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High blood levels of the hormone melatonin
may explain why blind women have significantly lowered risks for
breast cancer compared with sighted women.
Melatonin, which is secreted by the pineal gland in the brain and
plays an important role in the body's sleep cycle, is thought to
influence the secretion of estrogen,
which in turn influences breast cancer risk.
The investigators found that totally blind
women had a 36% lower risk of breast
cancer compared with sighted women. Women who became
blind relatively early in life (before age 65) appeared to be especially
protected against breast cancer, with incidence rates 49% below
those of sighted women.
Only total blindness -- not
visual impairment -- seemed to protect against breast cancer. According
to the authors, this supports the theory that increased nighttime
exposure to artificial light reduces melatonin levels, altering
estrogen secretion rates and upping risks for breast cancer.
Blind women are by definition unreceptive to light, however, and
may maintain high melatonin production at night regardless of external
light conditions. Kliukiene's team believe this may be the mechanism
whereby blind women are protected from breast cancer.
British Journal of Cancer March
2001;84:397-399
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