BOSTONExcessive sun exposure is a known risk factor for the development of skin cancer, but sun exposure appears to have a protective effect against a variety of other cancers, according to speakers at a symposium on sunlight at the 168th National Meeting of the American Association for the Advancement of Science (AAAS).
Sunlight’s product, vitamin D, long recognized as crucial to a number of vital bodily functions, also protects against prostate, breast, and colon cancer, and probably other solid tumors, said panel member Michael F. Holick, PhD, MD, professor of medicine, dermatology and physiology and director of the Vitamin D, Skin and Bone Research Laboratory, Boston University School of Medicine. In agreement was William B. Grant, PhD, an independent scholar from Newport News, Virginia.
Dr. Holick cited physiological evidence and Dr. Grant presented data from an ecological study to argue that insufficient vitamin D is related to higher cancer incidence and mortality.
Kenneth Kraemer, MD, research scientist in dermatology at the National Cancer Institute, however, reminded the audience that sunlight is clearly implicated in the genetic damage leading to basal cell and squamous cell carcinoma and, to a lesser extent, melanoma.
The link between cancer protection and vitamin D is the enzyme 25 hydroxy vitamin D 1-hydroxylase, Dr. Holick said. This enzyme is vital for changing the inactive form of vitamin D, produced when the skin reacts to sufficient sunlight, into the activated form that can be used by cells. Among other functions, he noted, activated vitamin D appears to down-regulate cell growth, acting as a "brake" on incipient cancers.
The hormone was previously thought to occur only in the kidney. It is now known, however, that prostate, breast, skin, and colon tissue also produce it, Dr. Holick said. He suspects that it also occurs in other tissues as well. A variety of organs thus have the ability to take circulating vitamin D and turn it into the activated form, he noted.
Sunlight and Cancer Rates