Via internet wordt melding gemaakt van het feit dat Johnni Hansen, een bekende Deense onderzoeker naar de werking van melatonine o.a. een nieuwe studie heeft gepubliceerd onder maar liefst 7.025 vrouwen die aantoont dat 's nachts werken het risico op het krijgen van borstkanker met 1,5 keer vergroot in vergelijking met mensen die overdag werken. Ik heb dit bericht en twee andere studies die deze bevindingen bevestigen en al in 2000 en 2001 in toonaangevende medische tijdschriften zijn gepubliceerd onder elkaar gezet. Vooral de al of niet voldoende aanmaak van melatonine wordt als mogelijke oorzaak gezien van dit verhoogde risico. Wie meer wilt lezen over de waarde van melatonine leest de studie analyses van arts-bioloog drs. E. Valstar op deze pagina en pagina onderzoek en voeding over gebruik van melatonine of leest zijn boek 'Voedingsinterventie bij kanker' waar hij ruim aandacht aan de genezende werking van melatonine besteedt.

Hier achter elkaar drie berichten/studies over het verhoogde risico op borstkanker door weinig zonlicht en nachtwerk.

Scientists have produced more evidence that night work can damage health. 
An analysis of data from Denmark suggests women who work at night may be at an increased risk of developing breast cancer. 

The study follows research which suggests working at night increases the risk of heart disease. 

Dr Johnni Hansen, of the Institute of Cancer Epidemiology in Copenhagen, studied the medical and employment records of 7,035 women between the ages of 30 and 54 who had been diagnosed with breast cancer. He found that women whose work involved night shifts were 1.5 times more likely than daytime workers to be diagnosed with breast cancer. 

Dr Hansen found that the longer a woman regularly worked at night, the higher her risk of developing the disease. The reason for the link is unclear. But some studies have suggested night workers are more likely to consume alcohol, a known risk factor for breast cancer. 

Artificial light 

Another theory is that the risk is increased by exposure to artificial light. It is thought artificial light acts to suppress production of a hormone called melatonin, which is normally produced in the body at night. Research has suggested low levels of melatonin may either stimulate the growth of cancerous cells in the breast, or encourage the production of the female sex hormone oestrogen, which has been widely linked to breast cancer. Dr Hansen argues that the experience of blind women provides evidence to back this theory up. Blind women have approximately a 50% smaller risk of developing breast cancer than other women. As these women cannot sense light visually, it has no impact on their melatonin production. 

Professor Gordon McVie, director general of the Cancer Research Campaign, said: "This study is completely inconclusive, and the increased risk is not enormous. "But it is a big enough study not to be swept under the carpet." Prof McVie said the fact that people who live in the Mediterranean area do not suffer higher rates of breast cancer suggested the melatonin theory may not be correct. He said several other hormones could equally be implicated in any possible link between night work and breast cancer 

The research is published in the journal Epidemiology.

Melatonin and Breast Cancer Risk 

A new study by researchers at the Fred Hutchinson Cancer Research Center has found that women who work the night shift may face an increased risk of breast cancer. The reason: Nighttime sleep loss or light at night may suppress the production of melatonin, leading to an increase in estrogen release. Estrogen has been linked to an increased risk of breast cancer. 

Writing in the Journal of the National Cancer Institute, Scott Davis, Ph.D. and colleagues reported on their investigation of the relationship between breast-cancer risk and exposure to light at night as determined by sleep habits, bedroom lighting and night-shift work. 

"Although there have been a number of studies looking at the health effects of night-shift work, from heart disease to stomach ailments, this is the first that has looked comprehensively at both graveyard-shift work and light at night as independent risk factors for breast cancer," Davis said. 

Davis' team interviewed more than 800 Seattle-area women who had been diagnosed with breast cancer. They assessed each woman's exposure to light at night and history of shift work, among other factors, during the 10 years prior to their breast cancer diagnosis. 

They found that women who worked the night shift during the decade before their breast-cancer diagnosis had a 60 percent increased risk for breast cancer compared with those who did not work the night shift. In addition, the risk of breast cancer significantly increased with each additional hour per week of night-shift work. 

The researchers believe that the link between sleep, light at night and breast cancer may involve melatonin, a hormone produced by the brain's pineal gland. Production of melatonin peaks at night during sleep. One theory is that nighttime sleep deprivation or exposure to light at night somehow interrupts melatonin production, which in turn stimulates the ovaries to kick out extra estrogen. 

Another study published in the same issue of the Journal examined the relationship between rotating night-shift work and breast cancer risk in more than 78,500 women who participated in the Nurses' Health Study. 

Dr. Eva Schernhammer of Harvard Medical School and colleagues found that women who worked at least three night shifts per month, in addition to their usual day and evening shifts, had a moderately increased risk of breast cancer compared to women who did not work rotating night shifts. 

However, Schernhammer cautioned that "further research is needed to determine if melatonin suppression is the reason for the increased breast cancer risk." 

In an accompanying editorial, Dr. Johnni Hansen of the Danish Cancer Society in Copenhagen wrote that "regardless of the underlying biological cause for the apparent increased risk of breast cancer among women who work at night ... there is an urgent need for further exploration of the relationship between exposure to light at night, shift work and cancers that may be influenced by melatonin." 


Journal of the National Cancer Institute, October 17, 2001; 93:1513-1515, 1557-1568 

Graveyard shifts may raise risk of breast cancer 

By Warren King
Seattle Times medical reporter

Women who work graveyard shifts may have an increased risk of breast cancer, a new study by the Fred Hutchinson Cancer Research Center shows. 

Hutchinson Center scientists found that women who regularly work in the middle of the night could face an increased risk ranging from 60 percent to 130 percent. The risk increased with the number of hours and years worked on the shift, which includes the earliest hours of the morning. 

"This study tells us it's important to keep looking at this (finding) carefully. But we're not at the point where we can make recommendations about changing behavior ... We wouldn't say quit your job," said Dr. Scott Davis, director of the study, which is reported in today's edition of the Journal of the National Cancer Institute. 

Conducted with more than 1,600 women from King and Snohomish counties, the study is consistent with four previous research projects from other institutions regarding breast cancer and the graveyard shift. 

The Hutchinson study focused both on the shift work and exposure to light, including sleep habits. Light exposure during normal sleeping hours disrupts the body's circadian rhythm, the biological clock that affects how the body functions. 

Scientists know the brain's production of a key regulating hormone, melatonin, peaks in the dark of night when we are sleeping. Davis and his colleagues think exposure to light during those hours decreases melatonin production, which, in turn, stimulates increased production of estrogen, which has long been linked to breast cancer. 

"Melatonin regulates other hormones in the body, including the production of estrogen. It may also affect other reproductive hormones," said Davis, who is also chairman of the epidemiology department at the University of Washington School of Public Health and Community Medicine. 

A study by researchers at Brigham & Women's Hospital in Boston, also published today in the Journal of the National Cancer Institute, similarly found a link between late-night work and breast cancer. 

The work histories of 78,000 nurses were studied over 10 years. The Brigham researchers found nurses who worked rotating night shifts at least three times a month for one to 29 years had about an 8 percent increased risk of breast cancer. Those who worked the shifts more than 30 years had a 36 percent increased risk. 

The Seattle scientists, from the Public Health Sciences Division of the Hutchinson Center, interviewed about 800 women about their history of shift work and exposure to light during the 10 years prior to breast-cancer diagnosis. Each was in her job a minimum of six months. They were compared with about 800 other women of similar age who had not had the disease. 

The work hours of study subjects at increased breast-cancer risk varied widely. It was difficult to define averages and patterns, Davis said. 

But generally, he said, a woman had a 60 percent increased risk if she worked at least 1 percent of her total hours on the graveyard shift during the study period. That is the statistical cutoff point, he said. 

Women who had worked the shift 5.7 hours a week or more had a 130 percent increased risk, or more than double the normal risk. 

Years on the shift made a difference, too. For example, the researchers said, women doubled their risk for the disease if they worked at least 4½ years on shifts when they were awake at least from 1 to 2 a.m. three nights a week. That is about the time when melatonin production is at its peak. 

"We found that a graveyard shift often is more than eight hours," said Davis. "Hopefully, our future research will tell us more about risks and patterns of shift work." 

Overall, about 1 in 8 women in the United States will develop breast cancer during their lifetime, experts estimate. Nearly 239,000 new cases will be diagnosed this year, and about 40,000 will die of the disease, according to the American Cancer Society. 

The Hutchinson researchers also found about a 40 percent increased risk of the disease among the relatively few women who regularly slept in bedrooms with light bright enough to read. But the results were not statistically significant, said Dana Mirick, a co-researcher. Other levels of light did not carry a risk. Nor did occasionally turning on the light, as when getting up to go to the bathroom. 

The Hutchinson Center research is also consistent with several previous studies that involved blind women's risk of breast cancer. Those who were completely blind had significantly reduced risk of the disease. The perception of light is what regulates the production of melatonin, Davis explained. 

Davis said it is possible that the stress of late-night shift work could increase breast-cancer risk. Many jobs involving shift work are stressful, including factory work, medicine, nursing, police, fire and rescue work. Stress is known to suppress the body's ability to fight off disease. 

Previous studies also have shown that late-night shift work increases the risk of gastrointestinal problems, cardiovascular disease and premature births. 

Davis and his colleagues next plan to look specifically at how levels of melatonin and estrogen are affected by disruptions in the body's normal sleep patterns. 

He said future studies may also examine whether women with a family history of breast cancer are more vulnerable to the disruptions and why some people adapt well to late-night shift work. 

"We're building on a progression of evidence," he said. 

An editorial in the cancer journal called the new studies and other research on the subject "alarming ... regardless of the biologic cause." 

"There is an urgent need for further exploration of the relationship between exposure to light at night, shift work, including timing during the night, and cancers that may be influenced by melatonin," said Dr. Johnni Hansen, of the Danish Cancer Society. 

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