As I write this at 2AM sitting under annoying fluorescent lights, am I increasing my risk of developing breast cancer? Maybe, according to a recent study showing that melatonin-depleted blood can spur the growth of mammary tumors:
The increased breast cancer risk in female night shift workers has been postulated to result from the suppression of pineal melatonin production by exposure to light at night.
Venous blood samples were collected from healthy, premenopausal female volunteers during either the daytime, nighttime, or nighttime following 90 minutes of ocular bright, white fluorescent light exposure at 580 µW/cm2 (i.e., 2,800 lx). Compared with tumors perfused with daytime-collected melatonin-deficient blood, human breast cancer xenografts and rat hepatomas perfused in situ, with nocturnal, physiologically melatonin-rich blood collected during the night, exhibited markedly suppressed proliferative activity and linoleic acid uptake/metabolism. Tumors perfused with melatonin-deficient blood collected following ocular exposure to light at night exhibited the daytime pattern of high tumor proliferative activity. These results are the first to show that the tumor growth response to exposure to light during darkness is intensity dependent and that the human nocturnal, circadian melatonin signal not only inhibits human breast cancer growth but that this effect is extinguished by short-term ocular exposure to bright, white light at night. These mechanistic studies are the first to provide a rational biological explanation for the increased breast cancer risk in female night shift workers.
While we know a good deal about factors that can contribute to breast cancer risk–including genetics (such as mutations in the BRCA1 and BRCA2 genes) and lifestyle choices (late or no childbearing, high fat diet, lack of exercise), many environmental risks for breast cancer remain controversial. Even the effect of cigarette smoking on breast cancer development remains uncertain, as does the environmental light idea. This new study, however, provides stronger evidence than previous epidemiologic studies that had linked the number of hours under artificial light to the development of breast cancer.
This isn’t a new hypothesis; indeed, it’s been bounced around for a few decades, and has been supported by a number of studies carried out in animals. At the heart of the connection are sleep cycles and the production of melatonin, which they discuss briefly in the article:
The duration of melatonin production during the night is directly proportional to the length of the dark period. The alternating light/dark cycle entrains circadian melatonin production to a 24-hour cycle, whereas ocular exposure to light during darkness rapidly suppresses melatonin production depending on the intensity, wavelength, and duration of the light exposure. In experimental systems, melatonin plays a fundamental role in the regulation of seasonal reproduction, circadian rhythm activity, retinal physiology, cardiovascular effects, immune function, and cancer.
Many tumors are dependent on a fat called linoleic acid in order to grow. Melatonin, however, interferes with the tumor’s ability to use linoleic acid as a growth signal, which causes tumor metabolism and growth activity to shut down. In figure 5 of their paper, the authors showed that tumor growth was repressed when tumors were “fed” melatonin-rich blood, while neither melatonin-poor blood collected either during the day or during the night following 90 minutes of exposure to white light had this effect. They also determined this was due to the action of melatonin by adding melatonin to the melatonin-poor blood, and blocking melatonin in the melatonin-rich blood. As expected, adding melatonin suppressed tumor growth, and blocking melatonin increased it, suggesting this is indeed an effect mediated by this chemical.
They conclude with possible ways to modify this potential risk factor:
Thus, strategies to preserve the integrity of the circadian melatonin signal (i.e., avoidance of bright light at night, intelligent lighting design, circadian-timed physiologic melatonin supplementation) coupled with modifications in nocturnal dietary fat intake may offer a unique approach to the prevention of breast cancer, and perhaps other melatonin-sensitive cancers, in our increasingly 24-hour society.
A problem with this is that currently, melatonin supplements aren’t regulated by the FDA, since they are dietary supplements and not officially “drugs.” If this connection is supported by further research, perhaps womens’ health advocates will need to make this a priority. While I simply prefer to work at night (I’ve always been more productive after everyone else is asleep), many women don’t have the luxury of choosing their shift–and their health shouldn’t suffer because of it.