Asian people have a lower prostate cancer risk than Western people, mainly due to living conditions, because as soon as Asian people move to the West and take over Western habits, their risk of prostate cancer increases. Not only prostate cancer risk is lower in Asia, but also that of other hormonal dependent cancers such as breast cancer. The oriental kitchen uses more soybeans containing high levels of isoflavones.
Effect of isoflavones in in-vitro and animal studies
According to a review study by Andres some in-vitro studies demonstrated that low genistein levels enhanced the growth of estrogen sensitive prostate cancer cells but only inhibited their growth at physiologically nonachievable high levels. Other studies showed a protective effect, even at lower levels, especially when different types of isoflavones were simultaneously administered . Kyle found that isoflavones were effective when the cancer cells were incubated over a longer time. Researchers at University of Florida found that Novasoy, a commercial isoflavone concentrate extracted from soy, inhibited the growth of prostate cancer cells through the modulation of cell cycle progression and the differential expression of androgen-regulated genes .
Animal studies are less conflicting and show that genistein reduces tumor growth of implanted androgen-sensitive prostate cancer cells, although one study demonstrated that genistein promoted tumor growth . Isoflavones also reduced the growth of induced prostate tumors in rodents. Protective effect of isoflavones was more pronounced when the rodents were treated their whole life instead of only during puberty or adulthood. Test conducted on Lobund-Wistar rats (rats that produce higher levels of the hormone testosterone, which is known to promote the development of cancers) revealed that when they received a natural diet containing soy meal had 10x lower risk to develop prostate cancers than those receiving the same diet but with milk protein instead of soy protein .
Protective effect of isoflavones in human studies
One meta-studies found that soy consumption reduced the risk of prostate cancer by about 25%, but other epidemiological studies in Western and Asian countries did not show an association . A Japanese study conducted at Sapporo Medical University School of Medicine, Japan, demonstrated that isoflavones significantly decreased the risk of prostate cancer and concluded that isoflavones might be an effective dietary protective factor against prostate cancer in Japanese men . The scientists came to this conclusion after examining the associations between nutritional and other lifestyle factors and the prevalence of prostate cancer in a case-control study. Men in the highest category of isoflavones intake (>90 mg/d) had a 58% lower prostate cancer risk than men in the lowest category (<30 mg/d). Hamilton-Reeves and coworkers at the University of Minnesota, examined the potential protective effect of soy protein isolate, with low and high levels of isoflavones, on prostate cancer risk in men at high risk for developing advanced prostate cancer. They found that soy protein isolate consumption suppressed androgen receptor expression in the prostate and could be beneficial in preventing prostate cancer. Messina suggested that a daily dose of 120 mg isoflavones may be useful in prostate cancer prevention, but recommended consumption of soy foods rather than isolated isoflavones supplements, because other soy components such as soy protein, fibre and saponins may offer additional health benefits . A recent study conducted at the Nara Medical University investigated the association between prostate cancer risk and isoflavone intake in a phase II, randomized, double-blind, placebo-controlled trial . The researchers found that treatment with isoflavones had no influence on the level of Prostate-Specific Antigen, but biopsies showed that isoflavones intake reduced the incidence of prostate cancer, but that this difference was not statistically significant. However, if they only considered patients aged 65 years or more, the incidence of cancer in the isoflavone group was significantly lower than that in the placebo group.
The view of Professor De Sy
In the article entitled “soy appears to be a magic cure”, published in the August 4th, 2004 edition of Knack, Professor De Sy claims that about two thirds of all cancers find their origin in factors from the environment (pollution, pesticides…) and living habits (smoke, nutrition,…). A lot of cancers can therefore be prevented by anticipating on these factors. Professor De Sy does research on the development of prostate cancer. Prostate cancer lends itself well to research because this cancer grows slowly in time: from the beginning of the development to the final stage of prostate cancer takes 20 to 30 years. Soy isoflavones have the property to slow down the development of prostate cancer. When isoflavones are digested, equol is formed. Equol blocks the functioning of dihydrotestosteron (DHT) resulting in a reduced prostate cancer risk. However, according to Professor De Sy large quantities of soy products are necessary to be effective against prostate cancer: one must drink 1.5 litre soya milk or 250 grammes of soya burger daily. Asians have also more bacteria in their bowels which are necessary for the production of functional substances. He advises to take isoflavonen in the form of tablets, because they contain a lot of active components. He firmly believes that when men start to take supplements from the age of 40 or 50, much less cases of prostate cancer would occur.
Soy and isoflavones seem to reduce the risk of prostate cancer in most studies, but some show tumour promoting effects. Larger randomized controlled trials are needed to determine whether soy or isoflavone can play a role in the prevention or treatment of prostate cancer.
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