Sun Exposure Cuts Cancer Risk at 16 Sites, Study Says

Allison Gandey

August 15, 2006

August 15, 2006 –- New research is rekindling the debate over sun exposure and cancer risk. Experts are questioning whether solar radiation helps patients by promoting vitamin D production and lowering their risk or harms those exposed by encouraging skin cancer.

The study, published in the July-August issue of Anticancer Research, suggests that solar ultraviolet B irradiance is associated with reduced risk at 16 sites of cancer through production of vitamin D. These cancers include 6 sites of gastrointestinal cancers, 3 cancers of female sites, 3 urogenital cancers, 2 types of lymphomas, and 2 upper aerodigestive tract cancers.

"The mechanisms whereby vitamin D reduces the risk of cancer are well known," lead author William Grant, PhD, from the Sunlight, Nutrition, and Health Research Center in San Francisco, California, said. "They include effects on intercellular adhesion, apoptosis, the inhibition of angiogenesis around tumors, and the inhibition of metastasis."

During an interview with Medscape, Dr. Grant said that short courses of sun exposure provide important health effects that have been overshadowed by doctors and the sunscreen industry. "Unfortunately, dermatologists are looking only at the skin. They have parochial point of view and have been part of the problem," Dr. Grant said. He suggests that about 15 minutes of sun exposure for fair-skinned people and about 3 times as long for those with darker skin tones can be very beneficial.

Oral Vitamin D Supplements Might Be Helpful

Even more important than sun exposure, Dr. Grant argues, is oral intake of vitamin D. He says previous studies have shown that 1000 to 1500 international units per day reduce the risk of cancer. He estimates that a year's supply of vitamin D can cost as little as US $15 and is well worth the investment.

But Barbara Gilchrest, MD, from the department of dermatology at the Boston University School of Medicine, in Massachusetts, says that many of these studies have been published in "second- and third-tier journals" and fail to account for confounding variables.

"This is a complicated, politicized area, I'm afraid," Dr. Gilchrest told Medscape. She says that while oral vitamin D supplements may well be beneficial, the evidence to date has been of poor quality.

In the present analysis, the researchers performed an ecologic study evaluating age-adjusted mortality rate data from 49 states plus the District of Columbia. They included other cancer-risk–modifying factors and found that a proxy indicator of smoking was associated with risk at 10 cancer sites, alcohol consumption with 9 sites, urban residence with 7, and Hispanic heritage with 6.

"These results provide additional support for the hypothesis that solar ultraviolet B, through photosynthesis of vitamin D, is inversely associated with cancer mortality rates and that various other cancer-risk–modifying factors do not detract from this link," the researchers conclude. They speculate that sun-avoidance practices after 1980 along with improved cancer treatment led to reduced associations during this period.

Premature to Conclude Sun Exposure Beneficial

Dr. Gilchrest told Medscape that it is too early to make such sweeping conclusions. "The authors are looking at averaged data over large areas and making statistical associations between how much sunshine there is in a given state and cancer mortality rates. This is very different from establishing cause and effect."

Dr. Gilchrest says the paper is a difficult read, presents weak data, and references mostly commentaries instead of rigorous well-controlled studies. "The researchers provide no data on sunscreen use, and they don't even mention the carefully done study presented in the New England Journal of Medicine in February of this year, which showed no benefit," Dr. Gilchrest said.

In that study, lead author Jean Wactawski-Wende, PhD, from the State University of New York at Buffalo, and colleagues showed that daily vitamin D supplementation had no effect on the incidence of colorectal cancer among postmenopausal women.

But during an interview with Medscape, Dr. Grant said that many of the well-controlled studies such as the one presented in the New England Journal of Medicine failed to use adequate doses of vitamin D. "How can you test the effectiveness of the vitamin D hypothesis without administering enough vitamin D?" He notes that this study used just 400 IUs instead of the recommended 1000 to 1500 IUs.

It is estimated that dietary sources provide about 250 to 300 IUs of vitamin D per day for most people. Proponents on both sides of the vitamin D debate call for additional study. The researchers write, "Solar ultraviolet B should be studied further with additional observational and intervention studies of vitamin D indices and cancer incidence, mortality, and survival rates."

Anticancer Res. 2006;26:2687-2699.
N Engl J Med. 2006;354:684-696.


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