Breast Cancer Risk Appears to Fall With Greater Sun Exposure

Washington—Could a walk in the sun reduce breast cancer risk? Two reports at the Department of Defense Breast Cancer Research Program “Era of Hope” meeting add support to hypotheses on the protective role of vitamin D.

Esther M. John, PhD, of the Northern California Cancer Center, Union City, and Esther C. Janowsky, MD, MPH, of the department of epidemiology, University of North Carolina School of Public Health, agreed that epidemiologic studies may indicate a role for vitamin D in warding off breast cancer.

Vitamin D is produced in skin cells upon exposure to sunlight. Its classic role is to maintain normal levels of calcium and phosphorous. But recent in vitro studies of 1,25(OH)2D (the prime vitamin D metabolite) show that it also inhibits breast cancer cell proliferation and promotes differentiation, Dr. John said.

Previous reports have shown a north-south geographical gradient for breast cancer in the United States. Mortality rates are higher in the Northeast than in the South and are inversely correlated with sunlight.

For her study, Dr. John drew from a cohort of women who took part in the first National Health and Nutrition Survey (NHANES I) from 1971 to 1975 and were followed prospectively until 1987. She looked at records of 4,881 white women, including 133 with breast cancer, to determine state of residency, personal estimates of sunlight exposure, actinic skin damage, and dietary intake of vitamin D.

In general, she found that breast cancer risk was reduced in regions with highest sunlight exposure. Recreational exposure produced a slight risk reduction, and occupational exposure produced a 35% reduction. Combined, the two factors accounted for a 46% reduction in breast cancer risk.

Similar rates of risk reduction were found in women who were born in areas of high solar radiation, or whose longest residence was there, or who had lived there for at least 20 years or half their life.

Dr. John found no trend of decreasing risk with increased dietary intake of vitamin D or use of multivitamins.

A Case-Control Study

Dr. Janowsky performed a case-controll study using 511 archived blood samples. She found that women with 1,25(OH)2D levels in the lowest quartile had a 2.7 times greater risk for breast cancer than those in the highest quartile. The effect was stronger among women aged 55 or more and among those with estrogen-receptor-positive tumors.

To evaluate potentially confounding effects, Dr. Janowsky analyzed her data both with and without the women with more advanced disease and possible hypercalcemia. “Breast cancer patients with high serum calcium levels may have decreased 1,25(OH)2D levels,” she said. “This study doesn’t tell us whether the low 1,25(OH)2D preceded the breast cancer or was a result of the breast cancer.”

How Much Exposure?

These studies are too preliminary to make any clinical recommendations, Dr. Janowsky said. “There’s no scientific basis for recommending an optimal time in the sun. I’m not a dermatologist, but regular moderate sun exposure without burning is probably ok.” Dr. John agreed: “Casual, everyday exposure is probably sufficient to reduce breast cancer risk.”

Both speakers noted that their findings apply primarily to white women since there were too few black subjects in their studies to make any inferences statistically valid. “Black women,” Dr. John noted, “have more pigmentation in their skin, and so produce less vitamin D.”

Neither researcher had any information on skin cancer rates for their study groups, but Dr. John found that while breast cancer risk was reduced among women with moderate to severe elastosis or keratosis, it increased among those with telangiectasia.

“The findings do not suggest that women should spend hours in the sun,” Dr. John said. “Casual exposure may be enough, but we don’t have the data to know how much or when in life it’s important to get this exposure—whether when young, old, or continuously.”