Jet-lag linked to cancer

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JET-LAG could cause breast cancer in women who fly frequently across time zones, a doctor suggests today. The phenomenon could explain why air hostesses have higher rates of breast cancer, an observation that has baffled scientists for three years.

In 1995, researchers from Finland reported that Finnish air hostesses had almost twice the rate of breast cancer of women in other jobs. The risk was highest for those who had spent 15 years in the job.

Several explanations were offered, including the possibility that the disease was linked with exposure to radiation at high altitude. Passengers on transatlantic flights receive approximately the same dose of radiation while in the air as people on the ground do in a year.

Now Dr Anthony Mawson from North Carolina in the US has offered a different explanation: that the increased risk of breast cancer may be linked with a deficiency of the hormone melatonin, produced by the pineal gland in the brain, which is linked with the body's biological clock.

When the body clock is disturbed - as it is severely in jet-lag - less melatonin is produced. The pineal gland secretes melatonin in response to darkness and reduces secretion during daylight. It is normally at its most active between 9pm and 8am but production is decreased by exposure to bright light during normal sleep time. Taking supplements of the hormone is thought to be the best available treatment for jet-lag, although melatonin is officially banned in Britain.

Writing in The Lancet, Dr Mawson says experiments have shown that melatonin slows the growth of breast cancer cells and there is other evidence that melatonin may be involved when normal tissue in the breast becomes cancerous, through its influence on sex hormones.

Dr Mawson says that shift workers in general, and air hostesses in particular, may be more prone to breast cancer because of these effects. He says the theory could be simply tested by measuring melatonin levels in air hostesses and seeing whether they correlate with jet-lag and the incidence of breast cancer.

Dr John Toy, director of clinical research at the Cancer Research Campaign, said scientists interest in the effects of melatonin on breast cancer was growing. But he warned against women rushing out to seek supplies of melatonin in the hope that it will help prevent them getting breast cancer.

"Melatonin is not harmless and can have serious side- effects, including low blood pressure, nightmares and sleep disorders," he said.