The Great Soy Debate

...The pros and cons of adding soy to your diet.

by Max Wettstein, Copyright 2005

    There seems to be a lot controversy lately regarding the pros and cons of soy.  I’ve been using soy milk, edamame, tofu, and a few soy-protein energy bars for about 6 years now to supplement my normal diet, with no noticeable negative effects, (I’m not a vegetarian, but wanted the benefits of soy).  It was right around this same time frame that soy products really became popular in nutrition circles.  Several studies of Japanese diets, rich in soy beans and soy products suggested a link to many health benefits including but not limited to, less obesity, lower cholesterol, lower blood pressure, lower cancer rates, as well as treatment of menopausal symptoms, when compared to these same rates in Western cultures.  Surely some of these benefits were a contribution of a diet that was much higher in fish and essential omega fatty acids as well.  Lately however, there is some buzz going around that too much soy may have negative consequences and even feminizing effects in men.

    The specific compound in soy that is at the center of interest is the Isoflavone.  Many plant foods are rich in isoflavones, mainly legumes, but soy beans especially are by far the highest in content.  What exactly is an isoflavone?  An isoflavone is class of micronutrient that falls under phyto-estrogens.  Phyto-estrogens are molecules found in plants and vegetables we eat that are very similar to the endocrine hormone estrogen produced in our bodies.  However phyto-estrogens are ‘weaker’ and generally produce only positive health effects when consumed.  Their molecular structure is so similar, that these phyto-estrogens will even bind to tissue-specific estrogen receptor sites in our bodies.  However, there are other synthetic estrogen-like compounds that make their way into our diets, through non-organic animal products, prescription drugs and steroids, and other toxins we’re exposed to, that can be harmful to our health and shift our endocrine system out of balance.  Phyto-estrogens and isoflavones can block these bad, synthetic estrogens.  Isoflavones are also known to be powerful anti-oxidants, binding up destructive free-radical molecules and combating oxidation. The two primary isoflavones found in soy are Genistein and Daidzein, and they are at the heart of all nutrition discussions involving soy and our health. 

    The benefits of soy isoflavones as an anti-oxidant are widely accepted, along with its LDL-cholesterol lowering power.  Also in favor of including soy in your diet, if you’re a man, is its contribution to prostate health.  The latest clinical studies reported in the Journal of Nutrition have shown that supplementing with 30 to 50 mg of soy isoflavones Genistein and Daidzein, or consuming other soy foods in your diet can moderately lower risk of prostate cancer.   We can accept these as established health pros for the purposes of this article so we can move on to the more controversial issues, such as possible feminizing effects in men, and estrogen-related cancers in women.  Discussing the latter is really beyond the scope of this article and my realm of comfort for that matter.  But I would like to comment that in every nutrition journal and clinical abstract I researched prior to writing this, I found nothing stating that consuming soy may increase risk of or aggravate estrogen-related cancers in women, such as breast cancer.  In fact in one study posted on, of Japanese women who had a diet rich in soy isoflavones and phyto-estrogens, this group of women had 4 times lower rates of breast cancer than women in the U.S., and their menstruation cycles were longer on average, a factor associated with lower breast cancer rates. 

    It should be clear by now that a major factor in the role soy isoflavones play in the body is directly related to the hormone estrogen, produced by both men and women.  Once again this is because isoflavones are in fact a class of phyto-estrogens, so they can act as estrogens would in our bodies once eaten.  It is for this reason that controversy abounds that eating too much soy will cause men to develop breast tissue, experience a decrease in lean body mass, and more significantly possibly increase risk of estrogen-related cancers in women.  It is true that eating large amounts of soy foods every day - we’re talking 4 servings are more, (50 mg or more of isoflavones every day) – will increase levels of estrogen in your body, possibly enough to slightly lower testosterone levels in men.  But clinical studies have shown that isoflavones/phyto-estrogens seem to only produce the positive effects of the estrogen hormone, and none of the negative effects.  In fact as a man, unless you are a professional bodybuilder or professional athlete, chances are you would not even notice a slight increase in estrogen, which could only happen if you were super-supplementing with soy isoflavones every day.  What you should consider as a man approaching his fifties and beyond, is that soy isoflavones also lower levels of the nasty metabolite of testosterone, Dihydrotestosterone, (DHT).  And it is DHT that can aggravate prostate problems and accelerate male-pattern baldness in aging men.  Soy isoflavones may also block the enzyme aromatase from converting testosterone to estrogen.  And for you middle-aged gals considering supplementing with soy isoflavones, clinical studies have shown slight-to-moderate relief from menopausal symptoms, including but not limited to fewer ‘hot flashes’ and lowered risk of osteoporosis.

    The jury is still out on soy, and it is a hot topic right now in nutrition science.  But based on the latest research soy should not be avoided and perhaps included as part of a healthy diet in moderate amounts – about 40 mg of isoflavones daily.  Soy can help you lower LDL-cholesterol, is a powerful anti-oxidant, is a great source of protein, (especially for vegetarians), is good for prostate health, and can decrease menopausal symptoms – this much is generally accepted in the nutrition community.  This should go without saying, but if you have an estrogen-related cancer, or your family has a history of it, obviously speak with your Oncologist or GP and follow your doctor’s recommendation.

    Lastly, for those of you more athletic types who supplement your diet with protein powder, soy is a good choice to vary with whey or egg-white.  Whey has a biological value, (BV), of 104.  Egg-white has a BV of 100, casein a BV of 77, and soy protein powder averages a BV of about 74.  Whey is the best for quick absorption and synthesis, and has the most anabolic and immune benefits, while egg-white is secondary, followed by casein.  But with soy protein you get the isoflavones and anti-oxidants and it is slower digesting, so it can be a good anti-catabolic alternative, and soy is one of the few protein powders on the market today that is not animal-derived.  Soy milk has come a long way too, and is lactose, hormone, and antibiotic free, is calcium fortified, and usually ‘organic’.  - A good alternate to cow’s milk.  Silk is what I use and it tastes great – good in your coffee too.

    Note: In October 1999, FDA approved a health claim that can be used on labels of soy-based foods to tout their heart-healthy benefits.  The agency reviewed research from 27 studies that showed soy protein's value in lowering levels of total cholesterol and low-density lipoprotein.  Food marketers can now use the following claim, or a reasonable variation , on their products: "Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease."  


(Found on

    - “Dietary isoflavones: Biological effects and relevance to human health” – Journal of Nutrition, 1999; 129:758-767

    - “Hormonal effects of soy in premenopausal women and men” – The American Society for Nutritional Sciences, 2002;       132:1154-1158


-, “The (partial) vindication of soy”, by Will Brink


Back to Max Wettstein Health and Fitness Library