The Truth About Soy + Cancer
Some people are afraid to eat soy because of myths about its connection to chronic disease. This three-part series cuts through the falsehoods to serve up the latest research supporting soy’s benefits for a variety of conditions, including cancer prevention!
Soy is a highly controversial food with a lot of fear-mongering surrounding its consumption.
I’ll be honest before I became a Registered Dietitian Nutritionist and started getting my information from peer-reviewed research studies instead of unreferenced blog posts, I was afraid of soy too.
There are a lot of people out there making a lot of noise about it with no evidence to back up their claims – and this obscures the facts.
Not to point any fingers…
The truth is soy not only NOT harmful, it’s actually incredibly beneficial for many conditions.
By barely skimming the surface of the research, you can quickly find an outstanding amount of evidence supporting this undisputable fact. In fact, more than 2000 soy-related peer-reviewed articles are published yearly.
But I get it. The claims against soy are scary and often convincing when they’re coming from popular nutrition “experts”. So I decided to go through the common claims against soy, one by one, to get the facts.
I’ve spent the past month doing a deep dive into the science of soy and found decades of good epidemiological studies and clinical trials supporting soy’s benefits for a variety of conditions including cardiovascular and metabolic disease, cancer, diabetes, menopause, and obesity.
Meanwhile, there’s also a ton of research shutting down the many claims against soy including reports that it has a detrimental effect fertility, thyroid health, acne, and PCOS.
However, there are some areas of soy research that are lacking, which I investigated as well.
The more I researched, the more information I found, which is why I’m breaking this topic up into a 3-part series. Today, we’re going to discuss the basics of soy and look at the association between soy and cancer.
POPULATION STUDIES ON SOY AND CANCER
Populations who consume a lot of soy like China and Japan have much lower rates of prostate and breast cancer than the United States. Meanwhile, Chinese and Japanese migrants to the US show similar rates of these conditions when consuming a typical Western diet but not when eating their traditional diets. One study showed that Japanese and Chinese women had a 60% increased risk of breast cancer if they were born in a Western society.
This shows that it’s not simply genetics providing protection against these diseases but likely the type of food consumed in these countries.
But before we dive into the studies on cancer, I want to explain the biological effects of soy.
BIOLOGICAL EFFECTS OF SOY
Soybeans contain a combination of carbohydrates, fat, and protein.
Soy is a great source of calcium and iron and despite its content of so-called anti-nutrients, studies show that absorption of these vitamins is actually quite high and usually comparable to animal products.
Some of the proteins found in soy have been shown to have their own anti-cancer properties, but it’s the phytochemicals known as isoflavones that we hear the most about.
There are two biologically active isoflavones found in soy: genistein and daidzein.
The amount of isoflavones in different soy products varies widely from high amounts in soy flour (150-170 mg/100 g) and soy protein isolate (91 mg/100 g) to lower amounts in soy milk (1-3 mg/100 g) and tofu (25-30 mg/100 g). The amount depends on growth conditions and processing.
Another factor that comes into play is our own microbiome’s processing of soy. Some individuals break down the isoflavone daidzein into a metabolite known as equol, which has higher activity than daidzein and may modify the effects of soy in different people.
Studies show 50-60% of Asians are equol producers while only 30% of Westerners have this ability.
These isoflavones are known as phytoestrogens, meaning they are structurally similar to the estrogen our body produces, and therefore, they have the ability to bind to our body’s estrogen receptors.
This is one reason some people (incorrectly) think that soy is bad.
They claim that soy binds to estrogen receptors and triggers cell growth, much like in hormone-sensitive cancers like breast, prostate, and ovarian cancer.
However, research shows that isoflavones are actually bettered described as selective estrogen receptor modulators, meaning they selectively activate different estrogen receptors in different organs.
Isoflavones have also been shown to preferentially bind to what’s known as estrogen receptor beta, which has anti-growth properties. In comparison, estrogen receptor alpha is a promoter of cell growth and it is this receptor that is activated in estrogen-receptor positive breast cancer.
In other words, soy isoflavones likely combat the effects of our own bodies estrogen in certain tissues and support in others, which you’ll see in a few moments is likely why they are beneficial for cancer prevention.
The European Food Safety Authority states that isoflavones do not negatively affect the breast, thyroid or uterus. And the North American Menopause Society also agrees that they do not increase the risk of breast or endometrial cancer.
However, as mentioned before, isoflavones are only one component of soy. Soy also contains bioactive compounds such as saponins, protease inhibitors, and phytic acid which may confer their own benefits.
So although many studies look at the health of effects of isoflavones and equate those to whole soy products, it’s not exactly a fair comparison.
So now, let’s dig into the specifics on soy and cancer.
Soy and Cancer
In addition to its effects on hormone response, soy has independent properties that directly affect cancer.
Soy has shown to regulate apoptosis, aka cell suicide, cell proliferation (which is an increase in the number of cells) and cell survival. It’s also shown to inhibit angiogenesis (the growth of blood vessels in tumors) and metastasis (the spreading of cancer). Soy also possesses antioxidant properties.
These are all important factors in the initiation and progression of cancer and are part of the reason for soy’s anti-cancer effects.
The reason people previously thought that soy potentially caused cancer was because of mixed results in animal studies.
A few old studies on mice showed that isoflavones were able to stimulate the growth of existing mammary tumors. However, we now know that animals metabolize isoflavones differently than humans and the human data does not support this outdated animal research.
What research has shown is that high consumption of soy is associated with about a one-third reduction in breast cancer risk. And some case-control studies have shown that the risk reduction is increased up to 60% when soy consumption begins earlier in life.
A recent meta-analysis of 40 randomized-controlled trials concluded that eating soy may also be associated with a lower risk of breast cancer reoccurrence and mortality.
Two more large population studies showed that soy food intake was associated with a significantly reduced risk of breast cancer reoccurrence in cancer survivors and a reduced risk of death, regardless of estrogen-receptor status.
In one study, a high intake of soy isoflavones was associated with a 60% reduced risk of cancer reoccurrence in women who were also taking the cancer drug tamoxifen. Finally, other studies have confirmed that these benefits extend to both Asian and non-Asian women.
However, studies of isolated isoflavone supplementation have not shown such beneficial results. In the European Food Safety Authority’s recent risk assessment including 43 human studies, they found no association between supplementation of isoflavones and cancer risk. Therefore, to reap the cancer-fighting and preventing benefits of soy, it appears best to consume it in a whole food form.
Soy consumption is also associated with reduced risk of prostate cancer.
A 2009 meta-analysis of 15 epidemiological studies showed that soy consumption was associated with a significantly reduced risk of prostate cancer.
Another 2014 meta-analysis of eight randomized controlled trials also found that soy consumption was associated with a 51% reduced risk of prostate cancer. It also found no effect on male sex hormones, another common concern associated with soy.
In fact, the rumors about soy affecting fertility are so rampant, that I made it the main topic of my next video!
So we’ll leave here today.
- Consuming whole soy food is likely better than supplementation for preventing cancer.
- The benefits of eating soy are NOT exclusive to Asian populations
- The earlier you start eating soy, the better.
I hope you guys enjoyed this article and that it has alleviated some of your concerns surrounding soy.
As I said mentioned, my next two videos/articles will cover more soy myths including the claims that soy causes infertility, so-called “man boobs,” PCOS, acne, and thyroid dysfunction.
Check out the next two posts in the series! >>
I’ll also be diving into the issue of genetically modified soy. G-M-Oh my!
If you have any questions, please leave them in the comments section below. And make sure you SUBSCRIBE to my YouTube channel so you don’t miss the next videos!
Want more nutrition myth-busting? Check out these posts:
- The Truth About Coconut Oil
- The Research Behind Fasting + Fasting Mimicking Diets
- The MIND Diet – An Eating Pattern to Protect Your Brain
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Weigh In: Do you eat soy? What myths have you heard about it?