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A lot of debate swirls around one specific protein—soy.

Questions like…

Is eating soy safe or dangerous?

Does it cause or prevent cancer?

How much should you really eat?

Let’s take a deep breath and untie the knots which surround the subject of soy.

The Big Question

Soy is one of those rare plants that mimic the profile of animal proteins with comparable concentrations of the nine essential amino acids.

As such, it is described as a ‘high quality’ (or complete) protein.

But does that similarity mean that soy poses equivalent health risks as we have seen in animal proteins when it comes to the IGF-1 cancer-promoting hormone?

To answer the question, let’s first recap what we know about protein consumption and cancer.

A Brief Protein Review

As we have discussed at great length in our article The Protein Question, eating animal protein amplifies the levels of the cancer-promoting growth hormone IGF-1 in our bloodstream.

At the same time, most plant proteins do the exact opposite—i.e. they lower IGF-1 levels, suggesting that they may be an excellent way to protect yourself from hormone-dependent cancers.

A study illustrated this when participants who ate a plant-based diet for as little as 11 days dramatically lowered IGF-1 levels, which slowed or reversed prostate cancer growth in vitro.

And a person who follows a plant-based diet (over a full year) may squash cancer cell growth a whopping 8 times more than someone eating a Western pattern diet.


The reduced risk of cancer in plant-based eaters can be explained by two factors. Following a plant-based lifestyle:

  1. Causes your cancer-promoting IGF-1 levels to drop.
  1. Increases your IGF-1 binding protein (IGFBP) levels.

What exactly is an IGF-1 binding protein?

When IGFBP binds to IGF-1, it essentially makes IGF-1 unavailable and unable to stimulate cancer growth. In effect, IGFBP acts like the body’s ‘emergency brake,’ slowing the cancer cell growth process down.

Those who eat a plant-based diet over the long term (14+ years) appear to have 50 percent less IGF-1 in their bodies and 150 percent more IGF-1 binding protein as compared to people who consume the typical Standard American Diet.

Back to the Big Question

So now let’s return to the question of soy.

As the plant-based protein that most resembles animal protein, will soy also increase the level of the cancer-promoting hormone IGF-1?

The answer at first blush appears to be yes.

In a recent study, vegans who consumed between 7-18 servings of soy foods per day for a year experienced an increase in IGF-1 levels similar to those who ate meat.

However, a couple of caveats should be mentioned here.

First, this study had participants eating a LOT of soy.

An equivalent of 4 quarts of tofu per day, all year long.

This is much more than most of us would ever consume in the ‘real’ world.

So if you are following a varied, plant-based diet you would probably be ‘mixing it up’ with other types of beans and whole grains (not just soy).

Second, we also need to remember that an increase in IGF-1 levels is only one part of the equation.

The other part is the presence of IGF-1 binding protein because a high level of IGFBP plays a crucial role in inhibiting IGF-1’s activity.

And that is exactly what seems to be the case with soy – it contains isoflavones, which greatly bump up the production of IGF-1 binding protein, thus counterbalancing the risks associated with higher IGF-1 levels.

How Much Soy Is Safe to Eat?

So in this last study, people who consumed tons of soy (7-18 servings a day) had their IGF-1 levels go up.

But what happens if you eat less?

Is there a ‘safe’ level when it comes to soy consumption?

A ‘magic number’ where we can gain the benefits of soy without any ill effects?

Let’s look at what the research tells us.

A 2-year study shows that when eating two servings of soy per day (i.e. tofu, soy milk, soy nuts or concentrated soy isolate found in protein bars or protein powder), your IGF-1 levels seem to not budge.

Equally, another report suggests that three servings per day will not affect IGF-1 levels.

However, a third study where participants ate 5-10 servings per day did show an increase in IGF-1 levels. (And we already know what happens when eating 7-18 servings per day.)

Given all of that, we can deduce that in order to keep your IGF-1 dial from moving in the wrong direction, you should aim to eat no more than three (maybe 5) servings of soy food per day.

How does that translate to your plant-based diet?

One serving of soy foods (~7 g protein) would correspond to one cup of soymilk, 1/2-cup cooked soybeans, tempeh, tofu, edamame, or soy-based meat substitutes, or 1/3-cup soy nuts.

As a side note, certain compounds (goitrogens) in soy, cruciferous vegetables (e.g. broccoli, kale, Brussels sprouts, cabbage, cauliflower), strawberries, peanuts and flaxseeds may disrupt thyroid hormone production by interfering with iodine uptake, but only when eaten uncooked, in large amounts, and combined with inadequate iodine intake. Anyone with a good source of iodine may eat soy foods in the recommended amounts without it causing thyroid problems.

Soy and Breast Cancer

Now let’s turn our attention to one of the most important topics related to soy consumption: breast cancer.

We have seen that when soy is eaten in large quantities it may lead to increased IGF-1 levels.

We have also discussed that this increase may partially be counterbalanced by soy phytonutrients, which seem to boost IGF-1 binding protein, thus impeding IGF-1 to do its work.

So all of this begs the question, does soy cause cancer?

And more specifically breast cancer?

The evidence is clear: soy foods (not supplements) seem to protect against breast cancer.

Study after study shows that women who eat soy enjoy a much lower risk of breast cancer as well as a better chance of surviving it compared to those who eat little or no soy.

Furthermore, women who are at increased genetic risk when it comes to breast cancer may benefit the most from eating soy.

Three separate studies demonstrated that breast cancer survivors who eat soy appear to experience a much lower chance of cancer coming back (i.e. recurrence)—at least 36 percent lower risk to be exact.

In one study, 100 percent of the soy-eating breast cancer survivors were alive after two years while 30 percent of the non-eating soy women had died. After five years, 90 percent of the soy eaters were still around, while 50 percent of the non-soy eaters were gone.

One of the explanations, why soy seems to improve breast cancer survival and decrease recurrence, is that the phytoestrogens in soy seem to ‘compete’ for the same receptors as estrogen.  Given that estrogen appear to stimulate the growth of more than half of all breast cancer cases, when some of these soy compounds bind to estrogen receptors, they ‘block’ estrogen from doing its job.

Soy may also encourage the expression of BRCA (BReast CAncer) genes.

BRCA are cancer-suppressing genes responsible for DNA repair. Even though BRCA1 and BRCA2 are present (and intact) in most women, they can be switched ‘off’ or ‘down’ in breast cancer cases. The phytonutrients in soy apparently have the effect of turning that protective BRCA switch back on.

In fact, a study published just two months ago showed that long-term consumption of soy foods seems to cause cancer suppressor genes (i.e. those helping prevent cancer) to be turned on/up while oncogenes (i.e. those causing cancer) appear to be turned off/down in breast cancer tissues.

Puberty, Menopause, and Soy

The age of puberty continues to decline dramatically.

This is important because girls who start puberty earlier are at greater risk of breast cancer later on in life due to an increased exposure to estrogen.

For every year puberty is delayed, the risk of future breast cancer is reduced by as much as 7 percent.

And soy may help stave off the onset of puberty.

In a study of 3,000 girls, researchers observed a very strong association between meat intake and an earlier onset of puberty. However, those girls who drank two cups of soymilk per week seemed to delay it; on average they developed breasts at least eight months later than their non-soymilk drinking counterparts.

Similarly, in the majority of studies covering menopausal women and soy foods, soy consumption was found to significantly reduce hot flashes.

Soy: the Best Bean?

Without a doubt, legumes should have a welcome place in your diet. They are power-packed with nutrition and flavor.

People who eat legumes live longer. They have lower blood pressure, blood sugar, cholesterol, triglycerides, and better kidney function.

As we have discussed at length in our articles The Brilliant Bean and Eat Your Beans!, there is a great variety to choose from including beans, chickpeas, split peas, soybeans, and lentils.

Soy is an excellent choice.

But it is certainly not the only option—or even necessarily the best one.

Eat an assortment of legumes and feel free to include soy in the mix.

Keep things varied, and you will never go wrong.

The bottom line is this.

If you are currently eating soy foods, you may continue to do so. If you avoid soy out of fear that it might be harmful to your health, you can rest assured that integrating it into a varied, plant-based diet is fine.

But wherever you are on the soy continuum, just remember to keep your consumption to less than three servings a day.

Rosane Oliveira, DVM, PhD

President & CEO, Plant-Based Life Foundation | Dr. Rosane Oliveira combines a lifelong passion for nutrition with 25 years of genetics research to create programs that help people develop healthy habits on their journey towards a more plant-based lifestyle. She is a Visiting Clinical Professor in Public Health Sciences and was the founding director of the first Integrative Medicine program at the UC Davis School of Medicine. She completed her postgraduate studies in Brazil and did her postdoctoral training in immunogenetics and functional genomics at the University of Illinois at Urbana-Champaign.