Soy on testosterone: Does consuming soy make you a soy boy?

Should an Alpha Energy Male avoid soy so that he doesn’t become a soy boy?

Perhaps. Now let’s look at the nuance.

Quick recap on soy and phytoestrogens

There are a few different kinds of phytoestrogens found in different foods, with flax and soy being the two richest phytoestrogen foods. Flax is rich in lignan and soy is rich in genistin and daidzin.

Genistin and daidzin (isoflavones in soy) are converted by gut microflora to produce bioactive aglycones, genistein and daidzein. These aglycones can be further metabolized to equol in the intestine in up to 50% of the population.

Equol is one of the most potent isoflavones, with a higher affinity for the estrogen receptor than its precursors and a longer half-life than both genistein and daidzein (R).

The isoflavones genistein and daidzein resemble 17-β-estradiol both structurally and functionally. Although they are less physiologically active than estradiol, they bind to estrogen receptor-α to exert an estrogen-like effect (R, R).

Soy consumption in Asia

The consumption of soybeans and soy products varies in different countries, but in Asia, soybeans are a popular legume, and their intake there is higher than in the United States or Western Europe. 

In East and South Asian countries, the isoflavone intake is around 20-50 mg/day on average, compared with only around 0.15-0.3 mg in the United States (R).

According to the Japanese government’s Food Safety Commission, the maximum safe daily isoflavone intake is 70-75 mg/day (R).

The Agence Française de Sécurité Sanitaire des Aliments has indicated that the highest isoflavone (aglycone equivalent) intake that is not considered to have any health risk is 1 mg/kg body weight/day (R).

Risks of soy and phytoestrogen intake

Many animal studies have shown that soy consumption lowers testosterone and increases estrogen.

This might lead to gyno, loss of libido, erectile dysfunction, fat gain, etc.

However, some studies say that these findings from animal studies are not applicable to humans, because of differences in isoflavone metabolism between rodents and humans and the excessively high amount of isoflavones to which the animals were exposed (R).

So should be we worried then?

Soy consumption in humans

A recent meta-analysis in 2021 that included 41 studies found that regular soy consumption had no negative impact on total and free testosterone, estradiol, estrone or SHBG levels (R).

Given that regular soy consumption is low with an average isoflavone intake below 100mg daily, this might result in minimal hormonal changes.

Very high soy intake

There are a few case studies of people who crashed their testosterone from consuming a lot of soy. These boys/teens/men also experienced side effects such as gyno, loss of libido and erectile dysfunction.

Case study 1 – 1.2L soy milk daily for 3 years

A 54-year-old man had been drinking approximately 1.2 liters of soy milk (equivalent to approximately 310 mg of isoflavones) per day for 3 years. He then developed erectile dysfunction and gynecomastia and had very low LH, FSH and testosterone (as seen in the May column).

He stopped drinking soy milk on his own in June of that year. When he retested in August, blood tests showed an increase in LH, FSH and T.

After 6 months, his malaise, gynecomastia, and loss of axillary hair improved gradually, but the erectile dysfunction did not improve completely (perhaps to age, low free testosterone and nutritional deficiencies).

Case study 2 – almost 3L of soy milk daily

A 60-year-old developed bilateral gynecomastia, erectile dysfunction and decreased libido after drinking almost 3 liters of soy milk daily.

He had no changes in testicular size, no headaches, no visual changes, and no change in muscular mass or strength. Testicular ultrasonography; computed tomography of the chest, abdomen, and pelvis; and positron emission tomography were normal.

His estrone and estradiol concentrations were 4-fold above the upper limit of the reference range.

After he discontinued drinking soy milk, his breast tenderness resolved and his estradiol concentration slowly returned to normal (R).

Case study 3 – soy boy

An 8-year-old boy developed unilateral gynecomastia after eating 168–238g of soy/week (estimated 273–440mg of phytoestrogens/week) (R).

He consumed various soy-containing food products that amounted to at least 168 g of soy-derived protein per week. His intake specifically consisted of imitation chicken soy nuggets 10–12/day (12g soy flour/4 nuggets) for 4 days/week, imitation ground beef soy crumbles 2 days/week (9g soy flour/serving), and vegetarian soy chili 1–2 cans for 2 days/week (15–19g soy flour or soy protein/can, depending on which of the two brands he regularly chose to consume).

After stopping all the soy, his unilateral gynecomastia was completely gone after 10 months.

Although the amount isn’t that much on a daily basis, it’s likely that he was a hyper-responder to soy.

Case study 4 – Soy eating vegan

A 19-year-old type 1 diabetic was eating a vegetarian diet (with large quantities of soy-based products) and lost his libido and experienced erectile dysfunction.

He had low total and free testosterone, which normalized after 1 year of stopping the soy consumption. Luckily he was also able to find his libido (after losing it) and regain his sexual function as well (R).

Soy intake on DHT

Isoflavone supplementation as Trinovin (each tablet contains 40 mg isoflavones (phytoestrogens) extracted from red clover and includes genistein, biochanin, daidzein and formononetin) in men for 3 weeks didn’t lower testosterone or DHT (actually DHT increase slightly) (R).

In this study, they compared milk protein vs soy protein with low and high isoflavone content (1.64 ± 0.19mg and 61.7 ± 7.35 mg isoflavones/d respectively) for 57 days (R).

After 57 days, testosterone was more or less the same in all the groups, but DHT was reduced in the soy protein groups regardless of the isoflavone content.

Another study compared the effects of whey vs soy in combination with exercise on hormones for 12 weeks. As you can see in the graph below, soy protein intake with training prevented the increase in testosterone compared to whey (R).

It’s likely not the isoflavones

The decrease in testosterone and DHT seems to be independent of the isoflavones. Consuming a lot of soy flour or soy protein will lower testosterone and DHT when consumed for a long period of time.

Also, keep in mind that the dose makes the poison. If you consume some soy here and there, it will likely not do much.

Long-term use of soy

Although the 2021 meta-analysis I mentioned above didn’t find that soy consumption had any effect on hormones, almost none of the studies were longer than 3 months.

This poses a problem since most of the negative hormonal effects only take place after 4 months (R).

In this study, 18 months of supplementing either 19.2g of casein or soy protein isolate (containing ~70 mg/day total isoflavones and ~24 mg/day genistein) reduced circulating testosterone and SHBG, but not free testosterone or estradiol (R).

The soy group experienced a 13.8% decrease in T and a 26.2% decrease in the testosterone-to-estradiol ratio after 18 months.

The effects only became significant after 4 months.

Summary

Soy doesn’t seem to lower testosterone in the short term (3 months). Also, it’s not the isoflavone content that seems to be responsible for the negative effect.

However, long-term consumption (>4 months) of even 1 scoop of soy protein can start to lower testosterone and DHT. It can also cause side effects such as poor mood, ED, gyno, low libido and tanked androgens. This is especially true for how much and for how long someone consumes it.

After stopping, things can resolve on their own, but it can take up to 1 year.

If you have soy every now and again (such as soy sauce with your sushi once or twice a week), you should be fine.