Soy, beans and nuts contain phytoestrogens, oestrogen-like compounds found in plants. The phytoestrogens in soy are mostly isoflavones, while beans and berries contain lignans.
The new study, published in the journal Cancer Causes and Control (Vol. 17, pp.169-180), claims to be the largest such study in the western world. The diet of 1499 volunteers with recently diagnosed prostate cancer was compared with the diets of 1130 healthy control volunteers using dietary questionnaires.
A smaller group containing 209 cases and 214 controls underwent blood tests to measure the amount of the phytoestrogen enterolactone.
"High intake of food items rich in phytoestrogens was associated with a decreased risk of prostate cancer. The odds ratio (OR) [the risk compared to a standard of 1.00] comparing the highest to the lowest quartile of intake was 0.74," wrote lead author Maria Hedelin from Department of Medical Epidemiology and Biostatistics, Karolinska Institutet.
The case-control blood test group also showed that those with increased blood levels of enterolactone had a 70 per cent lower risk of prostate cancer.
"Our results support the hypothesis that certain foods high in phytoestrogens are associated with a lower risk of prostate cancer," said Hedelin.
The researchers could not conclude if the protection from prostate cancer was due to the phytoestrogens alone or a combination with other substances. They also recommended against taking supplements with high concentrations of artificial phytoestrogens, stressing that no clinical trials had been performed and such high doses could have side effects.
An animal study published in Biology of Reproduction (2004, Vol. 70, pp. 1188-1195) claimed that the metabolite of the soy isoflavone daidzein stopped the effect of the male hormone dihydrotestosterone (DHT), which has been linked to prostate growth and male baldness.
More study if required to identify which phytoestrogens are active, and further work is needed to identify the mechanism of protection.
Over half a million news cases of prostate cancer are diagnosed every year world wide, and the cancer is the direct cause of over 200,000 deaths. More worryingly, the incidence of the disease is increasing with a rise of 1.7 per cent over 15 years.
The lowest incidences of the cancer are found in China, Japan and India, with experts linking this to a high dietary intake of soy products. A recent meta-analysis from the International Journal of Cancer (2005, Vol. 117, pp. 667 - 669) reported that men who regularly consumed soy-containing products had a 30 per cent lower risk of the cancer.