Soja en prostaatkanker.*
Uit een Japanse studie onder ruim 43.000 mannen blijkt dat het eten van soja en sojaproducten de kans op prostaatkanker, afhankelijk van de hoeveelheid soja, wel tot 50% kan verlagen. Deze verlaging viel vooral op bij mannen ouder dan 60 jaar en niet zo zeer bij jongeren. Onduidelijk bleef nog of de kans op uitgezaaide prostaatkanker al dan niet hoger was. In Japan worden veel meer soja en sojaproducten zoals miso, natto en tofu gegeten dan in de Westerse wereld en is het aantal mannen met prostaatkanker veel kleiner dan in het Westen. Soja en sojaproducten bevatten veel isoflavonen die de bescherming geven tegen het ontstaan van prostaatkanker.
Soy
Found Protective Against Localized Prostate Cancer Only
The largest study examining the relationship
between the traditional soy-rich Japanese diet and development of prostate
cancer in Japanese men has come to a seemingly contradictory conclusion: intake
of isoflavone chemicals, derived largely from soy foods, decreased the risk of
localized prostate cancer but increased the risk of advanced prostate cancer.
The prospective study of 43,509 men, published in Cancer Epidemiology,
Biomarkers & Prevention, suggests that the effects of isoflavones on
prostate cancer development may differ according to disease stage, say
researchers at the National Cancer Center in Japan.
One possible explanation is that isoflavones may delay the progression of latent
prostate cancer only; once tumors lose estrogen-receptor beta expression and
become aggressive, isoflavones may fail to protect against the development of
advanced cancer, and might even increase the risk of progression, possibly by
reducing serum testosterone, researchers say. It is also possible that advanced
and localized prostate cancer may be different tumor subtypes, which may react
differently to isoflavones.
"The present findings provide no clear understanding of when or how
localized cancer will develop to aggressive cancer, and of the related effect of
isoflavones," said the study's first author, Norie Kurahashi, M.D., of the
Epidemiology and Prevention Division of the National Cancer Center.
"Given that Japanese consume isoflavones regularly throughout life, we do
not know the period during which the effects of isoflavones on prostate cancer
are preventive, and further research is required to find that out, including
well-designed clinical trials," she said.
Until those studies are done, the researchers recommend that Japanese men
continue to consume isoflavones through their food and not through supplements.
"Consumption of isoflavones from traditional Japanese food throughout life
may protect against the incidence of prostate cancer, but we cannot recommend
the use of isoflavones from supplements for people who do not regularly consume
these chemicals, because the relationship between isoflavones and the risk of
advanced prostate cancer is not yet clear," Kurahashi said.
Isoflavones act as both strong antioxidants and plant-based estrogens. Soybeans
are the most common source of isoflavones, especially genistein and daidzein,
which have been shown in some animal studies to exert a protective effect
against prostate cancer.
Japanese men eat significantly more soy-based foods than do Western men, and the
incidence of prostate cancer is much lower in Asian countries than in Western
countries. Still, reviews of latent, or clinically insignificant, prostate
cancer findings in autopsy reports have revealed no difference between the
populations so scientists have theorized that isoflavones stop latent cancers
from developing further.
But because smaller epidemiological studies in Japan have reached differing
conclusions about the protective effects of soy on prostate cancer development,
this research team conducted the most comprehensive analysis to date. They
polled thousands of men age 40-69 about their consumption of 147 foods, the most
popular of which were miso soup (primarily made from fermented soybeans), natto
(also a product of fermented soybeans) and tofu, made from soy milk. Japanese
consume miso soup more frequently, usually daily, than other soy foods, and miso,
natto, and tofu account for about 90 percent of the population's consumption of
daidzein and genistein, according to Kurahashi.
The researchers then followed participants from 1995 through 2004 and found that
307 men were diagnosed with prostate cancer. In this group, 74 cases were
advanced, 218 were confined to the prostate organ, and 15 were of undetermined
stage.
They concluded that intake of genistein, daidzein, miso soup and soy food had no
overall link to diagnosis of prostate cancer. However, they calculated that the
risk of developing localized prostate cancer was 50 percent lower in men who ate
the most isoflavones compared to men who ate the least - meaning that men in the
top category ate between two and three times as much isoflavone-rich food.
However, in a discovery they cannot explain, they also calculated that the risk
of developing advanced prostate cancer was twice as high in men who consumed two
or more bowls of miso soup a day than in men who ate less than one bowl of soup.
They also found that the protective effect of isoflavone-rich food was strongest
in men who were older than 60: the more isoflavones they ate, the more they
reduced their risk of developing localized prostate cancer. "Isoflavone may
be protective for localized prostate cancer only in men aged more than 60 years,
and may not have a protective effect in the early stage of prostate cancer in
younger men," the researchers conclude in their study.
The inconsistencies in the finding - that isoflavones decreased the risk of
localized prostate cancer, but not the risk of advanced prostate cancer - could
be errors in food measurement, or could be due to the fact that the number of
participants who developed advanced prostate cancer was small, said Kurahashi.
Or, as researchers speculate, isoflavones could interact with the estrogen
receptor on prostate tissue enough to inhibit production of testosterone, which
can fuel prostate cancer. When tumors lose all of their estrogen receptors and
stop responding to isoflavone-induced hormonal interference, they grow
aggressively.
"A broad body of research is required to clarify the timing and period of
isoflavones' preventive effect on prostate cancer development," Kurahashi
said.
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(April 2007)
(Opm. Meer over soja
en over prostaatkanker.)