Today I'd like to partake in some serious nutrition myth-busting.
As a dietitian who specializes in cancer and plant-based diets, I'm constantly asked about the safety of soy foods. Today, I hope to ease your mind by sharing the quality scientific evidence we have related to this plant-based staple.
Soy can be found in foods such as tofu, tempeh, miso, natto (a traditional Japanese fermented food), soy milk, edamame, soy nuts, soybean oil, protein powders, and vegetarian meat substitutes.
Most questions I get on soy have to do with it's isoflavone content. These isoflavones are phytoestrogens, meaning they can bind to estrogen receptors on our cells.
Many women with estrogen receptor positive cancers worry that soy phytoestrogens could bind to cancer cells and lead to cancer growth. (In actuality, we have learned that phytoestrogens don't lead to cancer growth and may even block our own estrogen from being able to bind to hormone-sensitive cancer cells.)
Additional concerns related to the safety of soy foods include potential feminizing effects in men, thyroid health, cognitive function, and nutrient absorption.
Because of these concerns, soy has been one of the most heavily researched foods. This means we have lots of research to work with, but also increases chances of outlier studies, where conflicting results could be due to chance or poor study design.
When analysing soy research, (or any research topic) it's important to look at the overall conclusions and place highest emphasis on the highest quality studies. For example, it's important to focus primarily human studies because we now know that animals metabolize soy isoflavones much differently than we do. (Much of the negative soy research done in the past used rodents and soy amounts greater than typically eaten.)
Women who consume moderate to high amounts of soy (usually defined as 1-2 servings daily) have a lower risk of breast cancer than women who don't eat soy, this is especially true when soy is consumed early on in life.
Consuming soy after breast cancer diagnosis is also safe and potentially beneficial. Most studies indicate that eating soy is unrelated to cancer recurrence, even in the case of hormone sensitive cancers. (8, 9, 10, 11) Some studies have also shown decreased mortality and better cancer outcomes when soy is consumed. (12, 13)
Feminizing Effects in Men:
Have you heard the myth that soy can cause man boobs? Yeah, not true.
At least not unless you're having 12-14 servings of soy daily. (There are two case reports of men experiencing feminizing effects at this level. Both returned to normal after discontinuing soy intake.)
Controlled human studies (a better source of information than case studies or rodent studies) indicate that soy has no effect on circulating estrogen/testosterone levels in men and does not cause feminization at levels at or even above the soy intake of a typical Asian male. (14)
Eating soy has no effect on the thyroid for healthy adults. (15)
Isoflavones can potentially inhibit the production of thyroid hormones, but this generally only occurs with iodine deficiency. People who eat a lot of soy and/or subclinical hypothyroidism should ensure that they are getting enough iodine. (16)
Eating soy may necessitate a higher dose of thyroid medications for those with clinical hypothyroidism because the soy protein block absorption of the drug, but there is still no need to avoid soy consumption.
Concerns have been raised about the potential of soy consumption to cause cognitive decline later in life but we now know that negative research is likely due to confounding variables.
Studies raising this concern involved Indonesian tofu-eaters, who tended to have less money and education. It's also important to note that in Indonesia, formaldehyde is used regularly during tofu processing. (17, 18)
Interestingly, an Indonesian study showed that tempeh improved memory, while tofu had slightly negative effects. (Formaldehyde is not typically used in tempeh processing.) (23)
Like all beans and grains, soy does contain compounds that can bind to minerals and decrease their absorption. Soy is particularly high in a compound called phytic acid, which could potentially block absorption of essential minerals like calcium, iron, zinc, and magnesium.
Eating 2-3 servings of soy daily is unlikely to lead to a mineral deficiency. Studies indicate that the iron from soy is absorbed at a high rate, rare for plant-based non-heme iron. (24) Numerous studies show that soy does not negatively impact bone density - and some show that it could even be beneficial for bone health.(25, 26, 27, 28)
The Bottom Line:
A number of studies show that 1-3 servings of soy daily is completely safe for most people. Soy foods are also an important component of a plant-based diet, as they provide protein, essential micronutrients and phytochemicals.
For the biggest benefit, opt for minimally processed soy foods, like tofu, tempeh, edamame, and miso. Choose fewer ultra-processed soy foods (i.e. frozen meatless hot dogs/breakfast sausages/burgers/etc.) and opt for organic soy if possible.
Ready to add more healthy soy dishes to your day? Here are some recipes to get you started:
Southwest Tofu Scramble
I hope that I was able to ease your mind a little bit and help you feel comfortable about eating soy. Feel free to leave your questions in the comments below!
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