Step Out of the Shade for a Dose of Vitamin D
Hanging out in the sun, letting your face acquire that nice, ruddy glow, used to feel good. Then came all those depressing public health messages telling us that the sun was dangerous, that we should feel guilty about even the slightest tan. Well, fellow sun worshipers, the sad truth is that, as a general rule, we should still practice “safe sun” much of the time.
But there’s a new ray of hope--dare we say “sunshine”?--in the form of a modest but significant shift in medical thinking toward the view that some unprotected sun exposure may actually be a good thing.
“In my opinion,” says Dr. Robert Stern, chief of dermatology at Boston’s Beth Israel Deaconess Medical Center, “it’s probably true that for people over 40, even people who have had a non-melanoma skin cancer, we have oversold the idea of having to be sun-phobic. For them, modest exposure has little risk.”
For kids, cautions Stern, it’s another matter--excessive childhood exposure to sunlight has been linked to later basal- and squamous-cell skin cancers, as well as to melanoma, a more serious form of skin cancer. The rationale for the some-sun-is-good point of view, supported by a number of recent articles in medical journals, is that the vitamin D made in the skin in response to ultraviolet B radiation may protect against certain diseases, including cancer of the breast, colon and prostate.
Sunlight may even be an effective treatment for some diseases. In a recent study published in the journal Lancet, Dr. Michael F. Holick, an endocrinologist and leading vitamin D researcher at Boston University School of Medicine, and others showed that exposing people with mildly high blood pressure to UV-B can lower blood pressure, perhaps by correcting an underlying vitamin D deficiency.
Before we get in too deep here, let’s be clear. It’s vitamin D that has the real benefit, not sunlight per se, which means you can take vitamin D supplements, especially in the winter if you live at higher latitudes and especially if you have dark skin (which makes less vitamin D). It’s very tough to get enough vitamin D from your diet unless you consume lots of fish liver oil, the flesh of fatty fish such as salmon, and fortified milk and cereals.
Actually, vitamin D is not a vitamin at all in the normal sense but is a steroid-like hormone made--after exposure to the UV-B rays from sunlight--from a precursor of cholesterol in the skin. After an inactive form of vitamin D is made in the skin, it is transformed in the liver and kidney to the active or hormonal form called 1,25 dihydroxy vitamin D. Indeed, several teams of researchers have recently found that the organs in which vitamin D seems to reduce cancer risk, such as the breast, prostate and colon, can also make their own stores of the vitamin’s active form, an important finding.
Like other hormones, vitamin D works by fitting into specialized receptors on cells in many organs of the body and has numerous biological effects, the most important one being to aid in the absorption of dietary calcium. When a person has enough vitamin D in his or her system, the intestines can absorb 30% of the calcium available in the diet; without enough vitamin D, this drops to 10%, Holick says. The consequences of insufficient vitamin D can be serious. When the body can’t absorb enough calcium from the diet, it steals calcium from the bones to restore proper levels in the blood, a process that weakens bones, often leading to osteoporosis. Low levels of vitamin D can also lead to weak and achy muscles, as well as generalized bone pain, symptoms often misdiagnosed as fibromyalgia.
Just as important as its effects on calcium and bone is the fact that vitamin D helps regulate many basic cell processes, says Dr. David Feldman, an endocrinologist and vitamin D researcher at Stanford University School of Medicine. By acting on specific regions of DNA called vitamin D response elements, it helps control the biochemical signals that tell cells when to divide, when to stop dividing and when to die--all processes that are crucial in both normal and malignant cells. Vitamin D as a supplement has also been shown to be extremely effective at preventing Type 1 diabetes.
But perhaps the most intriguing evidence of vitamin D’s importance comes from studies of sun exposure and cancer.
In the March 15 issue of Cancer, William Grant, by day an atmospheric scientist at NASA’s Langley Research Center in Hampton, Va., and by night an independent researcher, published a study showing that the geographic distribution of many cancers varies with UV-B exposure.
Since the early 1980s, Grant notes, scientists have been gathering evidence that some types of cancer--most notably, cancer of the breast, colon, ovary and prostate, as well as non-Hodgkin’s lymphoma--are higher in Americans who live in the least sunny regions. “What I did was basically take two maps and put them together,” Grant says of his latest study. This showed that in addition to the cancers already known to vary with UV-B exposure, there appear to be many others (bladder, esophagus, kidney, rectum, stomach and uterus) that also increase as sunlight decreases.
Other researchers, too, have found links between sun exposure and cancer. And this March, researchers from the National Cancer Institute found that Americans living in sunny areas were significantly less likely to die from (not just get) cancers of the breast, ovary, prostate and colon. Not surprisingly, the team found, high levels of sun exposure were also linked to the milder (non-melanoma) types of skin cancer too.
The bottom line? If you’re extremely fair-skinned, all it takes for your skin to make enough vitamin D is about 15 minutes a day in the sun, without sunscreen, at noon, says Holick. If you’re extremely dark-skinned, it may take considerably more than that. How much sun you should get depends on your skin type and sensitivity to sun--if you burn readily, you may be able to tolerate only five minutes in the sun, and that would be enough.
And if you’d rather just take vitamin D supplements? That’s fine--the general guidelines are 200 IUs a day if you’re 50 or under, 400 if you’re between 50 and 70, and 600 IUs if you’re over 70, says Tufts University epidemiologist Susan Harris. Some researchers even recommend 800 to 1,000 IUs a day. The risks of overdosing are small, adds Reinhold Vieth, a biochemist and vitamin D researcher at Mount Sinai Hospital in Toronto, who believes it would take tens of thousands of IUs for long periods to become a problem. Still, to be on the safe side, many researchers suggest limiting vitamin D intake to 2,000 IUs a day.
*
Judy Foreman is lecturer on medicine at Harvard Medical School. Her column appears occasionally in Health.
