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Soy and Cancer, To Eat or Not to Eat?
According to the American Institute of Cancer Research, about 67% of cancers could be prevented by following a healthy diet, being active, maintaining a healthy body weight and avoiding tobacco use. When it comes to a healthy diet, some things seem straight forward; more vegetables and fruit in general, more crucifers like broccoli, more fish, less processed foods, even more vitamin D seems acceptable but nothing is more controversial in the diet and cancer dialogue than that of soy. To eat, or not to eat? That is the question.
What is soy?
Soy is a legume and is related to peas, alfalfa and clover. Early records show it was first cultivated in north-eastern China more than 3,000 years ago where it was traditionally made into fermented miso, tempeh, soy sauce, and tofu and eaten boiled as edamame. Of course recipes and preparations have evolved over time with slight variations from region to region.
As a legume, soy is rich in protein and contains all eight essential amino acids making soy a suitable food source of protein. Soy beans also contain carbohydrate, the plant form of the omega-3 fat alpha-linolenic acid and several vitamins and minerals. Soy also contains polyphenols called isoflavones which have been dubbed ‘phyto, or soy, estrogens’. What sets soy apart is that of the over 300 foods that contain phytoestrogens, soy is the richest source.
East versus West. How much soy is really eaten?
Soy is commonly consumed in China, Japan and Indonesia. While Asians don’t eat large amounts of soy like we in the West think they do, consumption is relatively higher. Compared to Westerners, the average intake of soy in Asian is about 40 to 65 g per day compared to less than 1 g in non-soy eating countries. To give an idea of how much that is, ¼ cup of boiled soy beans, roasted soy beans (‘soy nuts’) or just under 1/3 of a cup of firm tofu is 50 g,
What does the research say?
Concerns about the safety of soy in regards to breast cancer come from research that looked at the impact of isolated soy isoflavones on breast cancer cells in culture dishes and to a lesser extent research using mice; this kind of research cannot be applied directly to what happens when humans eat whole soy foods.
The majority of studies looking at the role of soy in breast cancer prevention have found that soy either has no impact on breast cancer risk and some have found soy consumption to be inversely related; in other words, eating more soy is associated with a lower risk of developing breast cancer.
The concern that most people have, and the question that fuels the soy and cancer debate, is whether or not soy should be avoided by those with a history of breast cancer. The bulk of the better quality research, prospective cohort studies involving tens of thousands of individuals, as opposed to case control studies, have found soy to be protective against breast cancer recurrence and reduced the risk for breast cancer mortality. None of found an increased risk.
Possible benefits of soy
Soy isoflavones have a very mild estrogen ‘like’ activity which allows them to bind to estrogen receptors on cells helping to block and temper the action of the body’s natural estrogen reducing the possible cancer promoting effects of estrogen such as cancer cell growth. This is the same mechanism as the drug tamoxifen.
• Soy isoflavones, like all polyphenols, help to reduce inflammation which is associated with increased cancer risk.
• Soy also helps to block the formation of new blood vessels which helps to prevent early cancer cells from receiving the nutrients they need to grow; a process referred to as angiogenesis.
• Soy also increases the production of sex-hormone binding proteins in the blood which help to bind natural estrogen, keeping it ‘locked up’ of sorts, thereby limiting the amount of estrogen circulating in the blood stream.
• Lastly soy helps to reduce the activity of the enzyme aromatase; the enzyme that is need for the synthesis of estrogen. Reduced aromatase activity means less estrogen production.
What to do?
The best research suggests that soy may reduce the risk for breast cancer; at the very least, soy has been shown to increase the risk. Regarding the safety of soy on breast cancer recurrence and mortality; soy has been found to be safe with some of the studies demonstrating a protective role.
Supplements of isolated soy flavones are not the same as eating whole soy beans or products made from them. The bulk of the research looking at soy flavones supplements has not been good. It is for this reason that Health Canada has issued a warning not to use isoflavone supplements for anyone who currently has, or has a history of breast cancer or for those of whom have a predisposition to breast cancer.
Doug Cook, RD MHSc CDE is a Registered Dietitian/Nutritionist and Certified Diabetes Educator who currently works at St. Michael’s Hospital in Toronto, and as a nutrition consultant. He practices a holistic and integrative approach providing science-based guidance on food and diet along with nutritional supplements and natural health products where appropriate. He co-authored Nutrition for Canadians for Dummies (Wiley, 2008). In addition, he has served as the nutrition expert for the Ministry of Health’s website: healthyontario.com. Visit his website: http://dougcookrd.com.