01/12/2005 - Breast Cancer linked to "light at night"
In women the risk of developing breast cancer is up to five times higher
in industrialised nations than in underdeveloped countries. In the UK, the
incidence is increasing at around 3% per year. Overall, nearly 50% of breast
cancers cannot be accounted for by conventional risk factors. In 2003, a report
prepared for the Health & Safety Executive highlighted the increased breast
cancer in night-shift workers (Ref 1).
Since the late 1980's, it has been suspected that people exposed to
excessive "light at night", such as night-shift workers, may be at a greater
risk of developing cancer, with breast cancer particularly implicated. A new
study has elegantly demonstrated that melatonin, a natural hormone produced by
the pineal gland at night time has a dramatic effect on cancer growth - a
hormone that is not produced if the person is exposed to light through the
night. EMF exposure has also been shown to reduce nocturnal melatonin.
Breast cancer tumours were transplanted into rats with no immune system
(known as "nude" rats) for the experiment. Young women volunteers were allowed
to sleep in total darkness.
In the early hours blood samples were taken automatically in a
manner which did not disturb the women (which would contain the normal amount
of nocturnal melatonin). This blood was injected into the rats, and the tumours
stopped growing. Blood samples were then taken from the same women after they
had been exposed to bright light which suppressed the production of melatonin.
When this blood was given to the rats the breast cancers continued to grow.
Finally, this second blood sample was supplemented with melatonin at a level
equivalent to those in the first blood samples. When given to the rats, this
again prevented the growth of the human breast cancers.
These findings are potentially highly significant, especially when placed in
the context of the increased incidence of leukaemias and other cancers in the
last 50 years. Magnetic fields from powerlines and electrical appliances have
been shown to suppress melatonin production in the general population, and it
could be this secondary effect that is causing the association between
powerlines and cancer. Also implicated is the large increase in cancers that
started to increase at the same time as the electrification of houses in
1) "Melatonin-Depleted Blood from Premenopausal Women Exposed to Light at
Night Stimulates Growth of Human Breast Cancer Xenographs in Nude Rats" -
David E Blask, George C Brainard et al, Cancer Research, vol. 65 (23), 1-11
Available from: the American
Association for Cancer Research - Cancer Research.
The increased breast cancer risk in female night shift workers has been
postulated to result from the suppression of pineal melatonin production by
exposure to light at night. Exposure of rats bearing rat hepatomas or human
breast cancer xenografts to increasing intensities of white fluorescent light
during each 12-hour dark phase (0-345 pW/cm2) resulted in a dose-dependent
suppression of nocturnal melatonin blood levels and a stimulation of tumor
growth and linoleic acid uptake/metabolism to the mitogenic molecule
13-hydroxyoctadecadienoic acid. Venous blood samples were collected from
healthy, premenopausal female volunteers during either the daytime, nighttime,
or nighttime following 90 minutes of ocular bright, white fluorescent light
exposure at 580 pW/cm2 (i.e., 2,800 lx). Compared with tumors perfused with
daytime-collected melatonin-deficient blood, human breast cancer xenografts and
rat hepatomas perfused in situ, with nocturnal, physiologically melatonin-rich
blood collected during the night, exhibited markedly suppressed proliferative
activity and linoleic acid uptake/metabolism. Tumors perfused with
melatonin-deficient blood collected following ocular exposure to light at night
exhibited the daytime pattern of high tumor proliferative activity. These
results are the first to show that the tumor growth response to exposure to
light during darkness is intensity dependent and that the human nocturnal,
circadian melatonin signal not only inhibits human breast cancer growth but
that this effect is extinguished by short-term ocular exposure to bright, white
light at night. These mechanistic studies are the first to provide a rational
biological explanation for the increased breast cancer risk in female night
shift workers. (Cancer Res 2005; 65(23): 1-11)