Previous studies have shown that a soy-rich diet can help prevent prostate cancer, but new research suggests that the benefits differ according to the stage of cancer development.

The largest study examining the relationship between the traditional soy-rich Japanese diet and development of prostate cancer has come to a seemingly contradictory conclusion: intake of isoflavone chemicals, derived largely from soy foods, decreased the risk of localised prostate cancer but increased the risk of advanced prostate cancer.

One possible explanation is that isoflavones may delay the progression of concealed prostate cancer only; but once tumours 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 localised prostate cancer may be different tumour subtypes, which may react differently to isoflavones.

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. Scientists have theorised that isoflavones stop latent cancers from developing further.

Every year in the UK, nearly 32,000 new cases of prostate cancer are diagnosed, and the disease is responsible for over 10,000 deaths, according to Cancer Research UK.

"The present findings provide no clear understanding of when or how localised cancer will develop to aggressive cancer, and of the related effect of isoflavones," said the study's lead author Dr Norie Kurahashi, of the Japanese 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 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 to exert a protective effect against prostate cancer.

They study 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 localised prostate cancer.

In the past, soy has been linked to lower cholesterol and fat levels (Read more: Magic Beans – Soya Prevents Obesity and Diabetes).

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