The Emerging Link between Light Exposure and Cancer
While American women have a life expectancy that is two-and-a-half times greater than women in some developing countries, they are five times more likely to develop breast cancer. According to Dr. Richard Stevens, a cancer epidemiologist at the University of Connecticut Health Center, as countries industrialize, the rates of breast cancer—both incidence and deaths—increase.
While possible explanations abound, a number of recent scientific studies suggest that nighttime exposure to light is one factor. With increased light pollution invading bedrooms at night, as well as more nocturnal lifestyles that keep people awake in artificial light during prime hours of darkness, it may be that people are simply not getting enough of a critical hormone.
Melatonin is released from the brain’s pea-sized pineal gland at night. Scientists have known for years that the hormone is light sensitive and can only be produced in the dark. It is now also a proven cancer fighter.
According to Dr. David Blask, a neuro-endocrinologist at the Bassett Research Institute in Cooperstown, New York, melatonin inhibits tumor growth by blocking the uptake of linoleic acid, which as one of the Omega-6 fatty acids is an important component of cell membranes and is essential to healthy organ function. Because the human body cannot produce linoleic acid, we must get this essential fatty acid from food. The trouble is that linoleic acid, which is found in most junk foods, is the most abundant polyunsaturated fat in the Western diet, and most Americans get more of it than they need.
"Tumor cells love linoleic acid," Blask explains. "They take up linoleic acid and use it for the calories because tumors need a lot of energy. It revs up growth pathways so that these cancer cells are really cranking. It stimulates cell division and the tumor grows."
In a groundbreaking clinical study, Blask and his colleagues collected three separate blood samples from healthy pre-menopausal women: during the day, at night and at night following 90 minutes of exposure to bright fluorescent light. (Fluorescent and halogen lights—those at the blue end of the spectrum—are the most disruptive to melatonin production.) The scientists perfused each blood sample directly through human breast cancer tumors grafted onto rats, using a technique whereby a single artery fed into the tumor, and a single vein exited it, allowing researchers to control the blood’s circulation.
The team found that the melatonin-rich blood suppressed tumor growth, while the melatonin-depleted blood—collected during the day and at night following light exposure—stimulated growth.
In order to verify that melatonin—rather than other hormones or contributing factors—is the critical link, Blask and his team took the research two steps further. First, they added melatonin to the depleted blood that was harvested from volunteers following nighttime light exposure. This brought the melatonin concentration to what it would ordinarily be at night without light exposure. They then ran this melatonin-rich blood through tumors and found that it had the same effect—inhibiting growth—as the naturally melatonin-rich blood.
The scientists then reversed the theory and took the naturally melatonin-rich blood that was harvested at night without light exposure, and added a molecule that blocks the receptors that receive the melatonin signals in tumors. "The tumor responded as though we’d perfused it with daytime blood or with nighttime light-exposed blood," Blask says. "The results of those two experiments," he adds, "are as close as you can get to absolute proof in science."
Trouble on the Night Shift
The Cooperstown experiment not only offers a likely explanation for higher rates of breast cancer in the industrialized world, but higher rates among night shift workers as well. Compiling data from 240,000 women over the last 20 years, Dr. Eva Schernhammer, a researcher at Harvard Medical School’s Channing Laboratory, has found a 36 percent increase in breast cancer risk among female night shift nurses.
Blask points out, "Our data does not prove that light at night is responsible for the increased risk of breast cancer for shift workers. There may be other explanations as well. What we can say with a high degree of confidence is that nighttime light suppression of melatonin is a new risk factor."
Because the research is still at an early stage, it has so far focused exclusively on breast cancer. The sheer number of unexplained cases, along with Channing Laboratory’s findings, has made breast cancer a natural research focus. Blask hopes to expand his inquiries to include other diseases; he is currently attempting to secure money for similar tests on prostate cancer.
Schernhammer also notes that 36 percent is a relatively small increased risk compared to family history, which has a 100 percent increase, and obesity, which carries a 50 percent risk increase. "Such a modest risk will not become a major public health concern, but from an individual standpoint, it matters," she says.
The American Cancer Society does not recognize nighttime light exposure as a risk factor, and categorizes shift work only as an uncertain risk. Dr. Elizabeth Ward, director of surveillance research at the American Cancer Society, explains, "There have been a number of positive studies, but they have all been published very recently. It would be useful to get a group of experts together who can take a look at the studies and conduct a systematic review of the literature. We can say there is increasing evidence."
Schernhammer agrees that there is "a lot of research that needs to be done," including determining "whether genetics has a role, whether there is a certain group that is particularly susceptible, and also defining more specifically who should be careful and who might be at more risk. With cancer, there’s not one single factor. Certainly it’s possible that melatonin is one piece of a larger puzzle."
Scientists are now trying to determine the relationship between age and melatonin, the precise role of light intensity and length of exposure, the implications for other types of cancer, and whether melatonin supplements would be appropriate for certain people, including night-shift workers orthose with advanced stages of cancer. Currently, most scientists and doctors recommend against takingmelatonin supplements preventatively, although some research indicates that melatonin combined with chemotherapy can improve survival rates for cancer patients.
While a number of questions remain, it is clear that shedding light on the world impacts not only our environment, but our health as well. "Only in the last 120 years have we changed our circadian rhythms on a wholesale basis," Stevens says. "Our environment now is totally different with regard to light."
BRIANNE GOODSPEED is a Massachusetts-based freelance writer and former E intern.