Now doctors say some sun exposure is good for you
Original Publication Date: 6/25/02
By JUDY FOREMAN
Well, fellow sun worshipers, the sad truth is that we should still practice "safe sun" — including hats and sunscreen, especially for little kids — much of the time.
But there's a modest but significant shift in medical thinking toward the view that some unprotected sun exposure may actually be a good thing, like 15 minutes or so a day in the summer for adults who tan well, less for those who burn easily.
Excessive childhood exposure to sunlight is linked to later basal and squamous-cell skin cancers, as well as to melanoma, a more serious form of skin cancer, cautions Dr. Robert Stern, chief of dermatology at Beth Israel Deaconess Medical Center in Boston. But, Stern said, "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."
In fact, modest exposure may have some benefits. The rationale for the "Some sun is good" view, supported by a number of recent articles in medical journals, is that the Vitamin D the skin makes in response to ultra violet B radiation (UV-B) may protect against certain diseases, including cancer of the breast, colon and prostate.
Other diseases, most notably multiple sclerosis (MS), also show a "latitude effect" — that is, they are less prevalent among people in sunnier climes — though, as with cancer, whether this is truly due to Vitamin D or to some other factor that varies by region is unclear. In rodents, high doses of Vitamin D can actually prevent MS.
Maps showing cancer rates across the United States offer some — albeit imperfect — evidence to support the sunshine theory. For instance, the incidences of colon cancer among white men and women and breast cancer in white women are higher in the less sunny northern areas, particularly in the Northeast. Prostate cancer rates for white men are also higher in the northern states, though high rates can also be found in pockets of the South. For blacks, rates for some of these cancers are also higher in the North, though the latitude effect is less pronounced.
Sunlight may even be an effective treatment for some diseases. In a recent study published in the British medical journal The 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.
Essential to Growth
But 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, in amounts ranging from 200 international units a day to 800 IUs a day, depending on your age. You should strongly consider supplements during the winter if you live at higher altitudes and 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, fatty fish like salmon, and/or fortified milk and cereals.
Actually, Vitamin D is not a vitamin at all in the normal sense, but a steroid-like hormone made from a type 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 an active, or hormonal, form called 1,25 dihydroxy Vitamin D. Indeed, several teams of researchers have recently found that organs such as the breast, prostate and colon also make their own active form of Vitamin D, a finding that supports the idea that Vitamin D may protect against some cancers.
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: It aids in the absorption of dietary calcium. When a person has enough Vitamin D in his system, the intestines can absorb 30% of the calcium available in the diet; without enough Vitamin D, this drops to 10%, notes Holick.
The consequences of insufficient Vitamin D can be serious. When the body can't absorb enough calcium from the diet, it steals the mineral 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 Vitamin D's effects on calcium and bone is its ability to help regulate many basic cell processes, said 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.
In the early 1990s, for instance, researchers showed that adding the active form of Vitamin D to cancer cells in the test tube inhibits their growth, a finding that has now been replicated in breast, prostate and colon cancer cells, as well as leukemic cells.
Lack of Sun and Cancer
Currently, researchers at several labs around the United States are testing whether a high-dose prescription form of Vitamin D called calcitriol can slow the progression of prostate cancer in men with the disease. This treatment could increase the risk of kidney stones, says Feldman, but it's promising enough that numerous drug companies are pursuing closely related versions (or analogues) of active Vitamin D that would be active against cancer without producing kidney stones.
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 noted, 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," says Grant 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. In 1999, a team led by epidemiologist Esther M. John of the Northern California Cancer Center in Union City reported on a study of more than 5,000 white women, 190 of whom developed breast cancer between the time they were first interviewed by government researchers in the early 1970s and 1992. The team correlated various measures of sun exposure and found that the women with the highest levels of sun exposure were the least likely to get breast cancer.
This March, researchers from the National Cancer Institute led by Dr. Michal Freedman, an epidemiologist, 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, her 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 have had skin cancer, check with your doctor before setting out to soak up the rays. Don't ever let your children get burned. But otherwise, indulge in a little sunshine and enjoy.
Vitamin D and Evolution
Vitamin D plays a role in one theory of the evolution of different skin colors among humans. In the scientific community, there's strong consensus that the first human beings were dark-skinned people from Africa who migrated outward from there. Dark skin is rich in melanin, a pigment that acts as a natural sunscreen, protecting against sunburn. But just like sunscreen of SPF 8 or higher, melanin reduces the amount of Vitamin D the skin can make. That's fine for someone in Africa who spends lots of time in the sun. Farther from the equator, however, people with light skin gain an evolutionary advantage. With less available UV-B, light skin probably evolved so that humans migrating northward would still be able to make enough Vitamin D, explained Reinhold Vieth, a biochemist at Mount Sinai Hospital in Toronto.
Indeed, people who failed to make this adaptation would have had a difficult time reproducing. Insufficient Vitamin D can lead to rickets, which causes defective bone growth. In women, this can mean such poor pelvic development that babies could not be borne — and the mother's genes would not be passed on.