Updated: Jul 21, 2021
Many sources online openly link soy to progression and development of various cancers. The most common one being breast cancer in women (and I'll make a separate article about this) but also prostate and colon cancer in men. Without doing proper research, it is hard to say whether any of these claims are true or not. It is easy to base assumption on hearsay and then accept that as what is true.
So without further ado, let's see what some of the research that asked this question found. One thing to bear in mind that any research on cancer is always observational. Because it would be unethical to design a trial in which people are supposed to get cancer, researchers always observe groups of people over time who have either develop cancer already or where they participate in certain behavioural patters (e.g. smoking or high alcohol consumption) and then scientists just observe what happens in 20-30 years. I want you to bear this in mind when someone says "XYZ causes or does not cause cancer" It is not as easy as that and we need to be careful with any claims we make that are based on nothing more than a belief.
Many large studies have tried to answer the question of whether soy is contributing to or protecting men from prostate cancer. A large systematic summary (Applegate et al., 2018) pooled in data from over 266-thousand participants and found that non-fermented products (e.g. whole soya beans) were very protective, where fermented soy products (tofu, tempeh, miso, natto) had no effect (neither protective nor detrimental). Another systematic review (Zhang et al. 2016). analysing data of over 17 thousand individuals found that regular consumption of phytoestrogens (included both fermented and non-fermented soy products) provided, on average, up to 23% protection from prostate cancer.
Overall based on the studies we have so far, soy products seem fairly protective from prostate cancer and are definitely not harmful in this case.
This cancer affects the final part of digestive tract called the colon (large intestines) as well as the rectum. It is the third most common cancer in men. Men living in USA, Europe, Australia & New Zealand are at greater risk than men in less developed regions of Africa and Asia which suggests there is a strong lifestyle factor, one of which is lack of dietary fibre. Numerous studies have tried to identify the link between CRC and lifestyle. So far, we can tell, with reasonably good accuracy, that general contributors are: alcohol, smoking, high salt and high processed meats (e.g bacon salamis, sausages etc.) consumption. The link between red meat and colorectal cancer is not as clear yet and still remains debated.
In terms of soy, a large study on men and women from India, Malaysia, Egypt, China, Japan, Singapore and Saudi Arabia (Azeem et al. 2015) found that regular consumption of soy has been protective. It appears soy is even more protective in non-Asian men. Due to the nature of the soy-rich diet in many Asian countries, this is where most of the studies are done and these are consistently showing protective effects of soy from colorectal cancer.
Following is a summary of different studies done across the world observing men and women and the relationship between soy & colorectal cancer (Yu, et al. 2016). This is not an exhaustive list, there are many more but a drastic majority are showing beneficial effects of soy products in colorectal cancer.
There was, however another major trial from Asia that looked at data from 22 studies accounting for over 965 thousand people across Asia & Europe. And while they found soy products being primarily protective from colorectal cancer, an interesting finding was that Japanese soup containing a fermented soybean paste called miso seemed to be linked to an increased rate of gastric (stomach) cancer. It appears that this is because of the high content of sodium and that fermented soybeans in Japan are often treated with nitrosamine, a known carcinogen compound.
Despite this one finding, soy products both fermented and non-fermented, remain largely protective from colorectal cancer and are an excellent safety policy for men & women with a familial history of this disease. Ideally, the focus should be put on whole soya beans, tofu or tempeh and where miso is used, it should not be excessively concentrated and should come from organically grown and untreated sources.
If you read my previous blog on soy & hormones, you would have by now heard about oestrogen (a sex hormone) and oestrogen receptors (docking stations for oestrogen). You may recall that there are two types of oestrogen receptors; alpha and beta. Alpha being somewhat more carcinogenic, and beta being somewhat protective (of course, it is not as simple as that 😉)
It has been found that lungs harbour a large amount of Oestrogen receptors, and thus it has been thought that if something in food could stimulate more docking of oestrogen into oestrogen receptors, it could be causing oestrogen-driven cancer. From this theory came the idea that soy could contribute to certain cancers (such as lung or breast cancer) because it was (incorrectly believed) that soy increases the amount of oestrogen in the body, but as you may have read in Part 1, this is not exactly true.
However, a small number of studies have explored the topic of “does soy contribute to lung cancer?” This is what they found. A large study conducted between 1997-2000 called the Shanghai Women’s Health Study monitoring health of 75 thousand participants found that those who consumed the highest amount of soy product has the best protection from lung cancer. The higher the daily consumption, the lower the risk of disease. Those women consuming on average more than 27g of soy per day had the risk reduced by 50% (THIS IS SIGNIFICANT!) Similar findings were replicated in another large trial ( Wu & Liu, 2013) but what has been interesting is that this protection is lost in smokers and in these patients, the soy consumption was not adding any level of protection from lung cancer, possibly due to the highly pro-inflammatory and carcinogenic effects of smoking on lungs where damage is more significant than any food can mitigate.
WHAT ABOUT GMO?
Genetically modified crops (maise, wheat, soy, potatoes etc.) have been suspected to be behind the rise of allergies and even disruptions to the human microbiome. Equally use of aggressive glyphosates that have been linked to newborn disease is a problem as well. But it is important to understand that this topic does not only relate to soy. Many other products are being treated in a similar way.
In Europe & UK, there is still a largely ban on GMO crops with the exception of Spain & Portugal that grow GMO corn called MON 810. (Link) In most countries, foods labelled as Organic should protect the consumer from heavy use of pesticides and from genetic modification
In the US, the growth of GMO crops is more widespread, and pretty much most of the following products are GMO-grown: soy, potatoes, sugar cane, canola, apple, papaya, alfalfa, squash. This is an unfortunate truth for readers based in America. But there is hope. According to the US Department of Agriculture, products labelled and certified as Organic cannot be subjected to genetic modification, and as such, the recommendation is to invest in organic produce. In terms of soy, this means buying organic tofu, tempeh, miso and, where possible organic soybeans. That being said, even if the budget does not allow organic consumption all the time, the benefits of consuming natural foods usually outweigh the cost.
In terms of Asia, Africa and the rest of the world, I am not too familiar with the local guidelines and have not researched this bit as I know the majority of the readers are based in the US, Canada and Europe. For those based in other countries, I'd encourage you to become familiar with local policies about GMO and Organic produce to determine what is safe.
SUMMARY OF KEY POINTS
So far there is no evidence that soy causes either prostate cancer, colorectal cancer or lung cancer. Majority of studies show protective effects and some show no effect (neither protective nor detrimental). Besides phytoestrogens (see previous blog to read more about them), soy products are a complete protein (contain all 9 essential amino acids) which makes them excellent protein source for vegans and vegetarians. Soy products are rich in various antioxidants and polyphenols all having positive health benefits. Soy is rich in fibre, calcium, magnesium, manganese, phosphorus and certain vitamins. It is also cheap which makes it an excellent budget food.
Finally, there is the question of GMO crops & glyphosate pesticides. Those living in Europe & UK are (for now) protected from this. Those living in US, Canada & LATAM would do well to invest in organic produce where possible for additional safety and those living in Asia, Africa, Oceania + AUS & NZ would do best to familiarise themselves with the regulations and agricultural standards in their country as this is something I have not explored.
Personally, I buy organic where I can and if my budget allows it. But if there is no option or if the cost difference is too significant or if the organic goods are imported across the entire world I opt for non-organic. Local produce is always preferable so that we respect the cost to the environment as well. In the end even consumption of even non-organic produce has a variety of health benefits usually outweighing any potential risks and regular consumption of natural plant foods is protective nearly from all disease known to man.
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SOURCES (GMO & PESTICIDES)
SOURCES (RESEARCH & STUDIES)
Applegate, C. C. et al. (2018) ‘Soy consumption and the risk of prostate cancer: An updated systematic review and meta-analysis’, Nutrients, 10(1).
Azeem, S. et al. (2015) ‘Diet and colorectal cancer risk in Asia - A systematic review’, Asian Pacific Journal of Cancer Prevention, 16(13), pp. 5389–5396.
Desmawati, D. and Sulastri, D. (2019) ‘Phytoestrogens and their health effect’, Open Access Macedonian Journal of Medical Sciences, 7(3), pp. 495–499.
Van Die, M. D. et al. (2014) ‘Soy and soy isoflavones in prostate cancer: A systematic review and meta-analysis of randomized controlled trials’, BJU International, 113(5 B).
Gong, Y. et al. (2012) ‘Soy food intake and risk of lung cancer: Evidence from the shanghai womens health study and a meta-analysis’, American Journal of Epidemiology, 176(10), pp. 846–855.
Hackshaw-McGeagh, L. E. et al. (2015) ‘A systematic review of dietary, nutritional, and physical activity interventions for the prevention of prostate cancer progression and mortality’, Cancer Causes and Control. Springer International Publishing, 26(11), pp. 1521–1550.
Hooper, L. et al. (2009) ‘Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: A systematic review and meta-analysis’, Human Reproduction Update, 15(4), pp. 423–440.
Lu, D. et al. (2017) ‘Meta-analysis of Soy Consumption and Gastrointestinal Cancer Risk’, Scientific Reports, 7(1), pp. 1–14.
Wong, W. C. W. et al. (2012) ‘Isoflavones in treating watchful waiting benign prostate hyperplasia: A double-blinded, randomized controlled trial’, Journal of Alternative and Complementary Medicine, 18(1), pp. 54–60.
Yu, Y. et al. (2016) ‘Soy isoflavone consumption and colorectal cancer risk: A systematic review and meta-analysis’, Scientific Reports, 6.