If you eat soy foods on a regular basis, you’ve more than likely heard or been asked the following:

“Aren’t you worried about getting cancer?”

“Soy is bad for your thyroid health!”

“Aren’t you worried soy will give you man boobs?!”

Soy and whether you eat it or not, is one of the most controversial topics in the realm of nutrition and there remains a lot of mixed messages out there! I totally get why people are confused, so lets break it down …

What are soy foods?

Minimally processed soybean products include:

  • Edamame
  • Soy nuts
  • Soy sprouts
  • Natto (fermented soybeans)
  • Tofu
  • Tempeh
  • Miso
  • Soy sauce
  • Soy milk

More processed, “modern” soy foods include:

  • Meat alternatives
  • Dairy alternatives (cheese, yoghurt)
  • Soy protein powders
Graphic courtesy of Examine.com

Why is soy so controversial?

Soybeans and soy products are the richest source of a class of isoflavones (a type of antioxidant), known as phytoestrogens.

Phytoestrogens are plant derived compounds that can exhibit some of the same effects that estrogen has on the body — it is because of this that soy can get a bad rap!

While it is true that soy has phytoestrogens that may act like estrogen in the body, they also have anti-estrogen effects! This means that phytoestrogens can actually block more potent natural estrogens and prevent them from causing harm.

Soy and cancer

Estrogen is known to induce breast cancer, so wouldn’t eating phytoestrogens from soy do the same thing?

This doesn’t appear to be the case! And like mentioned above, phytoestrogens can actually BLOCK more potent estrogens produced by the body preventing them from doing harm. Soy also has anti-oxidant and anti-inflammatory properties that can help reduce your risk of developing cancer.

This meta-analysis (combining data from multiple studies), of 35 different studies, reported that intake of soy was associated with a lower risk of developing breast cancer.

What if you have already been diagnosed with breast cancer?

This study showed that women who had been eating soy foods from a young age, had greater protection from recurrence of their cancer, than those who began eating soy foods in later adulthood.

Overall, meta-analysis studies have shown that soy intake is likely associated with better survival and decreased risk of breast cancer recurrence.

Soy and mens health

People, men in particular, tend to hear the word soy and automatically assume that they should shun soy foods in fear of becoming ‘feminine’ or preventing them from putting on muscle mass.

Let’s just say that you would have to be consuming a lot of soy everyday for a long time to have any “negative” health impacts — we’re talking like 6 cups or more of tofu per day (12 servings).

One serving of tofu, tempeh, soy beans or soy meats is 1/2 cup. The average number of servings consumed per day in Asian males (the population with the highest soy intake), is ~2-3 servings (1-1.5 cups).

Soy intake appears to be associated with a decreased risk of prostate cancer in men.

Soy and thyroid health

If you have hypothyroidism, also known as under-active thyroid, you may have been told to stay away from soy foods. This is because soy contains phytoestrogens and goitrogens, substances that can interfere with iodine uptake into the thyroid gland. Iodine is an essential nutrition for proper thyroid function, learn more about it here.

This review of 14 studies showed that those individuals with normal thyroid function and adequate iodine intake do not need to worry about soy intake affecting your thyroid negatively.

Even those with hypothyroidism and on medication don’t need to totally avoid soy foods!

Yes, you may need to be more cautious as soy can interfere with the absorption of thyroid medication, but work with your physician around meal timing and medication dosing to make sure you’re on an adequate dose. Work with your dietitian to make sure you’re getting enough iodine in your diet as well!

Overall, studies done suggest that soy does not have a significant impact on thyroid health, but as with many nutrition topics, more research is needed on the direct relationship between soy foods and thyroid function!

But, isn’t soy production bad for the environment?

The United States, Brazil and Argentina account for 80% of the worlds soy production and yes, farming of soy has undoubtedly contributed to mass deforestation due to increased demand!

But, here’s the thing. Over 78% of the soy produced is consumed by livestock, not us humans.

In Canada, about 60% of soy produced is GMO (made to withstand heavy pesticide use), but non-GMO, organic soy is readily available and easy to find! In fact, most of that GMO soy is being used in animal feed …

Bottom Line

  • Eating soy regularly has been associated with decreased risk of breast cancer development, decreased risk of recurrence and increased survival.
  • Eating soy foods does not reduce testosterone levels or have ‘negative’ affects unless consumed in excess (>6 cups tofu/day). Stick with the recommended 2-3 servings (max 3-5 servings), and you don’t have to worry!
    • 1 serving = 1 cup soymilk, 1/2 cup tofu, edamame, soy meat alternative).
  • Eating soy regularly appears to reduce the risk of developing prostate cancer.
  • Soy intake does not appear to affect thyroid activity in humans with normal thyroid function and adequate iodine intake! Be mindful that soy can interfere with the absorption of thyroid medication.
  • As with all foods, less processed soy foods like natto, tempeh, tofu and edamame, should included in your diet more often than their more processed counterparts like soy based meat alternatives or soy oil.
Resources Used: