I touched on this topic in a newsletter a while back on breast health. Soybeans and some soy products are backed by plenty of evidence for breast and hormonal health, particularly for women. However, there is a certain hysteria and polarisation of opinion in the general public around this topic, and many blog articles on both sides sharing the “facts”. I believe there are many misunderstandings, some of which I attempt to clarify here.
I was conditioned against soy myself for many years, despite leaning heavily on soy products in my earlier vegetarian years. I simply heard and read from many sources including sources I respected at the time, that soy was now considered dangerous- especially to men, baby boys, women with breast cancer, may cause breast cancer in some women, for thyroid health, and also that all soy was GMO. I figured some study or studies must have been done and there must be something to it. Then last year in my evidence-based university studies, I had to do an assignment on soy and research the evidence myself. I was surprised by what I found and it has changed my behaviour around soy.
Soy is what is called a phyto-oestrogen. That means it has plant (plant=phyto) oestrogen-like compounds which can attach to oestrogen receptors in humans and animals and exert weak oestrogenic effects. There are many phyto-oestrogenic foods and herbs, such as red clover, linseeds, oats, peanuts, barley, beans, alfalfa, rice, coffee, mint, ginseng, beer, fennel, apples.... you can see that they are widespread in our natural diet, but soy does have higher amounts than many, and these are called isoflavones.
Fueled by the media, people tend to assume that plant-oestrogens are going to be the same as “HRT” oestrogen which has been associated with an increase in breast and endometrial cancer rates in post menopausal women. Phyto-oestrogens can have both a weak oestrogenic effect or an anti-oestrogenic effect....depending on whether there is too much or too little oestrogen in the body. In other words, phyto-oestrogens generally have a balancing effect on oestrogen levels, increasing them when they are too low and decreasing them when they are too high. They also help to detoxify oestrogen from the body. When they are anti-oestrogenic, this can improve oestrogen excess symptoms such as PMS and heavy periods, although there is a possibility that excess soy may inhibit fertility by lowering oestrogen levels too far. However normal dietary levels of consuming soy once or twice a day, as is done in some Asian countries, has shown no effect on fertility.
It is important to understand the difference between studies done in laboratories and petri dishes, or even on rats, and well done human studies. Humans are not rats, and we have different, complex physiology. And we certainly respond differently as whole people, to isolated cells examined in laboratories. Many of the negative studies on soy were done on rats, chicks or in petri dishes. Animal studies often used unfermented soy as 100% of the diet and under not-normal conditions, which is not going to give a realistic result translatable to humans. On the other hand, many of the studies showing the benefits of soy were epidemiological studies- which means studies on whole populations of people over time. This is still not considered top tier evidence, which is the randomised placebo controlled trial, however it is a lot better than rats and petri dishes.
This is where the issue of traditional evidence arises for me. Before I researched this topic myself, I did wonder why Asian women seemed protected by their consumption of soy, and why it was embedded in their culture, if it was so bad for them. Japanese women consume around 8.7g of soy protein a day, Indonesians around 7.4g. They have around 1/3 less breast cancer than western women, and they have higher survival rates. Of course there may be other factors involved as well, especially the Japanese high iodine intake.
A meta-analysis is considered a high level of evidence where many studies on the one topic are analysed to find what the underlying evidence is. Single study results can be the result of chance, but when many studies are done, the overall results have more weight and can be more trustworthy. In this case, a meta-analysis was done in 2014 to determine if there was an association between eating a lot of soy, and breast cancer risk for post menopausal women. 35 studies were analysed and the result was that for both pre and post menopause, women in Asian countries have a lower breast cancer risk due to soy isoflavone intake. However in Western countries, there is no association- positive or negative. This may be because Western soy intake is generally too low to produce health effects, and also because the protective effects may occur earlier in life for Asian women who consume soy from childhood. A large study done in China shows that consumption of soy is protective after cancer diagnosis and treatment, significantly reducing risk of recurrence.
So after doing this research I am left with some thoughts on consuming soy. I am no longer at all concerned about a relationship between soy and breast cancer- I think that has been proven to be not true, and in fact, soy is likely to be protective, especially after occurrence. There is a slight possibility that eating soy may mess with some women’s hormones, but even this is actually unlikely at normal dietary levels, and in fact, is more likely to be balancing.
There are a few other questions, however. With epidemic levels of hypothyroidism occurring in western countries, does soy interfere with mineral absorption, or thyroid hormones? Well it turns out that the studies which initially fueled the concern were done on populations with suboptimal iodine levels, so they were susceptible to goiters anyway. The conclusion of a literature review of the available evidence was that there is no evidence that consumption of soy adversely effects thyroid function in iodine-replete individuals. Another study suggests that soy consumption may even preserve thyroid function in post menopausal women. So that issue has also been laid to rest.
Soy formula for babies is another controversial issue. Some babies are allergic or sensitive to dairy and soy formula has been used for several decades now for these babies. Therefore there is quite a bit of evidence around this issue. The original concern was due to a study on marmosets showing a decrease in testosterone with soy fed marmoset babies. However this has not been reproduced with humans. This study found no difference in the hormonal profile of babies fed breast milk, dairy formula or soy formula. This study shows that while the levels of isoflavones was significantly higher in soy formula fed infants, this did not effect the levels of hormones. This more recent 2015 study found that compared to breast or cow-milk formula, soy formula fed children have no difference in reproductive organ size at 5 years of age.
If you eat a low or no meat diet, soy can be a significant source of protein. Choose non-GMO organic soy- it is widely available in major supermarkets and health food outlets.
The most recent research shows that moderate consumption of fermented and non fermented soy such as tofu, soy milk and tempeh are not harmful and may even be beneficial, for most people. The beneficial effects include possible protection against breast and prostate cancers, bone health and cardiovascular health.
Eating fermented soy does not seem to be necessary as microbes in our gut will do that work for us, which is why soymilk and tofu are acceptable and commonly consumed soy products. However fermented soy(tempeh, natto, miso) are healthy forms of soy that are easier to digest.
Make sure you have sufficient iodine levels if you are consuming soy regularly. This may be a key to why some people or animals appear to be negatively affected by soy. Those cultures who tend to eat more soy also have higher iodine levels due to consumption of seafoods. You can increase your iodine levels naturally by eating seaweed regularly, or by taking a supplement.
I do NOT recommend soy supplements, or soy flour, or soy protein supplements, or soy isolates, or soy meat alternatives, because they are all highly processed products, may contain unreasonably high levels of isoflavones, and were not used traditionally. I highly recommend eating foods which are unprocessed, or minimally processed, such as tofu, tempeh, edamame and soymilk which are also backed by traditional usage. High levels of isoflavones, much higher than that found in a moderate daily consumption of soy products, can interfere with hormones and affect fertility.
I would not recommend soy-based baby formula in general because I believe milk has many nurturing qualities for babies, even if it comes from animals other than humans. Of course I recommend breast feeding, but for those who cannot or choose not to breast feed, a substitute needs to be found. Soy formula seems to be safe according to the current evidence spanning several decades, and I would recommend it for those babies who react to other formulas, but I do not feel completely comfortable about it.
There will of course be some individuals who react to soy, and I encourage you to listen to your own body. However I am confident that moderate consumption of soy foods as part of a broad and varied diet may be of benefit, and it is certainly a plant-based protein that has stood the test of time. I am a proponent of a largely plant-based diet, with small amounts of meat for those who desire. I believe soy can play an important health-promoting role in a modern, well balanced plant-based diet.