January 14, 2007 12:00

CANCER patients are being warned to avoid foods rich in soy because they can accelerate the growth of tumours.

The Cancer Council NSW will issue guidelines today, warning about the dangers of high-soy diets and soy supplements for cancer patients and those people in remission from cancer.

At particular risk are people suffering from hormone-dependent cancers, including breast and prostate cancer - the two most common types of cancer in Australia.

Cancer survivors are also being urged to avoid high doses of soy, as they may be more vulnerable to a relapse.

Research has found high consumption of soy products can also limit the effectiveness of conventional medicines used to treat the disease.

"There is evidence to suggest that women with existing breast cancer or past breast cancer should be cautious in consuming large quantities of soy foods or phyto-oestrogen supplements,'' a position statement from the Cancer Council says.

"Women with current or past breast cancer should be aware of the risks of potential tumour growth when taking soy products.

"The Cancer Council does not support the use of health claims on food labels that suggest soy foods or phyto-oestrogens protect against the development of cancer.''

Health experts are particularly concerned that breast-cancer sufferers who take soy or phyto-oestrogen supplements could feed the disease and reduce the effectiveness of their treatment.

Soy, which is present in soy beans, soy milk, tofu, tempeh and some breads, contains phyto-oestrogens that mimic the actions of hormones in the body.

This means it may interfere with cancer drugs such as Tamoxifen, which works by suppressing the female hormone oestrogen.

Men with prostate cancer are also being warned against high soy consumption, as phyto-oestrogens may imitate the male hormone androgen.

Although the Cancer Council has warned against soy supplements, it believes an occasional intake of soy food is still safe for cancer patients.

Cancer Council nutritionist Kathy Chapman said soy supplements could contain dangerously high doses of phyto-oestrogens.

"If you were a woman with breast cancer and thought, 'I'm going to radically change my diet and have very large portions of soy at every meal', it could be a problem,'' Ms Chapman said.

"For someone who has tofu once or twice a week and drinks a bit of soya milk, it's not so much of a problem.''

Soy has earned a reputation as a natural "superfood'' that cuts the risk of breast or prostate cancer, and is commonly included in women's health supplements.

This claim was based on findings that cancer rates were lower in Asia, where soy consumption is high.

But soy would lower the risk of contracting cancer "only a little'', according to the Cancer Council.

"While they may have a protective effect, there is also some evidence that phyto-oestrogens may stimulate the growth of existing hormone-dependent cancers,'' the council's statement said.

The risk of contracting other non-hormone-dependent cancers, including bowel cancer, would be unaffected by soy intake.

The Cancer Council was prompted to investigate the issue after being inundated with questions about the role of soy in cancer patients' diets.

"We felt we were getting a lot of calls on our hotline about this topic,'' Ms Chapman said.

Breast-cancer survivor Susie Musarra was surprised by the new evidence about soy.

The Sydney mother of two was diagnosed five years ago. She followed a healthy diet, containing plenty of fruit and vegetable juices, during chemotherapy treatment.

"It's really confusing, because you get a lot of conflicting information about what to eat,'' she said. "It's good to have this advice, because it helps you make an informed decision - and the Cancer Council is a reputable source.''