Large study concerning soy isoflavones and prostate cancer

[Guest guest]

PHILADELPHIA – 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 the March issue of

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 & #8722; 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 & #8722; that isoflavones decreased the

risk of localized prostate cancer, but not the risk of advanced

prostate cancer & #8722; 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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