Fact Buster

Q: Do soy foods increase your risk of cancer?

A: Research linking soy to cancer is limited and has been done on animals. Experts recommend 1-2 servings of soy a day as part of a balanced diet.

Our expert: Associate Professor Tim Crowe & Dr Kellie Bilinski



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Soy has been a staple in Asian diets for thousands of years, while for vegetarians and people with lactose intolerance it's a good source of calcium and protein.

When you think of soy you probably think of tofu, soy milk and other dairy replacements, edamame, miso, soy sauce and tempeh. But according to the CSIRO, up to 60 per cent of foods in the supermarket contain soybean, including breads, flours, oils, tuna, coffee and sausages.

While you tend to find many soy products in the health food aisle, soy's role in a healthy diet has been questioned over recent years and a quick internet search will find plenty of warnings about the dangers of soy  especially after it was linked to an increased risk of breast cancer.

But research has also found soy may actually help prevent breast cancer, along with stomach and prostate cancers, and heart disease. And dietitans, the Cancer Council and our national dietary guidelines still recommend we eat moderate amounts as part of a healthy diet.

So what should we believe?

Breast cancer link

Soy contains large amounts of phyto-oestrogens (or isoflavones) called genestein and daidzein, which are plant chemicals that mimic the hormone oestrogen, although on a much weaker scale.

One risk factor for breast cancer is high levels of oestrogen and concerns were raised when research found these phyto-oestrogens increased the risk of the cancer in rats.

But the problem with animal studies, explains dietitan and breast cancer researcher Dr Kellie Bilinski, is that rats have very different metabolisms to humans.

"So you really can't extrapolate those study results to humans," she says. "Plus, these rats were given huge doses that we'd never actually consume."

No studies on humans have found that eating soy as part of a healthy diet can be harmful. In fact, research on Asian women has found phyto-oestrogens actually have a protective effect against breast cancer.

"They can help to protect the cells in the body against its own oestrogen," explains Tim Crowe, associate professor in nutrition at Deakin University.

"Studies have found the critical stage is during adolescence when a woman's breast is developing, which is why Asian women seem to have lower rates of breast cancer. But we don't have that same level of consumption in the West."

Cancer Council says although the science on soy being protective against cancer is still inconclusive, it encourages people to eat soy-based foods "as part of a varied and nutritious diet".

"If there's going to be an adverse effect of phyto-oestrogens on hormone-sensitive tumours, it'd be at high concentrations  a lot higher than you'd find in food," Cancer Council CEO Professor Ian Olver says.

"So if people enjoy soy, there's no evidence saying you can't eat it in moderation."

Warning on soy supplements

Some of the confusion about soy stems from concerns around isoflavone supplements, which may cause problems for women with a particular type of breast cancer called oestrogen receptor positive.

Women who have this type of breast cancer usually receive a treatment called tamoxifen. There's some evidence to suggest that in high doses, like those found in supplements, phyto-oestrogens may actually reduce the drug's effect and promote cancer growth.

Cancer Council advises men and women against taking these supplements for preventing cancer and for breast-cancer sufferers to have soy foods in moderation.

Olver says while it's technically possible to get supplement-type dosages from soy foods, it's highly unlikely because people would need to eat such massive amounts.

Nevertheless, soy is still a healthy food that doesn't need to be avoided by anyone, Bilinski says.

"It's absolutely fine to have a few serves of soy foods a day. These natural phyto-oestrogens are antioxidants and we should be eating more of them."

Other cancer links

Some studies have also suggested soy may lower the risk of prostate and stomach cancers, although Cancer Council says the research is limited.

Phyto-oestrogens have also been linked to an increased risk of endometrial cancer in perimenopausal or post-menopausal women, but Cancer Council states "the evidence overall is inadequate to draw conclusions".

Its position statement says any evidence linking soy to pharynx, oesophagus, pancreas, breast, bowel and endometrium cancer is limited with no conclusion possible.

"For most cancers there just simply isn't evidence one way or the other," Olver explains.

"It's a mistake to focus on one disease and ignore others such as cardiovascular disease and diabetes, where a product may be quite beneficial."

Health benefits of soy

If you like to make tofu stir-fry for dinner or enjoy eating Japanese food, you'll be happy to hear that one to two servings of natural soy foods per day have been shown to lower cholesterol.

Soy is also high in protein, fibre, iron and omega-3 fatty acids, and low in saturated fat.

"For vegetarians and people who don't eat a lot of red meat  and we encourage everyone to decrease their intake  it's such a good food," Bilinski says.

"And if you don't eat dairy, soy can be a really good source of calcium as well."

Crowe recommends including good-quality soy milk, tofu, miso, edamame and tempeh into your diet. Although it is worth noting some popular soy-based foods  such as miso and soy sauce  are very high in sodium and you shouldn't have too much of these.

"Soy foods don't just contain isoflavones, they also contain hundreds of other beneficial chemicals for our health," he says.

"And if you're eating a fair amount of these foods, then you're probably putting less chips and pies in your mouth  and that's going to be good for your health."

Associate Professor Tim Crowe is from Deakin University's School of Exercise and Nutrition Sciences; Dr Kellie Bilinksi is an accredited practicing dietitian and spokesperson for the Dietitians Association of Australia; Professor Ian Olver is the CEO of Cancer Council. They spoke to Cassie White.