Soy troublesome?
There are contradictory opinions on soy and cancer risk, with some saying too much increases the risk of breast cancer while others say it may reduce the risk.
KUALA LUMPUR: Some say that eating soy can increase the risk of breast cancer, and a few women have been advised by their family members or friends to avoid consuming soy or too much of it.
Doctors and researchers say that there is no concern about consuming soy in relation to breast cancer, and in fact, it could reduce the risk of breast cancer.
Meanwhile, a toxicologist says consuming soy in reasonable amounts should not pose any problem, but cautions against any plant chemicals consumed in excess, such as extracts, on a consistent basis.
Doctors’ opinions
Oestrogen has been linked to the development of breast and other cancers, hence the concern that soy (which has oestrogen-like compounds) would react the same way in the body as oestrogen and cause breast cancer.
Animal feed, which contains a significant amount of soy, was also thought to be a hidden risk for women who consumed the meat.
However, National Cancer Society president and medical director Dr Saunthari Somasundaram said based on current scientific evidence, there is no evidence that suggests consuming any or too much soy bean causes breast cancer, whether in women who have never had breast cancer or in women who have been previously treated with breast cancer.
“That means it does not increase your breast cancer risk if you have never had cancer, or if you are a cancer patient, it does not cause your cancer to recur,” she said.
Dr Saunthari also said that currently, there is also no scientific evidence linking dietary soy to any cancers.
In fact, Asian women have significantly less incidence of breast cancer compared to Caucasian women even though the former consumed more soy, she said.
She said that most research into soy have shown soy being “protective” and improving cancer outcomes, but added that more research needs to be done for definitive conclusions.
The controversy on soy revolved around studies which showed that the isoflavones in soy led to increased cancers in animal and in-vitro studies, she said.
Isoflavones are a group of phytoestrogens that include genistein, daidzein and glycitein.
“The belief was these phytoestrogens could bind with oestrogen receptors in hormone sensitive cancers such as breast cancers, increasing the risk for the cancer occuring and/or affecting treatments, causing a recurrence of previous cancers.
“It has been shown in large epidemiological studies that these allegations are untrue.
“In fact, there is more positive evidence suggesting that dietary soy can actually reduce the risk of breast cancer in women at high risk and may also benefit women who have a diagnosis of breast cancer,” she said.
Dr Saunthari said other compounds in soy thought to be “cancer fighting” include saponins as the compounds may lower blood cholesterol, protect against cancer and affect blood glucose levels,
Consultant breast surgeon Prof Datuk Dr Yip Cheng Har said that some research findings showed that consuming excessive soy does lead to breast cancer while other findings showed otherwise.
“It is still controversial. It is currently not conclusive,” she said.
A researcher’s opinion
Cancer Research Malaysia (CRM) chief executive Prof Dr Teo Soo Hwang, who carries out breast cancer research, pointed out that breast cancer is not caused by just one factor, but multiple factors.
Most studies comparing thousands of breast cancer patients with healthy women have found that women who consume more soy are less likely to develop breast cancer, she said.
New evidence suggests that dietary soy intake is not likely to be harmful even for breast cancer patients, and may improve breast cancer survival, she said.
Dr Teo said that soy contains isoflavones, which are similar in structure to the female hormone oestrogen, and it was thought that the biologically active compounds may mimic the activity of oestrogen and lead to a higher risk of breast cancer.
Some studies in animals have also suggested that mice that consumed more soy had more cell growth, but the latest scientific research show that isoflavones have much weaker oestrogenic activity than naturally occurring oestrogen, she said.
“Overall, the evidence in humans suggests that soy, in the type and amount typically consumed in Asian diets, is not likely to cause breast cancer. In fact, it could potentially prevent the disease,” she said.
Dr Teo, who has a PhD in biochemistry and vast experience in breast cancer genetics, will be embarking on a research on the impact of soy on breast cancer risk with co-investigators University of Nottingham Malaysian campus lecturer and researcher in statistics Dr Ho Weang Kee, and CRM research associate and PhD student Nadia Rajaram.
On the amount of soy that one should consume, Dr Teo said the United States Food and Drug Administration recommends a diet of up to 25g of soy protein per day to maintain a healthy heart.
“This corresponds roughly to three glasses of soybean milk or two pieces of tofu.
“This amount of soy intake is also recognised as the safe upper limit for daily soy consumption by the Japanese Food Safety Commission in 2006,” she said.
She also cautioned against consuming too much soy as it may lead to bloating and digestive issues, weight gain, and in some women, high uric acid build up, leading to joint pains and gout.
“As with anything that we consume, mod- eration is key,” she said.
On whether removing soy skin makes any difference, Dr Teo said currently, there is no evidence to suggest its impact on health.
The main reason for hulling is to reduce the amount of fibre in the end product, and while the skin contains high levels of phytic acid, which could interfere with the absorption of other nutrients, the impact of this on breast cancer or other health outcomes are unknown, she said.
Despite arguments for soy’s protective effect, Dr Teo said that randomised controlled trials (the gold standard of causal evidence) have not been able to replicate these findings.
Dr Teo said that to answer the remaining questions on the relationship between soy and breast cancer risk, CRM is currently developing a randomised clinical trial to measure the effect of traditionally consumed soy products on biomarkers for breast cancer risk.
“We aim to recruit healthy women aged between 45 and 75 to participate in this yearlong study,” she said.
A toxicologist’s opinion
Universiti Malaya Medical Centre toxicologist Prof Dr Mustafa Ali Mohd warned against consuming too much plant extracts on a consistent basis as chemical compounds in plants can have health ramifications, if taken too much.
He said soy, like any plant extracts, if consumed consistently, could affect the body.
Dr Mustafa, who is from the department of pathology, said that based on some studies, researchers had found that in some women who had taken soy excessively, it could lead to breast cancer.
Soy bean contains genistein and diadzein and will bind to oestrogen receptors and prevent the “real” oestrogen from exerting its effects, and when the body thinks that there is no oestrogen in the body, the body will keep producing oestrogen in excess, he said.
“If this happens in a prolonged and persistent manner, the patient may get breast cancer because of excessive oestrogen,” he said.
However, Dr Mustafa said that if people consume soy products once or twice a week, it should not pose a problem.
“It would be a concern if they take it persistently as extracts, which have a high concentration, a few times daily,” he said.
Asked what the limit is, he said that no one knows yet the threshold.
The two compounds are also found in most legumes, and people should also not take too much legumes, although there are benefits, he noted.