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I didn’t really want to open this can of beans worms, but of all the nutritional topics this is the one that seems to generate the most heat and the most letters – and so I have, somewhat reluctantly, been persuaded to try to tell the soy story. I’d like to reassure my readers that I have no financial, emotional or other stake in soy or any of its alternatives, and will try to deal with the topic as even-handedly as I can, and as the science permits.

The debate about whether soy is good or bad for us has been going on for years, but the problem for consumers is that the academic debate between scientists can be difficult to understand. Additionally it is often drowned out by the shrill cries of the nutritional activists; with accusations and counter-accusations flying between self-proclaimed gurus on both sides of the Atlantic. At the scientific level, however, it is probably fair to say that there is now emerging a general (although not unanimous) agreement that soy is, on balance, a reasonably healthy food: not a cure-all, but not a life-threatening toxin either. It is, fundamentally, just another legume.

The health debate about soy really started when epidemiologists noted that in Asian countries where soy was regularly eaten, rates of certain cancers such cancers of the breast and prostate were less than a third of Western rates. Other scientists established that animals fed on soy-rich diets exhibited reduced rates of cancer (ie Hawrylewicz et al ’95) It was then discovered that soy contained a number of compounds that displayed anti-cancer properties in vitro, and in a variety of animal models. The most important of these compounds might be the isoflavones, although other compounds such as the glyceollins and the peptides Lunasin and BBPI also have significant anti-cancer properties.

The isoflavones are not only capable of killing cancer cells, but are also phyto-estrogens; that is, they have low estrogenic activity. In this they are far from unique, as many plant foods contain compounds with similar effects. Then, however, it was suggested that consumption of isoflavones could interfere with the therapeutic effects of estrogen-blocking drugs such as Tamoxifen, which are used to slow the growth of estrogen-sensitive cancers such as breast cancer.

The debate became more heated when environmentalists pointed out that intensive soy production (for animal feed and human food) was starting to cause environmental damage, and anti-soy activists began to claim that contrary to former opinion, soybeans as such were not and never had been a food staple. In Asia, they said, soybeans were always fermented or highly processed before they were eaten. They concluded that there must have been a reason for this, and that unfermented or incorrectly processed soybeans contained undesirable or toxic factors, to which Western consumers – and most alarmingly infants – were now being exposed. A couple of studies indicated that soy consumption could reduce fertility, and might even interfere with sexual maturation, and then the calls for action really started.

But let’s go back to basics. Are soy beans a food staple or not? Are they really so toxic that our ancestors somehow discovered they could safely only be eaten after fermentation? Or is this just another urban myth? To answer these questions I asked a number of Japanese and Chinese colleagues about traditional soy dishes and how they were prepared; and it soon became clear that contrary to reports, unfermented soybean dishes have a long and extensive history.

Fermented soy foods include miso, soy sauce and similar products – but these are only consumed in very small quantities, since they are quite salty. The main fermented soy food is natto, which consists of boiled and fermented soybeans. The short fermentation lasts 2 to 3 days, and produces enough vitamin K2 to confer significant protection against osteoporosis (ie Kaneki et al ‘01).

Natto is a regional dish, unlike tofu, which is eaten in far larger amounts. Tofu is not fermented and is only very lightly processed. Western accounts which talk about ‘the long slow boiling of soybeans to remove toxins’, reveal a profound ignorance of cooking methods; if soy or any other beans are overcooked they become starchy, making it impossible to express soy milk and therefore to make tofu.

Traditional methods of making tofu involve soaking the beans, grinding them, and then boiling the pulp for no more than 15 to 20 minutes to produce soymilk. A coagulant (nigari) is added, and the curds are pressed to make tofu and foods such as aburaage, atsuage, yuba and ganmodoki.. The whey is not thrown away because ‘it contains anti-nutrients’, but because nigari has a bitter taste. Traditionally, however, many Japanese households saved the whey for making the next batch of tofu.

Fresh young soybeans are widely consumed as edamame, a delicious dish which requires little more than a light steaming; and the whole dried beans are very frequently cooked, often together with hijiki (a type of seaweed), or mixed with steamed rice (daizu gohan).

Unfermented soybeans are, therefore, a significant part of the diet in a number of countries with a low cancer incidence. So will eating soybeans protect us from cancer? I doubt it. Cancer is so complex, and so multi-factorial, that it seems intrinsically unlikely that the addition of a single food will make a difference. Which would explain why the science (and there were at least 90 papers on soy and cancer published in 2007 alone) is so equivocal ….

During the last 12 months, a number of new studies produced positive results for soy.

One American group published findings suggesting that a high dietary intake of soy protected against breast cancer in postmenopausal monkeys (Wood et al ’06, ‘07); and an Ulster-based group showed that a soy metabolite produced in the gut reduces the invasive ability of cancer cells (Magee et al ’06).

Two teams at the Universities of Montreal and Myazaki found that soy isoflavones had antleukaemic properties (Raynaul et al ’07, Yamasaki et al ‘07). A fifth group at the University of Minnesota showed that when men consumed soy products, various biomarkers changed in a way predicted to reduce the risk of prostate cancer (Hamilton-Reeves et al ‘07), and a sixth team of researchers at the Karamanos Institute in Chicago (who I know and have a high regard for) showed that both soy isoflavones and lycopene have therapeutic activity in prostate cancer patients with PSA relapse disease and may delay the progression of both hormone-refractory and hormone-sensitive prostate cancer (Vaishampayan et al ‘07).

The clinical picture overall, however, was less clear-cut. On the positive side, one study carried out in Shanghai (Lampe et al ’07) found that higher plasma levels of soy isoflavones (corresponding to higher soy intakes) were associated with a reduced risk of both breast cancer, and benign breast cysts in women. A second Shanghai study came to a broadly similar conclusion (Cui et al ’07), as did a Korean analysis (Do et al ’07). Set against these studies (which were all retrospective) was a fourth retrospective study (Nishio et al ’07) which found that soy consumption had no effects in Japanese women; and a large prospective trial (Travis et al ’08) which found that increased soy consumption did not alter the risk of breast cancer in British women. And on the minus side, a Japanese study found that feeding commercial soymilk to rats enhanced the growth of chemically-induced mammary tumours, although the increase was only just statistically significant (Qin et al ‘07).

To put the above pieces of information into perspective, a large meta-analysis (Trock et al ’06) found that soy consumption might confer a possible slight protective effect against breast cancer, but the authors concluded: ‘This result should be interpreted with caution due to potential exposure misclassification, confounding, and lack of a dose response. Given these caveats and results of some experimental studies that suggest adverse effects from soy constituents, recommendations for high-dose isoflavone supplementation to prevent breast cancer or prevent its recurrence are premature.’

So what are the possible adverse effects? Well, as stated above, there is some evidence that isoflavones interfere with the actions of estrogen-blocking drugs such as Tamoxifen; but here again, the picture is not clear. In one experiment (Tonetti et al ‘07), combining soy isoflavones with Tamoxifen reduced the tumour-suppressing effects of the drug; and in another (Gallo et al ’07), high doses of soy reduced the effects of another estrogen-blocking drug. However, in two studies carried out at Harvard Medical School (Mai et al ’07a, Mai et al ‘07b), soy isoflavones enhanced the anti-tumour effects of estrogen-blockers.

So is it time to rejoice or to panic? My answer would be, neither. If there were a strong effect of soy on either reducing or enhancing cancer, it would be relatively easy to identify, and would have been documented by now. The fact that soy-rich diets consistently reduce the risk of cancers in animals fed on lab chow is fascinating, but its relevance to the less-controlled human situation, where soy is not consumed in isolation but as just one element in a generally diverse and dynamic diet, which contains a large number of other compounds that either protect against or cause cancer, is tenuous. It seems likely that other dietary factors (Power & Thompson ‘07) and genetic factors (ie Low et al ’06, Dai et al ’07) can both interact to determine how soy acts in the body, but that way leads to individual dietary programming (and consumer madness).

Hearts & Bones

There are other health issues where the impact of soy is slightly easier to make out.

In terms of cardio-protection, the ability of soy foods to lower LDL cholesterol has been charted by many (but not all) studies, and is sufficiently well recognised to have received FDA-approval as a health claim. For the sake of good science, however, I would have to add here that cholesterol reduction is not a very logical or effective way to reduce the risk of cardiovascular disease! In terms of bone protection the case is approximately as strong, with positive results in animal studies (ie Power et al ’07) and clinical trials (Marini et al ’07, Alekel et al 2000) showing modest protection against bone loss in post- and peri-menopausal women. A pretty decent meta-analysis (Ma et al ’07) came up with similar findings, although here again there are individual studies which found, in contrast, that soy extracts had no therapeutic effect (ie Evans et al ’07).

As with the cancer story, the truth is too complex to be uncovered by the typically narrow clinical investigations cited above. Bone economics (ie bone resorption and regeneration) are affected by multiple micro- and phyto-nutrients, and the idea that soy alone can have any consistent or dramatic effect is a pharmaceutical concept, and an essentially ludicrous one. The effects of soy on bone are determined by the presence or absence of a large number of other dietary factors such as flavonoids, beta glucans and vitamins D and K, and as these differ wildly in different consumers and in different parts of the world, it is no wonder that scientists have produced so many conflicting findings.

Nonetheless, the overall body of evidence indicates that soy consumption probably confers some protection against both heart disease and osteoporosis, in the majority of cases.

Fertility

There is only one area, in my view, where the effects of soy are almost unequivocal; and that is in the area of fertility. In 2006, a group of scientists at Kings College in London published a report showing that human sperm lose their ability to fertilise ova when exposed to soy isoflavones at levels similar to those found in the blood after consuming soy products (Fraser et al ’06). Soy is not the only culprit; other estrogenic compounds in our environment with similar sperm-disabling effects include 8-prenylnaringenin (a natural compound which occurs in hops and in beer), and nonylphenol, a synthetic compound found in paints, pesticides and cleaning products. This excellent paper strongly suggests that soy isoflavones can reduce human fertility; especially if other environmental estrogens are present.

Professor Fraser’s findings have been misunderstood by some writers (including at least one soy industry representative) who have said that high dose isoflavone supplements are exclusively consumed by women, and the impact of the lower doses of isoflavones ingested by men who consume soy milk, burgers or bread is less likely to be meaningful. But this misses the point; once sperm enter the female reproductive tract they are then exposed to whatever environmental estrogens the woman might have ingested. If she is taking isoflavone supplements, drinking large amounts of beer or using paint and cleaning products (or all three), then the chances of a successful conception will almost certainly be reduced.

Women might also, conceivably, be affected in other ways. A paper last year showed that female mice injected with genistein during the first 5 days after birth subsequently had abnormal development of their ovaries and sexual function in general (Jefferson et al ‘07). This study made the headlines, for obvious reasons – but is of doubtful human significance.

The sperm story, however, looks strong and is causing concern in some circles, given that infertility is already so prevalent. One in seven couples currently has trouble conceiving naturally, and the figure is set to increase to around one in three by 2015 due to the twin pandemics of obesity and sexually transmitted diseases. Theoretically, if large numbers of men and women were consuming soy products containing significant amounts of isoflavones, the infertility figures could become even more marked.

But is this really such a bad thing? Call me an old reactionary, but it seems to me that we are already over-populated. All around us we can see the environmental damage caused by excessive consumption and excessive waste created by our huge global human population; significant reductions in human fertility at this stage of our cultural development would be no bad thing, and a good deal less traumatic than other forms of population reduction. So on balance I am in favour of soy, although – given the high levels of fertility in those parts of the world where people do eat soy – I am not convinced that soy alone will solve our problems. A combination of soy, beer and cleaning products might, conceivably, do the trick!

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