Soy. It’s one of the most polarising of foods. A casual search of the Internet will uncover first one article lauding it for its health benefits, while the next article will class it as the food of the devil with eaters of it condemned to a hell of hormone-related disease. To help you make sense of the conflicting messages about soy, I’ll dig deeper into some of the key health areas linked to eating soy foods. And then I’ll clarify just how much credence you should give to both the health claims and the health alarms you may hear about it.
It doesn’t take much searching on the Internet to discover that soy is one of the most polarising and contentious of foods. So which side of this debate is right? Does soy deserve a health halo, or should you swear off the stuff for life? When you take away the cherry-picked science, alarmist and emotive language and the bro science, you’re left with a food that rightly can be a feature of any person’s diet.
All types of soy
Soy is part of the legume family and today is found in many foods. From the Japanese snack of edamame which is the soybean in its natural state through to soy milk and a variety of fermented foods, it can be enjoyed in many different ways. The nutrient composition of the soybean differs markedly from other legumes as it is much higher in fat, moderately higher in protein and much lower in carbohydrate. The soybean is notable not only for its protein content but the quality of that protein which puts it on a par similar to animal protein.
Soy foods can be divided into two groups: unfermented and fermented. Unfermented foods include soy milk, tofu, edamame, soy nuts and sprouts. Then there are the fermented foods which include miso, natto, tempeh, and soy sauce. Add to this list what are called ‘second-generation’ soy foods popular in the West. These include tofu burgers and hotdogs, soy-milk yoghurts and cheeses.
Soy milk is one of the most popular soy products in Australia. It is a water extract of whole soybeans where traditionally soybeans are soaked, heated and then crushed to extract the milky juice which is blended with water, vegetable oil, salt, and sweeteners like malted barley and sugar. Soy milk can also be made as a blend from soy protein isolate powder together with water, oil, sweeteners (like sugar or maltodextrin), gums, emulsifiers, vitamins, mineral salts and calcium. Soy milk is naturally low in calcium unless calcium has been added to it, but is rich in protein and B group vitamins.
Any mention of a vegetarian diet calls to mind tofu. Tofu is made in a process similar to cheese making which uses soymilk and a coagulant. Tofu is available in a variety of textures and densities and can be an excellent source of calcium if calcium has been used in the setting agent.
On the fermented foods front, tempeh and natto are the simplest soy foods to make. This is where boiled soybeans are inoculated with the fungus Rhizopus oligosporus (for tempeh) or the bacterium Bacillus subtilis (for natto) and left to ferment for several days.
Miso and soy sauce both involve inoculating boiled soybeans with the fungus Aspergillus oryzae for several days and then mixed with brine and lactic-acid bacteria for further fermentation. When the resulting paste is sufficiently ripe, it is blended and packaged as miso. Alternatively, the paste can be pressed to separate solids from liquids, and the liquids are filtered and pasteurised into soy sauce.
Soy and its health benefits
So, the big question: is soy good for you? On the positive health side, you’ll see many claims made about its benefits for heart health and even cancer risk and a long list of other health benefits. These health benefits are attributed in large part to a class of compounds found in soy called isoflavones, many of which can act as phytoestrogens. Phytoestrogens are compounds that can weakly bind to estrogen receptors throughout the body.
To start out broad, let’s look at what a recent review exploring the association of soy and isoflavones with human health found. This was an umbrella review which means it was a review of review studies – so it’s all a bit meta here. A total of 114 systematic reviews and meta-analyses covering 43 unique health outcomes were explored.
There is quite a bit to unpack from the review, but here is the summary. At the highest level of evidence rating, a beneficial link between soy consumption and cancer, cardiovascular disease, menopausal symptoms of hot flashes, bone health, cognitive function and visual memory was seen. The only harm considered of concern was a link to a higher risk of gastric cancer in men who drank a lot of miso soup.
On this review alone, you would have to see that most of the health concerns you have heard about soy are way off the mark or lack any context for its potential health benefits to offset them. You can pick any food you like and find research to show a negative impact on health. Just like what I did on a prior blog post on toxic broccoli that ended up going viral. In talking about the health merits of a food, what matters is what the balance of evidence says about it when comes to eating advice for the general population.
Now let’s drill down a bit further into some of the key areas for health and controversy when it comes to soy.
I’ll start with heart disease first. There does appear to be some benefit to eating soy when it comes to having a healthy heart and cardiovascular system. The findings from a 2017 meta-analysis of 17 observational studies suggest that eating more soy foods is linked to a significantly lower risk of cardiovascular disease, stroke and coronary heart disease.
But the evidence on soy and heart health is all really just from observational studies – research that has its own inherent flaws. When it comes to intervention studies, research finds that soy protein supplementation does lower the more harmful LDL-cholesterol and blood pressure so this could translate into less heart disease, but the effects of soy are quite modest.
There are though no concerns that soy increases the risk of heart disease. So it could also be that the benefits of soy may be from the dietary pattern eaten with it. If soy foods are replacing animal-based foods, this will decrease the amount of saturated fat eaten and up the fibre content of the diet – both of which can be a help for your heart. In other words, swapping out that bacon burger for tofu or tempeh is a heart-smart move. But having that burger followed by a bowl of soy ice cream for dessert probably won’t be as helpful.
Cancer is one area where soy features strongly, especially for breast and prostate cancer which have strong hormonal-related causes. This is where soy could be of benefit and that is because of natural plant chemicals found in it called isoflavones. These isoflavones are phytoestrogens.
Phytoestrogens can compete out the body’s own natural estrogen for binding to the estrogen receptor. Estrogen binding to its cellular receptor is needed to exert its physiologic functions. Phytoestrogens can attach themselves to estrogen receptors and activate them, but because their estrogenic activity is much weaker than human estrogen, the effect is less. So that all means less cell stimulation by estrogen. And if you had to describe cancer in a single concept it is simply cell growth and division that has got out of control.
Soy foods and phytoestrogens could all be a good thing when we talk about breast cancer risk. Much of a woman’s risk of developing breast cancer can be linked to her lifetime exposure to estrogen. Having an early menarche (which is the age when a girl has her first period) before the age of 12, going through a late menopause, not breastfeeding, having no children or having the first child over the age of 30 are all linked to a higher risk of breast cancer. And this comes down to longer lifetime exposure to estrogen as part of the normal reproductive cycle. Soy foods increase the length of the menstrual cycle by about one day. This may not sound like a lot, but over a reproductive lifetime, this minor effect on menstrual cycle length could help to decrease breast cancer risk.
From observational research looking at soy-food and isoflavone consumption and breast cancer risk, there is a significant association with a lower risk of breast cancer by about a quarter. Although a greater benefit is seen in women from Asian countries than in Western countries. So this could be partly related to the amount and diversity of soy foods eaten in Asian countries and also other lifestyle habits.
There is some limited suggestive evidence that soy foods may lower the risk of prostate cancer, but it not as strong as the link between soy and breast cancer risk. But mentioning this is a nice segue into my next topic which is one for the guys and that is testosterone.
Because of the estrogen-like effects of the isoflavones in soy, this food is treated by some men (especially the ones who are all about the gainz in the gym) in a class below rat poison. But do men need to avoid soy if they want to keep their hard-won muscles? And is soy going to kill off every bit of their manliness as it slowly feminises them in mind and body? The answer is: no.
A 2010 meta-analysis of 15 randomised-controlled trials looking at soy or isoflavones and its effect on reproductive hormones in men found no effect on total testosterone, free testosterone (which is the active form of testosterone in the blood), or the transport protein for testosterone called sex-hormone binding globulin. Guys, the next time someone tells you that you shouldn’t drink soy milk or eat tofu because it’s going to ‘feminise’ you physically, just refer them to this meta-analysis.
But that doesn’t mean that soy has zero effect in men. If you eat enough of it, just like anything, it can have an effect. And so far, two case reports have been written up in the medical literature of men who went just a little too crazy for soy foods.
In one case, a 60-year-old man who drank almost 3 Litres of soy milk a day for months on end reported erectile dysfunction, reduced libido and gynecomastia (which is swelling of breast tissue in males). The symptoms soon reversed when he stopped drinking soy milk. While in another case, a 19-year-old vegan male who had a very soy-rich diet suffered from low levels of testosterone and erectile dysfunction. These are both very rare cases and neither can be linked exclusively to soy foods as anyone having that much soy will be eating a lot less of other foods in their diet too.
When it comes to thyroid function, soy sometimes comes up as being a food to avoid for anyone with a sluggish thyroid gland (a condition which is called hypothyroidism). Soy does contain chemicals that are considered goitrogens – that is, they can interfere with thyroid hormone production by reducing the uptake and metabolism of iodine into the thyroid gland. Iodine is a key part of thyroid hormone after all.
But a review of 14 studies in healthy adults and people with hypothyroidism found little to no effect of soy foods or isoflavone supplementation on a range of measures of thyroid function. Still, the authors did state that there does remain a theoretical concern based on cell culture and animal studies that in people with compromised thyroid function or low levels of iodine consumption, that soy foods may increase risk of developing clinical hypothyroidism. But just whether they should avoid soy foods completely or just limit them is unclear.
But since this review came out, two long-term randomised controlled trials have been published (linked here and here) both of which involved woman taking isoflavone supplements. Neither study found any significant effect of the isoflavones on thyroid hormone concentrations, TSH levels or thyroid-antibody concentrations.
So, for anyone with thyroid problems, don’t get your diet advice from what you read on the Internet especially if it is prefaced by alarmist claims about soy that have no basis in scientific fact.
Soy and infants
Finally, to round things out, I’ll touch on the topic of whether a soy-based formula is suitable and safe for infants. But to start with, it must be said that breast is always best for an infant. But in the case where this isn’t possible and infant formula is needed especially if dairy or lactose intolerance is an issue, then is a soy-based formula okay to use?
The concerns about soy formulas come from its isoflavone content. An infant is going through rapid development stages that are sensitive to estrogen, but because of their small size, then use of an almost exclusive food source that is soy-based could expose them to higher levels of phytoestrogens than adults whose food sources are more varied.
What research there is in humans so far does not raise too many concerns, but the absence of evidence is not evidence of absence. The human studies so far indicate that soy-based formulas do not impair the growth of healthy full-term infants, but in premature infants, the case may be different where there has been a link made to it causing rickets which a bone disease caused by a deficiency in vitamin D.
Turning to animal models, adverse effects have been seen but it is unclear how much this can be applied to a different species, that being humans. So, in this case, it is not possible to give soy a clean bill of health for infants because of the lack of enough human research. So, some level of caution is warranted even though the risk is considered minimal, but minimal is different to none or negligible.
There is a good summary about the current state of research in this area put out by the United States Government’s National Institute of Environmental Health Sciences and I’ll let you read the assessment so you can make your own assessment.
What it all means
Let’s wrap up the soy story. Soy is a unique food that is widely studied for its estrogenic and anti-estrogenic effects on the body. And when you take away the cherry-picked science, alarmist and emotive language on the Internet and the bro science, you’re left with a food that rightly is deserving of a place in any person’s diet. Soy is a nutrient-dense source of protein that can safely be consumed several times a week and is likely to provide many health benefits. And yes, while you can find some evidence of harm, you can do the same thing for any other food you eat and that would mean you would starve to death if that’s how you made your food choices. What matters is the weight of evidence, and in the case of soy, it is tipped well in favour of a positive health benefit.