Soya and breast cancer
by Dr Justine Butler Senior Health Campaigner
Justine is a Senior Health Campaigner at the Vegetarian & Vegan Foundation. Published in national, regional and health journals she has given numerous presentations to the public, health support groups, patients and GPs. Justine holds a PhD in Molecular Biology from the University of Bristol.
Studies into the association of soya intake and breast cancer
The association between soya in- take and breast cancer risk is both complex and controversial. The crucial question is do soya foods affect the risk for breast cancer? Studies have yielded mixed find- ings and the result is confusion. Re- cent developments in the research should now offer reassurance to both practitioners and breast can- cer patients.
One in nine women in England de-
velops breast cancer at some point in their lives (14). However, breast cancer incidence rates in Asian countries are substantially lower. This does not reflect genetic differences because when people migrate from an area of low in- cidence to one of high incidence, they soon acquire the same incidence rate shared by the population into which they move (22,19). Their increased risk of breast cancer must then be attributed, at least in part, to environmental factors such as diet and lifestyle.
Isoflavones
The low incidence of breast cancer in Asian countries has led to a search for protective factors in the Asian diet. In recent years, attention has focused on soya protein which has been shown to have anti-cancer properties and im- prove cardiovascular and bone health (17). The soya isoflavones (genistein and daidzein) are a major group of phytoe- strogens that have attracted much inter- est. Isoflavones bind to oestrogen recep- tors and have the potential to produce both oestrogenic and anti-oestrogenic effects (3). However, they are much less potent (1). Some isoflavones are estimat- ed to be between 100 and 100,000 times weaker than oestrogens (11). Because isoflavones compete with
oestrogens for the binding sites of oestro- gen receptors (17), some scientists sug- gest that they may have a normalising effect on oestrogen levels (8). The theory is that they may partially block the strong oestrogen signal in those with high levels of oestrogen, for example women taking the contraceptive pill or HRT and they
12
may boost the weak oestrogen signal in people with low levels of hormone, such as postmenopausal women.
Breast cancer concerns
Despite the fact that isoflavones are much weaker than oestrogens, their potential to stimulate cell proliferation has raised concerns (17). The concern is that isoflavones may stimulate the growth of existing oestrogen-sensitive breast tumours. These potential oestro- genic effects have raised controversy about soya consumption, particularly in the case of postmenopausal women at high risk of breast cancer (12). However, the research supporting this theory is inconsistent and based mainly on in vitro and rodent experi- ments. These studies have produced mixed results and their relevance to hu- man breast cancer patients is unclear. To date there have only been two hu- man studies on this subject, the findings of which were also unclear (15, 4).
Reassurance
In contrast, a large body of evidence links soya intake to a reduced risk of breast cancer. One of the first studies to report this was the Shanghai Breast Cancer Study, the largest population- based study of lifestyle and breast cancer (16). This ongoing study, funded since 1996, has investigated genetic and lifestyle factors for breast cancer
risk and survival. Initially the study in- cluded over 1,400 breast cancer cases in women aged 25 to 64 between 1996 and 1998 in Shanghai and found that those who consumed the most soya as teenagers had half the risk of breast cancer as adults. This study suggested that the early consumption of soya foods could lower breast cancer risk later in life.
A year later, over 500 Asian-Ameri- can women with breast cancer were asked how often they ate soya foods (tofu, soya milk and miso). Results showed that those who consumed soya at least once a week during adoles- cence also had a significantly reduced risk of breast cancer (20). More recently, a Korean study found that women eat- ing at least one serving of tofu per day had a 74 percent lower risk. This effect was more pronounced in premeno- pausal women (5). In 2009, further analysis from the Shanghai study found strong evidence of a protective effect of soya intake among premenopausal (but not postmenopausal) women. Those consuming the most soya protein had a 59 percent lower risk of breast cancer (9). More recently, a study of more than 1,500 Asian American women found that eating soya foods during child- hood reduced the risk of breast cancer by 60 percent. The greatest protective effect was seen in those eating soya six
NHDmag.com June '10 - issue 55
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