the debate on soy: what you should know

April is National Soy Foods Month and I seem to be getting quite a few questions about it lately. Most of the questions and comments come from men who insist they can't eat soy due to it's feminizing effects.

Soy is quite the hot topic in the nutrition world and people seem to have strong feelings either for or against it. I've done my best to review the studies and give you the facts on what the latest research tells us about consuming soy, all opinions aside.

Read on for what the current research says along with the opinions of a few of my colleagues.

tofu health effects

What is Soy

Soy is a legume that originated in Asia but is now used heavily in the western world. Soy comes in variety of forms. You can eat it in the form of tofu, tempeh, edamame, miso, and nondairy milk. Whole soy foods are a great source of protein, unsaturated (heart healthy!) fats, potassium, fiber, iron, calcium, magnesium, B6, and Vitamin C. Soy is also added to food products in the form of soy protein isolate, soybean oil, or partially hydrogenated soybean oil.

Soy and Sex Hormones

What are Phytoestrogens?

The reason that men in particular are concerned with soy is because it contains a compound called isoflavones, which classify as phytoestrogens

Phytoestrogens are compounds that are naturally found in plants. In addition to soy, they are also found in coffee, tea, broccoli, oranges, and carrots. Phytoestrogens bind to the same receptors that estrogen binds to in the body. When they bind, estrogens cannot, and therefore minimize estrogen's effects (here's an easy to understand explanation).

My colleague, Rosanna Alvarez RDN, LD,CD, says that ""Just because isoflavones are considered the plant form of estrogen, does not mean it behaves exactly like estrogen when we consume it. Isoflavones are molecularly different than estrogen. Because the chemical structure is different so is the physiological effect."


Does soy impact testosterone and estrogen?

Both men and women have testosterone and estrogen. However, men seem to fear the isoflavones in soy, thinking it lowers their testosterone levels and therefore their 'manliness'. This meta-analysis found no effect of soy consumption on testosterone levels in men.

Brooke Mullen, MS, RDN, LDN of Well By Design Nutrition agrees,  "Based on my review of the literature, there are a few studies in rodents that have shown mixed results of soy protein intake and testosterone levels/hormonal effects (some show decreased, some increased, some no effect). However, we know rodent studies don't necessarily translate to humans, they simply introduce the need for studies in humans.

Of the human studies I have found, they show no effect of isoflavones and soy proteins on testosterone levels, or other estrogenic effects. Those that did show a positive correlation had men consuming unreasonable amounts of soy (a few liters or upwards of 20 servings per day).

Currently there is no strong supporting evidence that soy has any effect on male hormones, and the positive effects significantly outweigh any risk there might be."

This analysis, examining the results from 9 different studies, also showed no effect from isoflavone exposure on the estrogen levels in men. 


Soy and Cancer

"The research that is actually out suggests the consumption of soy reduces [up to 50%] the likelihood of men to develop prostate cancer," says Alvarez. This meta-analysis also supports that the consumption of whole soy foods is associated with a reduced risk of prostate cancer in men.

Soy consumption has also been associated with a reduced risk of breast cancer in women. One study, following over 5,000 female breast cancer survivors for three years found that soy food consumption was strongly associated with a reduced risk of death and cancer recurrence (more evidence, here).


Soy Has Many Health Benefits 

As mentioned above, whole soy foods provide protein, unsaturated fats, potassium, ALA omega-3s, fiber, iron, calcium, magnesium, B6, and Vitamin C. The isoflavones found in soy are also antioxidants.

Soybeans have more protein and less starch compared to other beans.

Soy has been shown to:

  • reduce LDL cholesterol in people with high cholesterol
  • decrease risk of colorectal, breast, prostate, and endometrial cancer
  • increase bone mineral density in post-menopausal women

"Studies show intake of traditional soyfoods (edamame, tofu, soymilk, miso, tempeh, natto) compared to isolates or flours, have a significant effect on lowering cholesterol, improve HDL/LDL ratio, and decreased risk of developing Type 2 Diabetes, as well as decreased risk of prostate cancer, " says Mullen.

We can also learn from Asian cultures who have been consuming soy for thousands of years. Many Japanese adults consume 1.5 to 3 servings of whole soy foods per day, including tofu, miso soup, and soy milk and have lower rates of cancer and heart disease.

The Blue Zones (a project that examined populations around the world who live the longest), found that many of the healthiest cultures focus on plant based diets and include whole soy foods. (Learn more here and here).


You can read more about the health benefits of soy here.

Final Thoughts and Take Home Message

Soy is now appearing in our foods in a variety of forms, including soy protein isolates and  partially hydrogenated soybean oils. These forms do not have the same nutritional benefits as whole soy foods like tofu and edamame. Soy can be consumed as part of a healthful diet, but experts recommend limiting your consumption to 3 to 5 servings per day.

"I encourage whole food sources like the "traditional" foods  rather than isolates, concentrates, or flours (i.e.: added as an ingredient to other foods)." - Brooke Mullen, MS, RDN, LDN of Well By Design Nutrition

"There is still a lot of research to be done, but what we know is that not all soy products are the same, and the ones with the most health benefits are the ones in their most natural form. Eat it in moderation and in it’s most natural form." - Rosanna Alvarez RDN, LD,CD

is soy milk healthy