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