The nighttime production of the hormone melatonin by the brain’s pineal gland may help prevent the growth of human breast cancer by blocking the tumor’s uptake of dietary linoleic acid, according to an article published in Breast Cancer Research and Treatment (June 2003).
Linoleic acid is an essential polyunsaturated fatty acid consumed at high levels in the Western diet. It has been shown to stimulate the development of breast and other cancers. A group of researchers at the Bassett Re-search Institute in Cooperstown, NY, believes that the growth of breast cancer and other malignancies is a net balance between stimulation during the day by growth factors such as linoleic acid and inhibition during the night by melatonin.
To further explore this relationship, the Bassett group exposed rats implanted with human breast cancer tissue to varying light situations. They found that those rats that were constantly exposed to bright light demonstrated a seven-fold increase in breast cancer growth as compared to rats on an alternating light-dark cycle. This is believed to be due to increased tumor uptake of linoleic acid from the lack of melatonin. Conversely, human breast cancers subjected to normal nighttime levels of melatonin demonstrated a nearly 70% decrease in growth, as well as decreased linoleic acid uptake and metabolism.
“This is the first biological evidence for a potential link between constant light exposure and increased human breast oncogenesis involving melatonin suppression and stimulation of tumor linoleic acid metabolism,” said the researchers in their article.
“You can essentially say that cancer cells are put to sleep at night by melatonin, but that they get their wake-up call during the day when there isn’t enough melatonin around to block linoleic acid’s stimulatory action,” lead researcher David Blask, M.D., Ph.D., told Life Extension magazine. “The melatonin signal at night is a key to the circadian regulation of cancer growth and we now know that 75-90% of human breast cancers have specific receptors for this signal.”
These findings may explain the link found in previous research between increased risk of breast and colon cancer in nurses who work rotating shifts. It is hypothesized that these nurses may be suppressing their melatonin production because of their increased exposure to light at night. In fact, research has shown that the risk of breast cancer is lower in blind women, who cannot detect light at night.
“Our study provides the first experimental evidence in a model system of human cancer to support the hypothesis that melatonin suppression in shift workers by light at night may be a major factor responsible for their increased risk of breast and colorectal cancer possibly because cells are taking up more linoleic acid than usual over the course of the day,” Blask explained.
“These findings help to make a stronger case that interactions between biological clock function, light-dark cycles, and diet will increasingly need to be taken into consideration by oncologists and others when making recommendations and decisions regarding cancer prevention and treatment strategies,” said Blask.*
—Carmia Borek, Ph.D.