Does Tofu Contain Female Hormones? (asked by Anonymous)
No. But they do contain some compounds that sometimes mimic female hormones such as oestrogen.
Soy, along with a few other components of our diet such as peanuts, lentils and other legumes, contains isoflavones. Isoplavones are polyphenolic compounds that are similar enough in their structure that they can mimic oestrogen, a female reproductive hormone. They are therefore classified as phytoestrogens. However, phytoestrogens are about one hundredth as powerful as real (human) oestrogens and appear in our diets in very small quantities – eating soy products is NOT the same as hormone therapies. You are not eating female hormones.
Soy is hugely popular worldwide. It has traditionally formed a major part of many Asian diets, and it’s popularity in the West has risen dramatically in the last 20 years; in 2008, soy food sales in the US alone topped $4 billion. Soy has gained popular appeal because of it’s many (supposed) health benefits, from lowering cholesterol, improving cognitive function, fertility and immunity, to reducing the risk of certain types of cancer and cardiovascular disease. But while a host of websites and other resources make bold claims about the health benefits of soy-based food, an equal number counter that soy is in fact very harmful to health, principally because of the “oestrogen-mimicking” compounds it contains.
The thing is, phytoestrogens are known as ‘oestrogen-mimicking’, but they don’t always mimic oestrogen. Sometimes, they do quite the opposite, blocking the action of naturally produced oestrogen. Phytoestrogens bind to some of the same receptors as oestrogens – sometimes they stimulate the receptor causing downstream hormonal changes and potentially stimulating the development and maintenance of female characteristics, other times they simply fill the receptor up and block natural oestrogen from binding. Isoflavones also have the ability to alter expression levels of sex hormone-binding globulin (SHBG) and influence the absorption and transport of sex hormones in the bloodstream.
Ok, so soy contains isoflavones which can interact with the production, transport and binding of sex hormones. But do we actually absorb isoflavones from soy when we eat it, and how do those isoflavones behave inside a real human body? Studies have shown that processing soybeans into soy protein as it is consumed in food (e.g. tofu) takes a lot of the isoflavones out – between 50% and 70% of isoflavones are lost, depending upon the exact type of processing. Tofu contains relatively high levels of isoflavones, although there is some variability between brands, while soy drinks are lower in isoflavaones.
Evidence suggests these remaining isoflavones do get into our blood stream and have a small, but measurable, short-term effect on physiology. Some forms of isoflavones are absorbed more efficiently by the digestive system, and can vary due to diet (consuming large amounts of fibre can reduce the absorption of isoflavones) and a person’s gut bacteria. Tofu consumption has been shown to slightly increase (9%) SHBG leves and reduce testosterone levels in men, and reduce levels of ovarian steroids and adrenal androgens in women, suggesting that isoflavones absorbed in the diet can indeed have an impact on our hormone levels. However, as is to become a running theme in this article, other studies replicating these experiments failed to find an effect of isoflavones on hormone levels in either sex.
Does Tofu Make You More Feminine?
Once inside the body, do isoflavones enact a feminising effect? The evidence says no. A 2010 meta-analsis reviewed placebo-controlled studies of soy foods and isoflavone supplements and found no measurable effect on testosterone concentrations in men. Isoflavone supplements did not influence sperm concentration, count or motility, or testicular or ejaculate volume. Basically, the men who ate soy were just as manly (read: fertile) as the men who didn’t, however you choose to measure it. So where did all the concerns come from? Well, early studies in rodents and cultured cells suggested there might be something to worry about. In mice, the isoflavone genistein has been shown to kill testicular cells, and studies in female rats suggested isoflavones might also reduce fertility. Further, early studies showed that developing mouse egg cells treated with isoflavones were more prone to miscarriage and reduced birth weights. Research in humans, however, has largely failed to replicate these findings. Because humans are not mice.
Ok, so isoflavones don’t cause problems for fertility in humans, but there are a few other health claims we need to address. Isoflavones have also been implicated as causing problems for child development, both pre- and post-natally. Furthermore, there is some limited evidence that soy consumption might be linked to cognitive deterioration with age. Animal studies have shown that consuming isoflavones can trigger premature puberty in juvenille mice and disrupt development in foetuses. However, very little is known about the impacts of isoflavones on human child development. Some scientists warn that isoflavone consumption might permanently influence childhood brain development and adult fertility. Although there is no evidence for harmful effects of soy or isoflavones on children, the AAP Committee on Nutrition’s official advice states that since there are also no known benefits to choosing a soy protein-based milk formula for infants, it is safer to stick with a more traditional cow’s milk formula.
Cancer – Culprit or Cure?
What we do know about isoflavones is that they seem to play a role in cancer risk, particularly for breast, endometrial and prostate cancers. These “hormone-sensitive” cancers occur in cells that have oestrogen receptors, which may explain why phytoestrogens can influence these cancers. Early research caused concerns when, again, it was found that rats and mice showed increased risk of cancer and faster tumour growth. However, subsequent research in humans has failed to consistently replicate these findings, and a large body of research now suggests isoflavones may in fact be beneficial in cancer treatment and prevention.
Exposure to oestrogen can increase the risk of breast cancer by stimulating oestrogen receptors in the breast tissue, however, isoflavones do not appear to share this effect. Observational studies comparing large populations of healthy women have failed to find any negative effects of soy on breast cancer rates, and in fact several studies indicate that soy diets may in fact protect against breast cancer. One of the key comparisons is between Asian and Western diets – Asian women have a far lower incidence of breast cancer, and studies have shown breast cancer risk in these women is negatively correlated with soy consumption. Similar trends have not been found in the US, however this may be explained by a lower overall consumption of soy there. (Asian women consume on average ten times more soy than women in the US).
More and more studies are now confirming a breast-cancer-prevention role for isoflavones – one study found a 36% reduction in breast cancer recurrence from consuming 10 mg of isoflavones a day (Roughly 100g of tofu – about 5 times more than the average American consumes daily) – although not all studies have been able to confirm this positive effect. As well as influencing cancers by binding to oestrogen receptors, isoflavones have been shown to have antioxidant effects in vitro, which might further explain their protective properties*.
* It’s actually far from certain whether dietary antioxidants really prevent cancer, but that’s a matter for another blog.
Other studies have expanded this view to include other hormone-sensitive cancers – the isoflavone genistein has been show to inhibit prostate, cervix, brain, breast and even colon cancer. In animals and cultured cells, isoflavones have been found to slow the progression of prostate cancer, and in humans, isoflavone supplementation slowed tumour progression. Genistein also makes cells more sensitive to radio-therapy, making well-timed isoflavone supplements an ideal complement to some cancer treatments. Soy proteins have also been shown to reduce the rates of endometrial and ovarian cancer in women, however other studies have failed to replicate these findings. Unfortunately, most studies of the role of isoflavones in cancer prevention have been observational, making it difficult to determine cause and effect. One study found that soy protein supplements did not reduce the risk of endometrial cancer in postmenopausal women, and another suggested that isoflavone supplementation had a positive effect that was not present for dietary soy protein.
The jury is still out on isoflavones and cancer prevention. There are numerous studies that report beneficial effects of isoflavones on cancer progression and treatment, although there are many more which fail to replicate those findings. Whether isoflavones reduce cancer and recurrance risk is also unclear, and data on this is less reliable since the majority of studies have been observational.
But isoflavones have a few other tricks up their sleeve. Dietary isoflavones may stimulate bone formation, and have been shown to have positive effects on some hormonal conditions. Isoflavones, particularly genistein, can reduce the frequency of hot flushes and other symptoms in menopausal women, although recent evidence suggests some women may be unable to metabolise isoflavone supplements to access their positive effects. Some research suggests that getting isoflavones directly from soy in your diet is better than supplements for treating menopausal symptoms.
Because of the possible cancer-preventing properties of isoflavones, along with numerous other claimed health benefits, you can actually buy isoflavone supplements to deliberately increase your intake of this phytoestrogen. They are marketed as helping with hormonal health issues and bone growth conditions. However, although we can be fairly sure isoflavones in our diet are not seriously harmful, these compounds have not been thoroughly tested at higher levels. Some research suggests that very high doses of isoflavones might have different, even harmful effects. For example, the isoflavone genistein, which has been linked with positive effects at dietary levels, has been found to be extremely toxic at high levels, and may cause DNA damage and cancer. The conclusion – diets rich in soy are safe, and might even be helpful. High doses of isoflavones in supplements or other sources are best avoided until we have more data.
Just never feed tofu to your pet mouse.
Want To Know More?
- Soy Isoflavones Linus Pauling Institute
- McCullough (2012) The Bottom Line on Soy and Breast Cancer Risk American Cancer Society
- Khan et al (2012) Soy Isoflavone Supplementation for Breast Cancer Risk Reduction: A Randomized Phase II Trial Cancer Prevention Research
- Ollberding et al (2011) Legume, Soy, Tofu, and Isoflavone Intake and Endometrial Cancer Risk in Postmenopausal Women in the Multiethnic Cohort Study Journal of the National Cancer Institute
- Hamilton-Reeves et al (2010) Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis Fertility and Sterility
- Bhatia et al (2008) Use of Soy Protein-Based Formulas in Infant Feeding Pediatrics
- Howes, Howes and Knight (2006) Isoflavone therapy for menopausal flushes: A systematic review and meta-analysis Maturitas
- Liu , Chang and Wiesenborn (2005) Antioxidant Properties of Soybean Isoflavone Extract and Tofu in Vitro and in Vivo Journal of Agricultural and Food Chemistry
- Kao, Lu, Hsieh and Chen (2004) Stability of isoflavone glucosides during processing of soymilk and tofu Food Research International
- Jackson et al (2002) Effects of processing on the content and composition of isoflavones during manufacturing of soy beverage and tofu Process Biochemistry
- Kurzer (2002) Hormonal Effects of Soy in Premenopausal Women and Men The American Society for Nutritional Sciences
- Mitchell (2001) Effect of a phytoestrogen food supplement on reproductive health in normal mal
Featured image is a composite of a public domain image and an image used under a creative commons licence. Original image of tofu by Fuzheado.