The high rate of breast cancer in industrialized countries has long puzzled medical researchers, but a team of U.S. scientists has discovered a possible explanation for why women in developed countries are at high risk of developing the disease.
The answer at first glance may seem unlikely: nighttime exposure to electric lighting.
In a major breakthrough, researchers have linked exposure to light at night to the growth in breast-cancer tumours. The tumours grew because artificial light interfered with the ability of women to create melatonin, the hormone that regulates the body's daytime and night rhythms.
The discovery holds major public-health implications because most women in industrial societies turn on lights at night in their homes and offices and may potentially be at risk from this exposure.
“Light, in terms of our experiments, stimulates breast-cancer growth activity, and obviously this is due to the ability of light to shut off melatonin production,” said David Blask, a scientist with the Bassett Research Institute in Cooperstown, N.Y., who led the team that made the discovery.
He said, “melatonin puts cancer cells, in particular breast-cancer cells, to sleep at night,” but if the levels of this hormone are diminished by exposure to light at night, cancers “become insomniacs” and grow all the time.
In recent years, there has been a flurry of research suggesting light at night may be a health hazard, causing illnesses ranging from chronic fatigue to depression.
But until now, there has only been circumstantial evidence linking it to breast cancer. For instance, women who regularly work overnight “graveyard” shifts have been found to have an elevated incidence of the disease, in some cases up to 60 per cent higher than those who work regular day shifts.
This new research, outlined last month in the journal Cancer Research, is the first experimental evidence to show that light at night can have an effect similar to a cancer-promoting chemical.
“Electric lighting as a driver of the breast-cancer epidemic worldwide — that's a dramatic big thing, and new,” said Richard Stevens, an epidemiologist at the University of Connecticut who has studied the health risks of light pollution.
The U.S. National Institutes of Health, which financed the study, hailed the results and said they offer “a promising new explanation for the epidemic rise in breast-cancer incidence in industrialized countries like the United States.”
Les Reinlib, a program administrator for the agency, said the discovery may hold promising avenues for preventing breast cancer with simple steps, such as changing women's exposure to light at night. He also said that melatonin, an inexpensive and widely available hormone supplement, should be studied to see whether it holds promise as an anti-cancer therapy.
Breast cancer is the most frequently diagnosed cancer among Canadian women, with about 21,600 new cases diagnosed annually. About 5,300 women die each year of the disease.
Dr. Stevens said the high breast-cancer incidence in industrialized countries, at about five times that of poor countries, has long intrigued researchers. “We have what I would call an epidemic of breast cancer and we don't know why,” he said.
Studies into possible explanations, such as the high-fat Western diet, pesticides, or industrial pollutants, have generally been inconclusive, suggesting that there is something else that is ubiquitous in affluent countries that is causing the disease.
Breast cancer is linked to genetics, early menstruation, and reproductive history, among other things, but about 60 per cent of those with the disease have no currently known risk factor.
