Updated: Jun 12, 2020
I’m sure we’ve all heard about the different “studies” that have shown how dangerous soy can be for you, or that you can only have so much before it affects your hormones, particularly if you’re male. Is there really anything to be concerned about, should you consume soy?
Okay, well first off: You don’t need to fear soy, in fact studies are suggesting there are benefits from including soy in your diet. Here’s the low-down on soy.
The Soy Nutrition Basics
Soy is a legume; soybean is where soy foods and products are derived from.
Soybean and soy foods like tempeh, tofu, soy milk and TVP all vary nutritionally, though there are many nutritional benefits with including them in our diet.
Soy is low carbohydrate, much of which is prebiotic goodness that benefits our gut health, as it provides food for the good bacteria in our intestines. As mentioned, this varies by source, and is why some soy products are considered high FODMAP, while others are considered low FODMAP.
Soy is a source of high quality, complete protein. This just means it contains all the essential amino acids required by our body and is well digested and absorbed.
Soy also contains lots of “good fats”- it contains monounsaturated and polyunsaturated fats, as well as omega-6 and omega-3.
When it comes to micronutrients, soy can provide substantial amounts of calcium, iron and zinc. These are particularly important in vegetarian and vegan diets. These nutrients also appear to be well absorbed, particularly the calcium. Again however, the content varies by the types of soy products e.g. when choosing tofu, you want to ensure it is calcium-set (e516, 516 or calcium sulphate in the ingredient list) and opt for fortified soy milks (look for 120mg per 100ml serve).
Consumption of Soy
Soy is traditionally consumed in Asian diets in a much higher quantity than Western diets. In Japan for example, the average intake of isoflavones from soy is 30-5mg/day, while in America, Canada and Europe, the typical diet provides less than 3mg/day. This is important to consider, as many studies will compare these diets for disease risk, in hopes of identifying which lifestyle and diet factors may increase or reduce risk. Soy is often associated with reduced health risk in this way.
What is a serve?
Given that we eat foods, not nutrients or isoflavones specifically, I thought it would be worth noting what “a serve” of soy, or the isoflavone content is considered equivalent to. A serve of soy is equivalent to 1 cup (250mL) of fortified soy milk, 100g of tempeh, 170g tofu, 150g edamame. The Japanese average intake is equivalent to about 3 serves of soy/day. This diet is higher in less processed soy goods.
Okay, but what about our hormones?
Soy contains isoflavones, which belong to a class of compounds called phytoestrogens. These occur in other foods, though are highest in soybeans. They look and behave in a similar manner to oestrogen, though they are not considered hormonally active. They bind to receptors much weaker than oestrogen and seem to preference different types of receptors- which means effects also seem to differ. They are often referred to as “selective” in this process.
Many concerns regarding soy and isoflavones affecting hormonal and reproductive health come from animal studies where very high doses are used- this not only definitely exceeds a normal, and comfortable intake of soy, but we know these compounds are metabolised differently by rodents, and as previously discussed, we can’t apply these to human health.
Studies on human health have provided no conclusive evidence that soy consumption negatively impacts human reproduction, development or hormonal health.
High intakes of soy may increase the length of a woman’s menstrual cycle by 1 day, though intake overall has been associated with positive effects on fertility.
When it comes to males, even intakes 3x higher than the average intake in Asian countries (and thus, far high than what is in Western diets) showed no significant effects on oestrogen or testosterone levels. Soy intake has not been found to negatively impact sperm health either. So, not negatively impacting reproductive health. Also, it does not result in “feminisation” of men.
There is evidence suggesting positive impacts on our health, however, so let’s look at them:
Soy and heart health
Soy has been associated with a 5% reduction in LDL (“bad”) cholesterol, reducing circulating triglyceride levels, as well as increasing HDL (“good”) cholesterol. It has also been associated with reducing blood pressure. The mechanisms for these effects are not exactly clear, while theories are being investigated. There is also evidence suggesting that soy intake may be protective of cardiovascular disease- which may be due to effects on cholesterol and blood pressure, in part.
More research is still needed in this area. It may be that the isoflavones in soy have positive effects, it may be the inclusion of more fibre or good fats, it may be that diets higher in soy foods are also lower in foods associated with increased risk of these conditions. Either way, the inclusion of soy seems to have heart-protective effects, reducing risk of disease.
Soy and Cancer
Breast cancer is commonly noted as a concern of soy intake, again associated with presumed hormonal affects. Breast cancer has been found to be lower in countries consuming soy often, as in Asian diets, and that the Westernisation of these diets is associated with an increase in breast cancer in the population. As such, it appears to potentially be protective of breast cancer.
Female childhood soy intakes of 1-1.5 serves a day has been associated with up to 60% reduction in breast cancer risk. If you didn’t consume soy as an adolescent, don’t fret, higher consumption of soy as an adult does then appear to provide protective effects too.
Moreover, breast cancer patients are often concerned that soy consumption will worsen their health or lead to a return of the cancer. A meta-analysis of over 11,000 women found that women with higher intakes of soy had 17% lower risk of death from breast cancer, but also that the cancer was 26% less likely to return. Generally, it is considered safe and may provide positive effects here, particularly in conjunction with certain medications*.
Prostate cancer had also been found to be lower in Asian countries consuming soy, compared to countries consuming Western diets. High soy consumption was actually associated with a 50% reduction in risk of prostate cancer. Though these studies are not strong enough to draw a conclusion and are rather suggestive of an effect, needing further investigation.
Soy and Thyroid Health
Much association with soy negatively impacting thyroid health comes from either in-vitro or animal studies, as well as the 1960s, where a soy-based baby formula fortified with iodine resulted in Goitre (enlarged thyroid). This was due to the iodine content, not the soy, however.
Soy has been found to have no adverse effects on thyroid function in men or women. Soy can interact with certain thyroid medications, as can other drugs, calcium, fibre, and may interfere with absorption, however. This is an interaction with the medication, not hormones*.
Soy and Menopausal Symptoms
In Asian countries consuming soy, hot flushes are not reported to be common. This, along with assumed oestrogen-like effects on the body, has been the reason many look into soy reducing menopausal symptoms. The results are currently inconclusive. There have been studies showing significant effects compared to placebo, but there have also been studies showing no effect. Studies are now looking at the concentration of specific types of isoflavones to see if this may explain the inconsistencies, as previously studies did not specify the content.
Okay, so now what?
As I’ve said before, nutrition science is tricky and the body of evidence regarding soy is still growing. We may not know the mechanisms of soy in generating potential effects, or the extent of effects on our health, soy has been conclusively been shown to be safe to consume.
The current recommendation is around 1-4 serves of soy per day to receive benefits and reduce risk of diseases, even if just from reducing other foods in the diet. In saying that, if you do not enjoy soy or especially if you are allergic or intolerant to soy, don't feel you need to consume it, particularly if only to reduce disease risk. There is no one food or nutrient that will inherently prevent or cause disease, it is always a diet overall that should be of focus.
The isoflavone content varies with the processing of soy products, being lower in more processed foods. This does not mean you need to avoid processed forms as they are still a good source of nutrition, it is always best for your diet to be lower in processed foods.
* As always, this is general information and not medical advice. If you have an existing medical condition or are on medications, it is always best to consult with a health professional to assess the potential effects a food or supplement may have on you, individually.
Soy, it's good news!
How much time do you have? You want to read even more? Okay, here you go:
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Munro, I.C., Harwood, M., Hlywka, J.J., Stephen, A.M., Doull, J., Flamm, W.G. and Adlercreutz, H. (2003), Soy Isoflavones: A Safety Review. Nutrition Reviews, 61: 1-33. doi:10.1301/nr.2003.janr.1-33
Bruce J. Trock, Leena Hilakivi-Clarke, Robert Clarke, Meta-Analysis of Soy Intake and Breast Cancer Risk, JNCI: Journal of the National Cancer Institute, Volume 98, Issue 7, 5 April 2006, Pages 459–471, https://doi.org/10.1093/jnci/djj102
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