.

Researchers have known for decades that vitamin D is crucial for healthy bones, but recent studies have shown that the vitamin plays an important role in myriad functions in the body.

It regulates insulin production in the pancreas as well as in genes that control cell growth, according to Michael F. Holick, a School of Medicine professor of medicine, physiology, and biophysics. It has a marked effect on vascular smooth muscle, important in regulating blood pressure and preventing type 2 diabetes, cardiovascular disease, and stroke. It also impacts the immune system, helping it to fight infectious diseases like tuberculosis and influenza and reducing the risk of autoimmune diseases, including Crohn’s disease, multiple sclerosis, rheumatoid arthritis, and type 1 diabetes.

“Every tissue and cell has a vitamin D receptor,” says Holick, who has been studying how skin makes vitamin D for more than 30 years. “We estimate that as many as 2,000 genes — up to one-sixth of the total human genome — are directly or indirectly regulated by vitamin D.”

And yet, says Holick, half of all people in the United States don’t get enough of the vitamin. “It’s estimated that about a billion and a half to two billion people worldwide are vitamin D–deficient,” he says. “It’s certainly the most common nutritional deficiency and likely the most common medical problem in the world.”

We get vitamin D from our diet and from supplements, but the principal source is the sun. When sunlight penetrates the skin, says Holick, it converts the prohormone 7-dehydrocholesterol to vitamin D-3, which enters the bloodstream. When it reaches the liver, it becomes 25-hydroxyvitamin D — the circulating form — which is what doctors measure to determine whether a patient is deficient. Next, it goes to the kidneys, where it is converted to the biologically active form, called 1,25-dihydroxyvitamin D. From there, it moves into the small intestine, interacts with a vitamin D receptor, and stimulates the absorption of calcium. “If you’re vitamin D–deficient, you absorb only 10 to 15 percent of the calcium in your diet,” he says. “If you’re vitamin D–sufficient, 30 to 40 percent.”

Vitamin D also goes to your bones, where it stimulates cells to remove calcium if you are not getting enough from your diet. “It does that,” says Holick, “because the ultimate function of vitamin D throughout evolution is to maintain blood calcium in a normal range. Only when you have enough calcium and phosphorus will you mineralize the skeleton. That’s why vitamin D has always been thought of as important for bone health.”

He says low levels of vitamin D can exacerbate osteoporosis and cause osteomalacia, a painful softening of the bones (called rickets in children). It can also put us at risk for a host of other problems, including multiple sclerosis and rheumatoid arthritis, high blood pressure, diabetes, and colon, prostate, breast, and other cancers.

Adults, he says, need 2,000 IUs (international units) of vitamin D a day, a level we can’t get from food. A glass of milk or vitamin D–fortified orange juice, for example, has 100 IUs. A common multivitamin might have 400 IUs. “We did a study last year in a healthy adult population,” he says. “We gave them 1,000 units of vitamin D-2 or vitamin D-3 through the wintertime. Not one person became vitamin D–sufficient.”

Holick argues that another reason we are deficient is that we don’t get enough sun; we spend too much time indoors or slathered with sunblock. It’s especially a problem for residents of northern latitudes.

“We showed that in Boston you can’t make any vitamin D in your skin from November through March, no matter how long you stay outside,” he says. “In the summertime, you can’t make any vitamin D at eight in the morning or at five in the afternoon.”

He recommends that adults take a supplement with 2,000 units of vitamin D every day and that teenagers and children take 1,000 units per day. He also advises spending some time in the sun, which puts him at odds with dermatologists. The American Academy of Dermatology recommends getting vitamin D through a healthy diet, which may include vitamin supplements, rather than spending unprotected time in the sun.

“I don’t advocate tanning,” Holick says. “But I do advocate sensible sun exposure. I tell people to wear some protection on your face. Arms and legs — 10 to 15 minutes of sun a couple of times a week between 10 and3, depending on the season, latitude, and skin pigmentation — is usually adequate.”

Holick, whose book The Vitamin D Solution will be published in April, says he’s begun doing studies on varying amounts of vitamin D in the body and its impact on health outcomes. “We’re asking the question, how does your body know it’s getting enough vitamin D?” he says.

The bottom line, he believes, is that there is no downside to increasing our intake of vitamin D: “If you think of it evolutionarily, it’s the oldest hormone on this Earth. I don’t think that this is going to be a flash in the pan.”

Cynthia K. Buccini can be reached at cbuccini@bu.edu.