Saturday, May 16, 2009

Soy Isoflavones Reduce DHT, Increase Testosterone

Soy isoflavones reduce DHT and increase testosterone levels in rats
Soybeans contain the isoflavones genistein and daidzein. (Photo by T. Hagihara)

There's a lot of speculation on how soy intake and hair growth are related, so in a series of posts beginning with this one, we'll be taking a look at what the studies have to say. Hopefully, it will become clear whether soy isoflavones really do anything, how much isoflavones is the optimal intake, and whether oral or topical is the way to go.

In the first study we'll look at, male rats were fed soy isoflavones in various amounts (link). After a week, their testosterone and dihydrotestosterone (DHT) levels were measured. Since reducing DHT levels seems to be an effective way to reduce hair loss, this should be an interesting study for people considering soy isoflavones as a remedy.

Composition of the soy diets

To find out how soy isoflavones and androgen levels are related, the authors conducted two experiments. In the first experiment, rats in the treatment group were given soy flour with their normal chow. In the second experiment, rats in the treatment groups were given either a soy methanol extract or semipurified soy isoflavones.

The isoflavone content of the soy flour was 1.92 mg/g. The isoflavone contents of the soy methanol extract and semipurified soy isoflavones were 3.38 mg/g and 218 mg/g, respectively. In the first experiment, the rats in the treatment group received 442.7 g/kg soy flour in their diet. In the second experiment, they received 20 g/kg of soy extract or 2 g/kg of soy isoflavones in their diets.

Long story short, the according to the authors, the actual soy isoflavone intakes of the rats were as follows: 19 mg/day in the soy flour group, 0.9 mg/day in the soy extract group, and 3.3 mg/day in the soy isoflavone group. The control groups consumed zero soy isoflavones.

Soy isoflavones and DHT

Rats on the soy flour diet had significantly lower DHT levels than rats on the control diet. Similarly, the DHT levels of the rats on the soy isoflavone diet were about 60% lower DHT than in the control group. On soy extract diet DHT levels tended to decrease, but the difference was not statistically significant.

soy isoflavones and DHT
The figure above shows the DHT levels for the soy extract and soy isoflavone diets compared to the control group. The observed decrease in DHT from the soy flour diet (not shown above) was similar to that of the soy extract diet, with the exception that the difference was statistically significant.

Soy isoflavones and testosterone

Rats on the soy flour diet had similar levels of testosterone + dihydrotestosterone (T+DHT) as the control group. Since their DHT levels were lower, however, this means that there was an increase in testosterone from eating the soy flour diet. In the soy isoflavone diet, this effect was even clearer; not only was the reduction in DHT balanced by an increase in testosterone, but the total T+DHT levels were much higher than they were before the diet.

soy isoflavones and testosterone + DHT
The figure above shows the T+DHT levels of the soy extract and soy isoflavone diets compared to the control group. Testosterone levels tended to increase and DHT levels tended to decrease also on the soy extract diet, but again, the differences were not statistically significant.

The fact that the soy isoflavone showed significant effects and the soy extract is possibly due to the differences in soy isoflavone content of the diets. The rats on the soy extract diet consumed only 0.9 mg/day, while those on the isoflavone diet consumed 3.3 mg/day. The two graphs shown here seem to support the idea that the effect is dose-dependent.

What is confusing, however, is that the soy flour diet showed a less pronounced effect than the soy isoflavone diet even though it had a much higher isoflavone content. Perhaps the dose-response is not linear but a bell curve. Unfortunately, the authors offer no explanation or theory for the results in the paper.


Soy isoflavones significantly reduced DHT levels and increased testosterone levels in male rats. An intake of 3.3 mg of isoflavones per day was the most effective of the three treatments tested. A lower intake showed similar but less pronounced effects, while a higher intake did not appear to further add to the effect.

For more information on hair growth, see these posts:

Tea Tree Oil vs. Korean Red Ginseng – Hair Growth Battle Conclusion
North African Plant Extract (Erica multiflora) Increases Hair Growth
Bioactive Form of Silicon (BioSil) Improves Skin, Hair & Nails in Photoaged Women
2% Nizoral Shampoo Increases Hair Growth More than 2% Minoxidil

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13 kommenttia:

Anonymous November 20, 2009 at 7:23 PM  

The calculation for 22.4 g/day would not be 0.024 * 850 but 0.0224 * 850 and that gives the answer 19.04 which is how the researchers came up with their result.

JLL November 21, 2009 at 1:43 PM  


You're right, it was my calculation that was wrong. Thank you for the correction!

MCT December 3, 2009 at 8:22 AM  

Nice post! I think there's a mainstream misconception that exaggerates the association between soy, femininity, and estrogen.

It's also been shown that soy isolavones also have the capacity to cause fat cell death. Somehow I doubt soy would be considered an "accepted food" within the paleo community, but nevertheless I plan on including it into my own fat loss program.

Jacey February 15, 2010 at 6:24 PM  

You may want to consider the following research:

JLL February 15, 2010 at 7:32 PM  


I'm aware of how the folks at Weston A. Price Foundation feel about soy. I'm not entirely convinced their fears are founded. Neither am I saying everyone should start eating soy -- just that if one is looking to reduce DHT and increase testosterone, soy isoflavones are one way to do it. I will go through to the articles you linked to again, however. Thanks for the comment!


Philip March 20, 2010 at 7:44 PM  

Jacey, I have seen lots of negativity about Soy (and those people are making a lot of money!) Perhaps some of their points are valid but looking at the net benefit ratio - I include Soy in my diet. Have seen lots of studies about how it affect positively gene expression.

Philip March 20, 2010 at 7:46 PM  

Also... if the Okinawan's are eating it - that cannot be a bad sign...

Anonymous April 1, 2010 at 5:19 AM  

I´ve found a supplement that says:

40% Soy Isoflavones....62.5 mg per pill (equivalent to 25 mg of isoflavones)

How many pills should I take a day?

Thanks in advance!


JLL April 4, 2010 at 2:36 PM  


In this human study the following amounts were used:

"The low-isoflavone soy protein isolate contained 0.02 mg isoflavones/kg body weight, while the high-isoflavone isolate contained 0.72 mg/kg body weight. Mean soy isoflavone intakes were 1.64 mg and 61.7 mg, respectively, with the most abundant isoflavones being genistein and daidzein."

Based on that, about two pills per day seems right. I would not take much more than that without reading up on potential harmful effects first.

Anonymous July 14, 2010 at 6:31 AM  

@PHILLIP "I have seen lots of negativity about Soy (and those people are making a lot of money!)"

They are? How?

Soy is cheap, abundant and used in so many foods now. Seems to me that all the companies that use soy in their products have more to gain by telling us it's perfectly safe to consume in high doses.

Anonymous November 20, 2012 at 5:52 PM  

it sure looks like soy is bad for the thyroid in women:

eating fermented soy once in a while is probably OK but I wouldn't eat soy on purpose hoping that it will do something positive

Mykell Miller June 22, 2013 at 5:35 AM  

I've heard all of this stuff about how soy is estrogenic and will increase your estrogen levels and feminize men's bodies.

Are you suggesting the exact opposite, gender-wise?

JLL June 25, 2013 at 11:49 AM  


I think the "soy = feminine" connection is vastly exaggerated and not really based on proper studies; indeed, many of the studies show the opposite connection.

Still, simply because soy seems to reduce DHT and increase T doesn't mean there can't be something else happening in the body that would be harmful to men. Perhaps the increased T is a reaction to something else, for example. All in all, though, it seems to me like a small quantity of soy isoflavones is a net positive for men as well.


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