Sun Not Always a Shady Character in Cancer
Vitamin D may keep internal tumors in check
FRIDAY, Feb. 15, 2002 (HealthDayNews) -- Americans are taught that sunlight is the enemy, at least when it comes to raising the risk of skin cancer. But getting plenty of rays may be a lifesaver by generating enough vitamin D to keep aggressive cancers of other organs in check, scientists now say.
Researchers have long known that for certain forms of internal cancers, including tumors of the colon and breast, death rates climb with rising latitude and fall nearer the equator.
For some cancers, the odds of dying are as much as twice as high if you live in New England than if your home is in New Mexico.
Differences in diet between Northerners and Southerners and other lifestyle factors have been offered up as the explanation for these trends.
But relatively recent evidence suggests that exposure to ultraviolet radiation from the sun -- not what people eat or how much they exercise -- may be more important at determining their risk of deadly tumors. The reason is that UVB rays stimulate the body's production of vitamin D, a key nutrient that appears to put a brake on cancerous cell growth.
William Grant, a physicist from Newport News, Va., has made an intense hobby of mapping cancer rates and patterns of sun exposure, both in the United States and abroad. Grant says his analysis of data collected between 1970 and 1994 suggests that as many as 30,000 cancer deaths and 70,000 tumor cases each year in this country are premature and result from inadequate exposure to the sun and a subsequent dearth of vitamin D.
Grant also identified 13 cancer types for which increased time in the sun appears to be protective, including tumors of the digestive organs, the kidneys, bladder and ovaries.
Grant, an independent researcher with no academic affiliation, presented his findings today at a meeting in Boston of the American Association for the Advancement of Science. The work will also appear next month as a paper in the journal Cancer.
Cedric Garland, a California cancer researcher who first proposed the UVB-vitamin D-tumor connection more than a decade ago, says the theory "is playing out in a very gratifying and satisfying way."
Many experts initially dismissed the notion that geographic variation in cancer death rates were due to anything other than differences in diet and exercise habits. But Garland, a professor of family and preventive medicine at the University of California in San Diego, says two large studies that examined these factors failed to explain the disparity.
Other scientists have suggested that tropical infections may account for lower cancer death rates close to the equator, since these diseases would probably kill off people before their tumors could. But again, "if it were strictly an artifact of [infection], the difference would disappear upon age adjustment. And it doesn't," Garland says.
Garland and other researchers who support his hypothesis say vitamin D seems to take tumors down a notch by freezing them in a non-active state of cell division, he says.
However, while most of the evidence suggests that vitamin D stalls the spread of existing tumors, some research hints that it may prevent certain cancers from forming in the first place.
At least one study, for example, showed that women with a history of skin damage, though not necessarily melanoma and other tumors, may have a lower risk of developing breast cancer, he says.
Store-bought milk is now fortified with vitamin D, and certain foods, like fatty fish, are rich in the substance. Even so, it's difficult for people to get enough of the nutrient in their diet. Safe exposure to sunlight -- that is, briefly and without burning -- is necessary for most people to get sufficient amounts.
But should you run to the tanning booth for a UVB fix? "Heavens, no," says Dr. Michael Holick, a Boston University dermatologist. Holick, who has conducted his own studies that support Garland's work, says just 10 minutes a day of sunlight on the hands, face and arms during the late spring and summer is enough for the typical New Englander. That much time is fine year-round for those living in lower latitudes.
"If you're in a southern latitude, sun exposure will result in vitamin D production year round," says Dr. Bess Dawson-Hughes, a Tufts University nutrition expert. "Further and further north, there is a longer and longer blackout period."
During the colder months, the angle of the sun is so low for people living in Boston, for example, that they can synthesize no vitamin D between about mid-October and mid-March. Residents of Edmonton, Alberta, which is 11 degrees latitude farther north, must rely on their diet and fat stores for their vitamin D about two months longer than that, Dawson-Hughes says.
As a result, people in northern latitudes should consider taking vitamin D supplements, which are "every bit as effective as sunlight exposure and a hell of a lot cheaper" than a trip to the tanning bed, Grant says.
What To Do
Vitamin D also plays a crucial role in healthy bone growth, and people who are deficient in the nutrient face developing osteoporosis.
To learn more about vitamin D, try the National Institutes of Health.
For a look at cancer mortality by geographic area, try the National Cancer Institute.