Soya beans are being touted in the press as a new miracle food for women, and maybe for men too. New research from Britain and the US, as well as Japan, provides strong evidence that women can substantially reduce the risk of breast cancer by consuming soya bean products, such as soya milk or tofu, the Japanese bean curd.
In the most convincing study to date, a team of researchers in Cambridge has shown that a diet high in soya protein can alter women’s hormones in a way that mimics the effects of the anticancer drug tamoxifen. Sheila Bingham, a senior nutritionist at the Medical Research Council’s Dunn Clinical Nutrition Centre in Cambridge and one of the team’s leaders, says: ‘It could be a viable alternative.’
The research is just the latest piece of evidence to point to the beneficial effects of a diet that includes soya beans, or ‘soybeans’ as they are often called in America. ‘Soybeans are a potentially important link between diet and cancer risk,’ says Stephen Barnes of the University of Alabama. Barnes and his colleagues showed four years ago that female rats fed soya bean products and dosed with cancer-causing chemicals were much less likely to develop breast cancer than those eating proprietary rat food.
The researchers even demonstrated a dose-related response: the more soya protein the rats ate, the fewer tumours they developed. Barnes’s team went on to show last year that soya extracts could also inhibit the growth of prostate cancer cells in laboratory tissue cultures.
And three years ago, a study carried out by Nick Day, now director of the Institute of Public Health in Cambridge, and his colleagues, revealed that premenopausal women from China and Singapore who had developed breast cancer tended to eat less soya protein than their compatriots who remained healthy. Men as well as women may benefit from a diet high in soya. Epidemiological surveys backed by laboratory studies suggest that eating soya products can lower the risk of both prostate and colon cancers.
Long maligned in gastronomic circles for its bland flavour, this unassuming tropical legume is rich in vegetable hormones known as ‘phytoestrogens’, so-called because they behave like weak oestrogens. Because of a quirk of evolution, these plant chemicals are close enough to the natural hormone’s shape to bind to the oestrogen receptors on cells in the human body.
However, these plant oestrogens do not set off the entire biochemical cascade that normally follows the binding of oestrogen proper. As a result, they can, paradoxically, act as ‘anti-oestrogens’, effectively diluting the impact of the body’s own production of the hormone.
‘We don’t really know yet what effect plant oestrogens are having on humans,’ cautions John McLachlan, director of the National Institute of Environmental Health Sciences in North Carolina. ‘But the signs are growing that they might actually be good for you because they protect you from exposure to too much oestrogen.’
Oestrogen, manufactured by both ovaries and testicles, albeit in differing amounts, is potentially dangerous because it is a potent stimulator of cell growth in certain tissues – notably the breast, ovary, uterus and prostate. The more often cells divide, the greater the risk that a genetic mishap will occur that could lead to the uncontrolled proliferation of malignant disease.
A major risk factor for breast cancer is thought to be the amount of oestrogen a woman is exposed to during her lifetime, and hence the number of menstrual cycles she experiences. Anything that reduces the number of cycles – such as late start to menstruation, early menopause or pregnancy – lowers the risk of breast cancer.
Japanese women have a longer menstrual cycle – about 32 days on average, compared to the 26 to 29 days that is typical in Britain – and a quarter of the risk of breast cancer. Vegetarians also tend to have longer or more irregular cycles, and a lower risk of breast cancer. Could diet be contributing to these striking differences?
The most telling link between plant oestrogens and breast cancer to date comes from the work carried out at the MRC in Cambridge by Bingham and Aedin Cassidy. Collaborating with Ken Setchell of the Children’s Hospital Medical Center at the University of Cincinnati in the US, the Cambridge researchers produced the first evidence that eating a diet high in soya produces physiological effects in healthy humans.
They studied six women in their twenties who spent several months living in the ‘metabolic suite’ of the Dunn unit where their diet and metabolism could be closely monitored. Part of the time, the women ate a normal Western diet low in plant oestrogens. Then they were given specially prepared meals that provided 60 grams per day of additional soya protein. As a result, their menstrual cycles lengthened on average by two and a half days. But when the women reverted to their normal diet, so did the length of their cycles.
Cassidy and Bingham found that the follicular phase of the menstrual cycle, which precedes ovulation at mid-cycle, lasts longer on a soya-rich diet, as a result of the suppression of the mid-cycle surges of two pituitary hormones, the gonadotrophins LH and FSH. This effect may be beneficial because it is the latter part of the cycle – the time between ovulation and the onset of menstruation known as the luteal phase – that puts the breast at most risk. During the luteal phase, levels of both oestrogen and progesterone are high, and this combination stimulates breast cells to proliferate four times as fast as they do during the follicular phase.
A diet that lengthens just the follicular phase could significantly reduce the total number of luteal phases a woman’s breasts are exposed to over a lifetime. As a result, the researchers conclude that eating soya protein daily can ‘lead to significant biological effects in premenopausal Western women which are potentially beneficial with regard to breast cancer risk’.
The active component of soya beans seems to be plant chemicals known as isoflavones. The Cambridge volunteers ate 45 milligrams of isoflavones a day – about a third or a quarter of the typical intake of the Japanese – and it seems likely that the more you eat, the greater the protection.
One of the most exciting suggestions to result from this research is that plant oestrogens may turn out to be an effective, and perhaps safer, alternative to tamoxifen – the controversial anti-oestrogen drug now on trial in Britain as a prophylactic for women who have a high risk of breast cancer (This Week, 24 July 1993). Tamoxifen has been linked to an increased risk of endometrial and liver cancers. ‘Soy protein could potentially be as effective as tamoxifen in preventing breast cancer,’ says Setchell.
The Cambridge research also showed that there were similarities in the biological effects of soya protein containing isoflavones and the potent synthetic anti-oestrogen, tamoxifen. The researchers suggest that it is time to consider soya protein as ‘an alternative approach to drug therapy for breast cancer prevention’.
Soya beans could also help to cut the number of heart attacks. Both tamoxifen and soya protein reduce levels of total serum cholesterol and the undesirable LDL cholesterol, which is closely linked to the risk of a heart attack. Cholesterol levels of the volunteers in the Cambridge study fell on average by 9.6 per cent – which the researchers say is ‘impressive given the relatively short duration of soy protein intake’.
What is now urgently needed, say researchers, are precise studies linking food, hormone levels and cancer outcomes, such as the EPIC project – the continuing European collaboration that will link diet to the health of 400 000 Europeans over a decade or more (‘Britain’s deadly diet’, New Scientist, 11 May 1991). ‘We intend to look at the cases of breast cancer and look for differences in the consumption of plant oestrogens or other compounds in food,’ says Bingham.
Only by collecting such detailed data can nutritionists hope to tease out the subtle influences of dietary components. It may even matter what sort of soya product you eat, for instance. Setchell and his colleagues suggest that fermented soya products – such as miso or tempeh – may be more potent than non- fermented foods such as tofu or soya milk. ‘There are lots of tentative suggestions, but it’s an open field at the moment,’ says Bingham. ‘We can’t make any firm recommendations.’
Nonetheless, it looks as though nutritionists will soon explain a longstanding epidemiological puzzle. Compared to Americans or Britons, remarkably few Japanese people develop cancer of the breast, colon or prostate, or suffer heart attacks. Yet if they move to the US or Britain and eat a Western diet, their risk of dying from these diseases rises dramatically to match that of their adopted country. One key ingredient in the equation now seems to be the humble soya bean.
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When Ken Setchell and his colleagues at the University of Cincinnati first found an unknown oestrogen in human urine, just over a decade ago, they thought they had found a new human hormone. What they had found, it transpired, was really a plant chemical which had been modified by bacteria in the human gut.
To date, some 15 oestrogen-like chemicals derived from plants have been discovered in human urine. Most of them fall into two broad families, the lignans and the isoflavones. Most high-fibre foods, including whole wheat, oats, barley, rye, sesame and flax seeds, are particularly rich in lignans, while isoflavones are found in high concentrations in tropical legumes, notably soya beans.
Isoflavones seem to have the strongest oestrogenic effect, and are converted by bacterial fermentation into a metabolite known as equol (so named because it was first discovered in mare’s urine, in 1932). Equol is a reliable indicator of isoflavone consumption: as a rule, the more plant oestrogen in the diet, the greater the amount of equol in the urine. Setchell reports that an average person from the West may excrete from zero to 300 nanomoles per day, while a vegetarian churns out 20 000 nanomoles.