According to rumors that spread on the Internet, soy is contraindicated when training muscle strength because it contains phytoestrogens (plant substances with effects similar to estrogens) that They reduce levels of testosterone, a hormone that participates in muscle development.
In reality, there is no rigorous scientific study to support this assumption. The only ones that suggest it have been carried out with rodents and with a small group of participants. Some individual case reports prove estrogenic effects, but taking disproportionate amounts of soy, such as 3 liters of soy milk per day or 360 mg of isoflavones (which would be found in 1 liter of soy milk plus 1 kilogram of tofu). These doses of phytoestrogens are not achieved with reasonable dietary consumption.
According to a study, the Japanese consume between 25 and 50 mg/day of isoflavones, and 10% of the Asian population reaches 100 mg per day.
Soy products do not affect testosterone levels
Faced with these unrealistic studies, there is a systematic review, which included 15 placebo-controlled studies and 32 additional reports with a total of 36 treatment groups, which found no effects of soy consumption on testosterone levels. Studies conducted after this meta-analysis showed similar conclusions.
You may wonder if the lack of effect is maintained by taking soy protein daily, something that a person who does muscle training and does not want to consume meat or fish may value. A German study offers an answer to this question: the participants, who were overweight, consumed every day 16 g of soy protein for 12 weeks. Testosterone levels were examined before and after the study and no decrease in testosterone levels was observed. The values even increased slightly.
Nor has any reduction in testosterone been found in diabetic men, who often suffer from hypogonadism due to diabetes, after taking 66 mg of isoflavones daily for three months, according to a study in which 200 men participated.
Scientists from the School of Sports, Rehabilitation and Exercise Sciences at the University of Essex concluded in 2021 that neither soy nor isoflavones reduce testosterone levels after meta-analysis of 38 studies carried out between 2010 and 2020.
soybeans are very suitable for muscle development
Scientific research concludes that soy protein promotes muscle development or at least is not worse in this sense than animal protein (for example a meta-analysis of 9 studies with a total of 266 participants).
It has been discovered that soy protein shows the same results in terms of strength and body mass after resistance training. A 2021 Brazilian study examined the effects of two protein sources on muscle mass and strength in healthy young men who performed resistance training twice a week for 12 weeks. Half of the 38 participants were vegan and the other half were omnivores, with an average age of 26 years. In addition to their usual diet, they received 1.6 g of soy protein per kilogram of body weight (in the vegan group) or 1.6 g of whey protein per kilogram of body weight (in the normal eating group). A significant increase in muscle mass was observed in both groups, regardless of the protein source.
A 2021 study with 123 Chinese people between 65 and 79 years old showed that in older people with low muscle mass, protein supplementation with soy, whey, or a mixture of both types of proteins can maintain muscle mass and physical performance. The type of proteins played no role. The results did not differ.
Relieves training-related muscle damage
Soy protein too can reduce exercise-related muscle damage and improve muscle recovery after intense training, according to an Indian study.
This was the result of a study with 40 male athletes (20 boxers and 20 cyclists) aged 18 to 28 who received soy protein as a dietary supplement or a placebo powder for four weeks and trained for 30 hours per week.
The dietary supplement consisted of 25 g of soy protein isolate powder, mixed with 300 ml of water twice daily and containing 21 mg of isoflavones.
In summary, it can be said that soy products or protein in usual quantities do not pose a threat to testosterone level in men and they do not hinder muscle development in any way, and even promote it, just like milk proteins.
However, if you consume kilos of tofu and other soy products day after day for months or years you can expect negative effects, not only on testosterone levels, but on your overall health. But this applies to almost all foods if you consume too much of them.
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