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Breast cancer linked to night shifts

The Danish government has begun paying compensation to women who have developed breast cancer after long spells of working nights

The Danish government has begun paying compensation to women who have developed breast cancer after long spells of working nights. This follows the ruling by the UN agency, International Agency for Research on Cancer (IARC), which ranks night working just one level below working with known carcinogens such as asbestos, diesel engine exhaust and ultraviolet radiation.

Ulla Mahnkopf, who developed breast cancer after working for 30 years as a flight attendant for SAS said: "When you think back now I can see that when I stopped flying it was like coming out of a shell."

Women who had a family history of breast cancer had their claims rejected.

Dr Vincent Cogliano of the IARC said that it believed that alterations in sleep patterns caused by working nights could lower the body's production of melatonin.

The idea that shift work might relate to risk of breast cancer derives from a hypothesis put forward in 1987 that exposure to light-at-night might lead to increased risk of breast cancer via impairment of pineal secretion of melatonin.

This hypothesis was based on a theory that breast cancer development might be promoted by pineal hypofunction. Several different potential biological mechanisms for the proposed effect of melatonin on breast cancer risk have been proposed - for example via effects of melatonin on oestradiol levels or on free-radical scavenging, or via immune modulation.

Said Dr Cogliano, "Melatonin has some beneficial effects in preventing some of the steps leading to cancer."

Cogliano said that the UN agency's finding was based on a wide range of studies. "The level of evidence is really no different than it might be for an industrial chemical," he said.

Professor Andrew Watterson, an occupational health specialist at Stirling University, said that the British government must follow Denmark in examining the subject. "I think we can say there is a big public health problem here," he said.

In 2003 the Health and Safety Executive (HSE) published a review of four studies that directly investigated the relationship between breast cancer risk and shift work. One of these was a case-control study carried out in Denmark. At the time few studies had been published on the relationship and although each of the four studies has some weaknesses, it was stated that none as of "poor quality" and they found significant results. The 2003 HSE report states: "Hence the overall evidence for an association (but not necessarily a causal association) between shift work and risk of breast cancer is now appreciable, although not definitive."

New shift work-study to establish if dietary MSC could reverse harmful effects of shift work
In February this year, researchers at the Cancer Institute of New Jersey (CINJ) announced a new study to investigate whether shift work predisposes employees to cancer by altering the body's response to hormones, and if so, whether a dietary supplement, methylselenocysteine (MSC), which contains the trace mineral selenium, can help.

Leading the study, Helmut Zarbl and his team recently found that chemical carcinogens also disrupt circadian rhythm, contributing to what Zarbl considers an imbalance in a protein that prevents cancer by regulating the cell's response to hormones. They found that a naturally occurring compound, methylselenocysteine (MSC), found in many foods, prevents cancer in rats by restoring circadian rhythm and the cell's response to oestrogens. MSC contains the trace mineral selenium, which a number of epidemiological studies have shown can reduce the incidence of several types of cancer.

Zarbl said. "We have an opportunity to determine whether a supplementation of the diet with the amount of MSC found in a single Brazil nut can prevent two of the most common forms of cancer. The implications from such findings could be far reaching."

During the course of the three-year grant period, the team will determine if shift work also disrupts the cell's response to oestrogens, and if this effect can be reversed by dietary MSC. The team will address these questions in two groups of participants. The first group, comprised of primarily hospital workers, will be asked to donate blood while working the day shift, and again after working at least one week on the night shift. This group will determine the effect of shift work on various biomarkers related to circadian rhythm and oestrogen response.

In the second phase of the study, 100 volunteers who engage in traditional "shift work" professions, such as firefighters, police, airline and factory personnel, will be recruited through the New Jersey Family Medicine Research Network. For 30 days, this group will take an over-the-counter MSC supplement. Blood taken from these study participants will be used to determine if the MSC supplement can reverse the harmful effects of shift work on circadian rhythms and oestrogen response. If so, the results will form the basis for a prospective study to determine if MSC supplements can prevent breast and prostate cancer in those who serve the community by working at night.

16th March 2009


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