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.
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.
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