Phytoestrogens and Strength Training – Isoflavones

Phytoestrogens and Strength Training - Isoflavones (FI)

As a reader of Strength Cooperative, you probably have specific strength training goals you are working towards, such as getting stronger in the big three lifts (squat, deadlift, and bench press), building some muscle, and maybe losing excess body fat. In order to do this, you train hard and smart in the gym, emphasize recovery, track changes in your body fat, and eat a nutritious diet. What if part of your diet was hindering your gains though?

Phytoestrogens are dietary estrogens derived from certain plant sources, such as flax seeds or soy, and since they affect the bodies endocrine (hormonal) system, there is a potential for phytoestrogens, along with other xenoestrogens, to have a direct effect on the bodies endocrine system, and by relation, testosterone levels. If you regularly eat foods which contain phytoestrogens, you might be lowering your natural testosterone levels without even knowing, and that will change how you respond to strength training drastically. Less testosterone = less gains.

There are four types of phytoestrogens, which are isoflavones, stilbenes, lignans, and coumestans, and in this article we will review the science behind isoflavones and determine whether there is a risk of disrupting your endocrine system by consuming foods high in isoflavones. Lets begin.



Isoflavones are the strongest of the four phytoestrogens in exerting estrogenic effects on the endocrine system, and are naturally found almost exclusively in legumes, particularly soybeans. In soybeans, isoflavones are bound to a sugar molecule, known as glycosides, but fermentation and/or digestion of soy products results in the release of the sugar molecule from the isoflavone glycoside, leaving an isoflavone aglycone. Considering this article will not cover isoflavone glycosides but instead only isoflavone aglycones, only those versions of isoflavones will be referenced. The soy isoflavone aglycones above are genistein, daidzein, and glycitein.

Isoflavones in Foods

Legumes, especially soybeans, are the richest sources of isoflavones in the human diet. A lot of common foods contain isoflavones, such as whole wheat bread (0.02 mg/100g), lima beans (0.03 mg/100g), or pinto beans (0.27 mg/100g), but the amounts are relatively benign. If lima beans or pinto beans are in the 132 lb weight class in terms of isoflavone content, then soybeans are the equivalent of super heavy weights. Soy based foods such as soy protein concentrate (102.07 mg/100g), soybeans (144.99 mg/100g), tofu (31.1 mg/100g), and other soy food products have isoflavone concentrations up to four orders of magnitude greater (10,000x) than other grains and legumes such as the foods listed earlier, and isoflavone concentrations 100x greater than certain beef products (1.86 mg/100g)! Additionally, the beef products tested likely have these levels of isoflavones because the cattle were fed soy, and unfortunately there is no information I can find on the isoflavone content of grass-fed beef from pasture raised cattle (but I bet it is 0.0 mg/100g or close). Read more of the isoflavone content of common foods for a complete view, but to sum up, soybeans are by far the primary source of isoflavone phytoestrogens in the modern diet. Of the three isoflavones tested, genistein is the most prevalent isoflavone, usually making up more than half of the total isofalvone content of soy products, with daidzein and glycitein making up the remainder in various percentages.

A high-soy diet is characterized at roughly 25 or more grams per day, and considering that only 0.88 oz of tofu or an 8 fl oz glass of soymilk places you at or over the “high-soy diet” 25 g/day figure, I wouldn’t be surprised if the daily consumption of soy is higher than 25 g/day for a significant portion of the population.

Since the early 1990’s, soy consumption in the U.S. has increased dramatically, with soy food sales rising from $300 million in 1992 to $4.5 billion in 2013. This data would imply an increasing consumption of soy products for the average American, and it’s unfortunate that a recent study hasn’t been done to characterize the amount of soy products/ingredients eaten on average daily basis by Americans. In addition, this data is also only referencing soy only products such as tofu or soy milk, and does not take into account the massive increase in soy consumption through commonly encountered soy food additives or ingredients such as “textured vegetable protein”, soy meal, or soy lecithin (an emulsifier).

Note – Soybean oil, a commonly consumed soy product in processed foods, does not contain any isoflavones (0.00 mg/100g).

The Effects of Isoflavones on the Endocrine System

There are three naturally occurring estrogens, estrone, estradiol, and estriol, and estradiol is the most estrogenic of the three, specifically 17-β-estradiol. All three isoflavones, genistein, daidzein, and glycitein, have the ability to attach to estrogen receptors, but genistein has the strongest affinity to attach by a wide margin. The affinity of the isoflavone genistein to the β estrogen receptor (the estrogen receptor which when bound exerts the strongest estrogenic effects upon the body) is comparable to the affinity of 17-β-estradiol (Kuiper et al. 1998, Morito et al. 2001). The affinity of other isoflavones, such as daidzein and glycitein is approximately 100-500 times lower than that of 17-β- estradiol.

In this study comparing the effect of dietary soy supplementation compared to estrogen and placebo on menopausal symptoms, a low does hormone therapy (HT) pill containing 1mg estradiol and 0.5mg norethisterone acetate (a type of birth control) was compared to a dietary supplement containing 90 mg of isoflavone and also to a placebo. The study found that after 16 weeks, menopausal relief from hot flashes, muscle pain, and vaginal dryness was significantly reduced in the HT group and  reduced equally as well in the isoflavone group. No change was observed in the placebo group. A typical birth control pill will contain 1 mg of estrogen, usually in the form of ethinyl estradiol, and this dosage is strong enough to have major hormonal impacts on women (who already have high amounts of estrogen in their system, especially compared to men). Looking at the ingredients, the HT pill that was used in the study is effectively birth control, and the isoflavone supplement was just as effective in hormone replacement therapy for the sixty healthy postmenopausal 40-60 year old women compared to the HT pill.

Another study showed that consumption of 65+/-9.4 or 129+/-16 mg/day of isoflavones was able to decrease estrogen synthesis in premenopausal women compared to the control group, who only consumed 10+/-1.1 mg/day of isoflavones. At first this might sound like a positive for guys who might want to reduce estrogen levels, but what it really means is that the supplemented isoflavones were so effective in producing estrogenic effects, that the bodies of premenopausal women (who have estrogen levels 2.5 to 7.5x higher than men) started to down regulate estrogen production as a result of isoflavone supplementation, because estrogen no longer needed to be produced naturally to the same degree. This is the same way the human body will down regulate or shut down testosterone production when taking supraphysiological doses of testosterone or other steroids.

Birth Control Pill

Consider the following from the recently published study, titled “The physiological actions of isoflavone phytoestrogens”:

“The effect of isoflavones also depends on the level of endogenous estradiol, since isoflavones and estradiol are competing for their binding on estrogen receptors (ER). In a state of high levels of endogenous estrogens (e.g. women in the follicular phase of the menstrual cycle), isoflavones may obstruct full estrogen activity by occupying a part of the ERs. On the other hand, in a state with low levels of endogenous estrogens (men, women in menopause, after ovariectomy, etc.), the estrogen activity of isoflavones may become manifest (Kuiper et al. 1998, Lephart et al. 2002, 2005).”

The bolded sections speak for themselves, and the fact that the study even mentioned how phytoestrogens could potentially affect the endocrine systems of men is amazing. The scientific community is seemingly in denial over the so far anecdotal deleterious effect of soy intake on men, and there is no rush to perform any studies examining the effects of isoflavone or any phytoestrogen consumption on male hormonal levels and strength training. If you are looking to optimize natural testosterone levels, or even just maintain the status quo and continue to make progress in the gym, soy intake could be a big problem.

At sufficiently high levels (over about 100 nmol/L for genistein), the effect of isoflavones may approach the effect of endogenous 17-β-estradiol at its physiological level (Kuiper et al. 1998), and it has been demonstrated that isoflavones are absorbed well. For adults who consumed soy products containing ~50 mg of isoflavones, plasma concentrations of 1.4–4 μmol/L isoflavones are obtained between 6 and 8 h in adults (studies 1, 2, 3). 100 nmol/L of genistein, the amount needed which may approach the effect of the potent estrogen 17-β-estradiol, is equivalent to 0.1 μmol/L, so via the three above isoflavone blood plasma concentration studies, consuming ~50 mg of isoflavones is able to increase plasma concentrations of isoflavones 10x higher than they would need to be to approach the effect of endogenous (naturally occurring) 17-β-estradiol at its physiological level.

Genistein has a longer half-life than other isoflavones (~8 hrs compared to ~5 hrs for daidzein), and genistein has a lower rate of urinary excretion than daidzein, at about 1/3rd of the rate. Remember, genistein is the isoflavone which has the same affinity to potent estrogen receptors as the most estrogenic version of estradiol, 17-β-estradiol, making it the most potent of the three soy isoflavones. Genistein is the most estrogenic, the most prevalent, and the slowest to be metabolized/excreted isoflavone. Sounds like a deadly combo.

Are you at Risk?

90 mg of isoflavones has the same estrogenic activity in pre- and postmenopausal women as a birth control pill based off of physiological measurements, and doses at ~50 mg also seem to be strongly estrogenic, but how much soy Soy Beans TOXICwould you need to consume to take in 90 mg of isoflavones? That question first depends on the isoflavone concentration of the food source, but considering the extremely low amounts of isoflavones in all other common food sources other that soy products, we can rule out whole wheat grains, non-soy legumes, meats, and basically every other non-soy food sources. A common soy based food, Soymilk, has a density of 1.03 g/ml, and a typical 8 fl oz serving of soymilk equates to 240 ml, which is equivalent to ~247.2 grams of soymilk per 8 fl oz. Considering the total isoflavone content of soymilk is 9.65 mg/100g, which is on the low side for most soy based foods (due to the high water content), a typical 8 fl oz glass of soymilk has 23.85 mg of isoflavones. Drink 3.77 glasses, or 30 fl oz of soymilk, and you’ll have consumed 90 mg of isoflavones, which is the amount used in both menopausal studies! Drink 16.7 fl oz, or one large glass of soymilk, places you at ~50 mg of isoflavones, which is the level shown to increase isoflavone blood plasma concentrations 10x higher than the amount needed to approach the effect of 17-β-estradiol at its physiological level.

Following the same mathematical procedure and the isoflavone content of tofu (31.1 mg/100g), then ~10.21 oz of tofu contains about 90 mg of isoflavones, and 5.67 oz of tofu will contain ~50 mg of isoflavones. 10 oz is a bit more than half a pound, but as a strength athlete who eats a lot daily in order to gain strength, it’s not hard to consume 10 oz. 88 grams of soy protein concentrate (102.07 mg/100g), or 2.75 standard scoops of protein powder, will place you at 90 mg of isoflavones. Consider that it is also easier to reach the 90 mg isoflavone threshold (which might actually be lower) when you combine different soy products, such as soymilk in the morning, tofu for lunch, and a post-workout soy protein shake, along with of course all the soy additives and ingredients consumed throughout the day common in processed foods.

Are you at risk? I would say YES. Anecdotally, one of my good friends used to consume a lot of this one nutritional shake his parents bought him, and it contained a lot of soy protein concentrate (102.07 mg/100g). As a college student low on money, my friend was drinking 2-3 of these shakes a day because they were free and easy to make. Over time he noticed that he had developed gynecomastia, also know as man boobs or “bitch tits”. Gynecomastia is defined as swollen male breast tissue caused by a hormone imbalance. He decided that soy foods were the culprit, and once he cut soy completely out of his diet, his gynecomastia reduced greatly, though not completely. The gynecomastia could have been the result of late stage puberty, but considering the above evidence, we both think it was his high soy intake that caused the issue to develop.

Final Thoughts on Isoflavones

The evidence is fairly compelling in regards to soybean isoflavones having a strong estrogenic potential and physiologically noticeable effects on the human body, especially in those with naturally lower amounts of estrogen (men, postmenopausal women, etc). Considering the possibility that easily consumed amounts of soy food products can deliver enough isoflavones potent enough to equal the estrogenic activity of a low-dose hormone replacement therapy pill, aka birth control, I would steer FAR FAR away from any soy products. If you consume soy foods for the other potential health benefits, then I would advise you to research further into those scientific claims, because it seems soy isn’t the anti-cancer/anti-heart disease “superfood” that many nutritionist make it out to be.

I recently took a testosterone test while I was still drinking a lot of a soy milk, and I came in right around average for total and free testosterone (681 ng/dL & 16.8 pg/mL), but considering the fact that I have cut soy completely out of my diet now for a few months, I plan on taking another testosterone test to see the effect, if any, that regular soy consumption had on my testosterone levels. These personal tests of mine will be by no means conclusive, but however the results turn out, it will be good food for thought. I’ll update this post once I have my new results.

Stayed tuned for part 2 of this series: Phytoestrogens and Strength Training – Lignans.

Note – Much of this article has been basically a review, piercing together, and interpretation of the science behind isoflavones and how they affect the endocrine system. What isn’t well known is the actual effect of isoflavones on strength training and body composition. This area of research is highly deficient. Additionally, while the research appears to be fairly conclusive regarding the estrogenic potency of isoflavones, the negative effects of isoflavone consumption, if they exist, might be of such insignificance that physiologically no noticeable difference would be able to be distinguished. I do not think this is likely, considering eating 10 oz of tofu or drinking 30 fl oz of soymilk, for example, is the equivalent of taking a birth control pill.


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Stefan Burns

Stefan Burns

Fitness, nutrition, and powerlifting buff. Created Strength Cooperative as a way for hardworking, natural lifters to share advice on how THEY got results in the gym.
Stefan Burns

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