Basking briefly in the sun every day may be an important way to prevent breast cancer, researchers said Monday. But they also warned, "Don't overdo it."
New studies indicate that vitamin D, a nutrient made by the skin during exposure to sunlight, can lower the risk of breast cancer by 30 percent to 40 percent and perhaps even more.
"We know now that a little bit of sun is beneficial, but it is not good to stay out there four or five hours," said epidemiologist Esther John of the Northern California Cancer Center. "We don't want to recommend that people go out and bake in the sun."
She said it is well known that excessive exposure to sunlight can cause skin cancer and premature aging of the skin.
John, in a study presented Monday at a scientific meeting of breast cancer experts, said that a study comparing the health habits of 133 breast cancer patients with women who did not have the disease found that exposure to sunlight significantly reduced the risk of breast cancer.
Sunlight lowers the risk, said John, because the skin uses ultraviolet rays from the sun to make vitamin D. This nutrient has been closely linked to protection against breast cancer in other studies, she said.
The study was part of a national research project that has been following the health of about 8,000 women since 1970. The research monitors the development of cancer and other health events and compares the effects of such things as diet and exercise of women who get disease and those who don't.
John said that the study confirmed earlier findings that women who live in the southern tier of states below Kansas tend to get significantly less breast cancer than those who live in the North.
The difference, she said, is sunlight and how the skin makes vitamin D.
Southern states have more year-round sunlight than Northern states do. As a result, people in the South get more natural ultraviolet ray exposure.
John said the study did not determine just how much sun exposure is needed, but she said it probably is less than what would cause skin damage.
"It is possible that all it takes is 10 or 15 minutes outside in bright sunlight to get a benefit," she said.
Sunscreens that block ultraviolet rays would also block the formation of vitamin D, she said.
Vitamin D can also come from the diet in fish oil, fatty fish, egg yolk and liver, all foods that few Americans eat, she said. Milk and some cereals and breads are fortified with vitamin D.