The soybean is a type of legume that originated in China around 1100 B.C, but it wasn’t until the 20th century where soy production really took off around the globe. 

It is an extremely nutrient dense food with high amounts of protein, fiber, B vitamins, potassium and magnesium. Considering it’s one of the few plant foods that has a higher protein to carbohydrate ratio, it should be a staple in any plant based diet.

Despite this, soy has received an undeserved bad reputation over the last few decades. The purpose of this article is to dispel many of the myths that have continued to propagate despite a severe lack of evidence.

As a note, this article is going to get into the weeds quite deeply so if you just want the ‘TLDR version’ simply skip to the summary.

Let’s dive in!

Why So Conflicting?

A lot of the controversy appeared to have occurred between 1998-2009 which explains why less than half of baby boomers consider soy a healthy food in comparison to 68% of generation Z-ers who weren’t exposed to this information as adults.

Funnily enough, I myself was in the same boat for a long time! Especially with regard to testosterone and it wasn’t until I put together this article that I discovered how much BS there was surrounding soy, specifically the isoflavones.

Isoflavones are compounds exclusively found in plant foods (such as soybeans) that can bind to estrogen receptors in the body. This is why they are commonly referred to as phytoestrogens (phyto means “from plants”) and why they have been erroneously vilified and accused of ‘acting like estrogen’ in the body.

Many of the studies demonstrating soy isoflavones to have negative effects came from early animal research that suffered from huge errors.

For instance, these compounds were studied in vitro (in petri dishes or test tubes) which is not how these compounds are consumed in the real world. Nutrients that are consumed together interact differently than when in isolation.

Finally, isoflavones are metabolised differently in animals vs humans. Research conducted in humans has consistently shown no adverse effects of isoflavones and in fact, have shown to be positively associated with:

• Reducing risk of cardiovascular disease.
• Improving oxidative stress & inflammation.
• Improving cholesterol markers.
• Lowering blood pressure.
• Improving hot flashes in menopausal women.
• Depression.
• Cognition.

With that out of the way, let’s get into the myths.

Soy & Breast Cancer

Considering the benefits of soy and soy isoflavones on managing inflammation and mitigating oxidative stress, it’s difficult to find a plausible mechanism as to how they would promote cancer growth.

One proposed theory was that because soy isoflavones are phytoestrogens, they can bind to estrogen receptors and promote the growth of specific cancers. These are known as “Estrogen Receptor Positive” breast cancers which use circulating estrogen to grow.

The thing is, soy isoflavones are between 10-14,000 x lower in potency in comparison to estradiol meaning if they were to bind, they would actually have more of a protective or neutral effect. This is why when we examine the evidence, women who consume soy appear to have much better outcomes overall.

“With an average intake of soy isoflavone above 17.3 mg/day, the mortality of breast cancer can be reduced by about 38-36%.”

A very recent meta analysis examined 81 prospective cohort studies and found that soy consumption was positively associated with a reduction in cancer by up to 10%. 

Another interesting finding was that for every 25g increase in soy consumption per day, cancer incidence were reduced by 4% meaning the individuals consuming the most had the highest level of protection.

Molly Adams, MD is a breast cancer survivor herself who promotes the consumption of soy for all of her patients.

You can read her thoughts on the very topic here.

Soy & Testosterone In Men:

Most of the early research here was done on rats which did show a clear correlation between isoflavone intake and a reduction in testosterone.

Additionally, a case study that looked at one individual who consumed 360 mg of isoflavones per day (almost 5 times the recommended amount) showed a large reduction in testosterone, an increase of estrogen, gynecomastia (man boobs) and erectile dysfunction.

More recently though, a meta analysis that examined 41 peer reviewed studies between 2010-2020 concluded that there just isn’t any strong evidence to support these claims.

“Regardless of the statistical model, no significant effects of soy protein or isoflavone intake on any of the outcomes measured were found. Sub-analysis of the data according to isoflavone dose and study duration also showed no effect. 

This updated and expanded meta-analysis indicates that regardless of dose and study duration, neither soy protein nor isoflavone exposure affects TT, FT, E2 or E1 levels in men.”

Soy & Thyroid

Current evidence shows no impact of soy/isoflavone consumption on T3/T4 (active and inactive forms) but it is mixed on Thyroid Stimulating Hormone (TSH).

Importantly, those who are suffering from hypothyroidism do need to be concerned as levothyroxine (medicine used to treat an underactive thyroid) absorption can be inhibited by soy if consumed in the same meal.

This doesn’t mean you need to avoid soy as for most individuals, levothyroxine (medication for those with hypothyroidism) is encouraged to be consumed on an empty stomach regardless. 

If this is a potential concern, aim to have your soy based meals around lunch or dinner.

What About Menopause?

One of the biggest changes that comes with menopause is the drastic decline of the sex hormones, estrogen and progesterone.

This shift can explain many of the symptoms that come with menopauses such as a loss of bone density, mood disturbances, hot flashes and sleep issues.

There have been studies showing no adverse effects of isoflavone supplementation along with others demonstrating women reporting improvement in hot flashes.

If you are going through menopause, the addition of soy can be of great benefit and shouldn’t 

be avoided.

Soy & Weight Loss

Diets that include soy protein are just as successful when it comes to weight loss when compared to whey protein. This is largely due to the positive impact on satiety, appetite regulation and helping with muscle mass retention.

Soy is the only plant based protein that has a complete amino acid profile making it a fantastic addition to a well rounded, whole food, plant based diet.

Our go-to choices include:

• Tofu
• Soybeans
• Tempeh
• Edamame Spaghetti
• Textured Vegetable Protein (TVP)
• Soy Mince
• Soy Milk/Yoghurt
• Soy Protein Powders

These are usually very minimally processed and contain a good amount of protein along with a range of vitamins and minerals. 

Try to limit the ‘mock meat’ type products as they can come with a lot of additional calories and sodium.

Can Too Much Soy Be Harmful?

The below was taken from the University Of California regarding soy.

“Numerous clinical studies have found that daily consumption of up to 50 grams of soy protein is not only safe, but may also be effective in improving risk factors for chronic disease such as

some types of cancer, diabetes, and cardiovascular disease.

There is little basis for concern that excessive amounts of dietary soy, even in those with or at high risk for breast cancer, will lead to adverse health effects.”

What does 50g of soy protein per day look like?

• 200g of firm tofu. (16g)
• 100g of tempeh. (20g)
• 100g of shelled edamame beans (12.5g)

As you can see, you need a fair amount to hit the mark!

Also, if you enjoy eating soy there is no need for concern regarding any upper limit. The only issue that can arise is a lack of other foods that may be of benefit but this is true for any food eaten to excess.

Summary

Soy consumption has consistently been associated with better health outcomes across a broad spectrum of markers such as:

• Cardiovascular health.
• Metabolic health.
• Bone health.
• Cancer prevention/ improved prognosis.
• Menopausal symptoms.

It does NOT impact your hormones in any negative way.

This includes testosterone in men.

You can still consume soy if you have a thyroid condition.

It is a great source of protein, fiber, potassium and a range of other nutrients depending on the source.

Even if you’re not vegan or vegetarian, soy should still be included as part of a healthy diet.

Get your sources from minimally processed items most of the time.

Most health organisations recommend at least 25g of soy protein with 40-80 mg of isoflavones per day but no adverse effects have been seen with higher intakes.

Based on the current body of evidence, there is no need to eliminate or avoid soy based foods unless you simply do not enjoy them!