Introduction
The number of reports demonstrating adverse effects of soy isoflavones due to their estrogen-like properties, has increased. It is very common to hear that men should avoid soy foods, due to the estrogenic properties of soy isoflavones.(1)
In this blog we will review whether soy affects men’s health, testosterone levels, Sex hormone binding globulin, erectile dysfunction, and fertility, also whether soy benefits men who are at risk of developing prostate cancer.
Does soy affect testosterone levels?
There are some concerns that soy isoflavones may reduce the production of the male hormone testosterone. As per the theory, in the journal of acupuncture and meridian studies, it was observed that isoflavones can inhibit the activity of enzymes involved in testosterone production, thus lowering testosterone levels and increasing estrogen levels.(2)
Some research shows that regularly eating soy products like edamame, tofu, soy milk, and miso may cause a drop in testosterone levels. For example, one study in The Journal of nutrition, found that 35 men drinking soy protein isolate for 54 days resulted in decreased testosterone levels. (3)
Four groups of 20 young men, were supplemented with 50 g of protein from whey protein concentrate (no isoflavones), soy protein concentrate (138 mg of isoflavones), soy protein isolate (49 mg of isoflavones), or a whey-soy protein blend (24 mg of isoflavones) in combination with 12 week resistance-training program. At the end of week 12 Body composition, testosterone, estradiol and sex hormone binding globulin (SHBG) were measured at baseline.(4)
There were no significant differences found in total or free testosterone, SHBG, estradiol, or estradiol-to-testosterone ratio between four groups of 20 young men after 12 weeks.(1)
Summary
Soy protein has the potential to reduce testosterone levels, but only when consumed in excess.
Effect on Sex hormone binding globulin
A meta analysis of 15 placebo-controlled treatment groups with baseline and ending measures and an additional 32 reports involving 36 treatment groups reported no effects of soy protein or isoflavone intake on testosterone, sex hormone binding globulin, free testosterone or the free androgen index.(5) These studies used a variety of soy foods and protein powders that provided 10–70 g of soy protein and 60–240 mg of isoflavones per day.(1)
The results of this meta-analysis suggest that neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men.
A study in the Journal of the International Society of Sports Nutrition reported similar findings in young athletes. Evidence in the study suggests an inverse relationship between soy protein intake and serum concentrations of male sex hormones.
Summary
Neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men. There is an inverse relationship between soy protein intake and serum concentrations of male sex hormones.
Does soy cause erectile dysfunction?
A 60-year-old man went to the endocrinology clinic for evaluation of bilateral gynecomastia of 6 months’ duration and he described a daily intake of 3 quarts of soy milk. He reported erectile dysfunction and decreased libido. This is a very unusual case of gynecomastia related to ingestion of soy products. Health care providers should review patients’ dietary habits to declare the analysis of medical conditions.(6)
Another case of a 19-year-old (type 1 diabetic) but otherwise healthy man vegan eating a soy-rich diet, indicates that soy product consumption is related to hypogonadism and erectile dysfunction.(7)
The two above mentioned case reports simply illustrate that consuming excessive amounts of essentially any food can potentially lead to abnormalities.(8)
Summary
Reasonable intakes of soy foods and soy isoflavones do not affect men’s testosterone levels, or estrogen levels, although case reports have documented adverse effects from incredibly high daily intakes of soy for 6–12 months.
Does soy affect fertility?
Soy doesn’t appear to impact fertility, either. In a Randomized crossover intervention, the effects of consumption of soy protein of varying isoflavone content on parameters of semen quality in healthy young men was determined. The trail was observed in healthy young men compared the effects on semen parameters of supplementing with 30 g of protein from milk protein isolate (no isoflavones), low-isoflavone soy protein isolate (2 mg of isoflavones), and high-isoflavone soy protein isolate (60 mg of isoflavones) Consumption of soy protein of low or high isoflavone content does not adversely affect semen quality in a sample of healthy adult men. (9) The changes seen after 2 months were not significantly different between groups for any of the parameters measured (semen volume, sperm morphology, sperm concentration, sperm count, and amount of motile sperm).
Another study was observed in the Clinical Trial, healthy volunteers took a supplement containing 40 mg of isoflavones daily for 2 months and donated blood and semen samples monthly for 2 months before and 4 months after supplementation. This is the first study to examine the effects of a phytoestrogen supplement on reproductive health in males. It was concluded that the phytoestrogen dose consumed had no effect on semen quality.(10)
Summary
When soy protein is consumed in excess they can reduce testosterone levels and interfere with fertility, from the daily consumption of 10–70 grams of soy protein or 60–240 mg of isoflavones no such effects have been observed.
Does soy benefit men who are developing prostate cancer?
Prostate cancer (PCa) is the second most commonly diagnosed cancer in men, accounting for 15% of all cancers in men worldwide. Asian populations consume soy foods as part of a regular diet, which may contribute to the lower PCa incidence observed in these countries. The meta-analysis demonstrated that soy foods and their isoflavones (genistein and daidzein) are associated with a lower risk of prostate carcinogenesis. Thirty observational studies were included for analysis of the potential impacts of soy food intake, isoflavone intake, and circulating isoflavone levels, on both primary and advanced PCa. These thirty studies reported that neither soy food intake nor circulating isoflavones were associated with advanced PCa risk, although very few studies currently exist to examine potential associations.(11)
A meta-analysis of two studies evaluate the evidence from randomized controlled trials (RCTs) on the efficacy and safety of soy/isoflavones in men with prostate cancer (PCa) or with a clinically identified risk of PCa. The study reported a significant reduction in risk (-51%) in the group given a soy supplement compared to the group given a placebo. A good safety profile is shown by this meta-analysis for soy/soy isoflavones supplementation. (12)
The more recent RCT used 60 mg of isolated soy isoflavones per day for one year,(11) whereas the older used 40 g of soy protein isolate (107 mg of isoflavones) or concentrate (6 mg of isoflavones) per day for six months (the results of both isoflavone dosages were combined in the analysis to increase statistical power, although both groups showed similarly low rates of cancer progression).(13)
However, men already diagnosed with prostate cancer may not benefit from consuming soy.
Summary
Soy may benefit men who are at risk of developing prostate cancer. Regular intake of soy protein in men may reduce the risk of developing prostate cancer in them. Meta-analyses report no significant association between soy intake and risk of progression to advanced prostate cancer in observational studies(11) and no significant effect of soy consumption on prostate-specific antigen (often elevated in men with prostate cancer) in RCTs of men with prostate cancer. A good safety profile is shown by the meta-analysis for soy/soy isoflavones supplementation.(12)
Conclusion
This isn’t to say that soy can’t have negative effects on testosterone and fertility when overconsumed. Two case reports have documented adverse effects with ≈360 mg of soy isoflavones per day for 6–12 months. Those effects included gynecomastia, erectile dysfunction, and reduced libido in a 60-year-old man drinking three quarts (2.8 liters) of soy milk per day,(6) and hypogonadism and erectile dysfunction in a 19-year-old vegan eating a soy-rich diet.(7)
Despite the fact that case reports have documented adverse effects from incredibly high daily intakes of soy for 6–12 months, appropriate intakes of soy foods and soy isoflavones do not affect men’s testosterone levels, estrogen levels, or fertility. Men who are at risk of developing prostate cancer might reduce their risk by eating soy foods, but soy foods do not appear to benefit men who already have prostate cancer.(1)
References:-
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- Is soy good or bad for you? – Examine
- Inhibitory effects of isoflavonoids on rat prostate testosterone 5α-reductase – PubMed (nih.gov)
- Soy protein isolates of varying isoflavone content exert minor effects on serum reproductive hormones in healthy young men – PubMed (nih.gov)
- Effect of protein source and resistance training on body composition and sex hormones – PubMed (nih.gov)
- Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis – PubMed (nih.gov)
- An unusual case of gynecomastia associated with soy product consumption – PubMed (nih.gov)
- Hypogonadism and erectile dysfunction associated with soy product consumption – PubMed (nih.gov)
- Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature – PubMed (nih.gov)
- Soy protein isolates of varying isoflavone content do not adversely affect semen quality in healthy young men – PubMed (nih.gov)
- Effect of a phytoestrogen food supplement on reproductive health in normal males – PubMed (nih.gov)
- Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis – PubMed (nih.gov)
- Soy and soy isoflavones in prostate cancer: a systematic review and meta-analysis of randomized controlled trials – PubMed (nih.gov)
- Effects of soy protein isolate consumption on prostate cancer biomarkers in men with HGPIN, ASAP, and low-grade prostate cancer – PubMed (nih.gov)
