Research vs Bro-science

Does Soy Lower Men's Testosterone? The Phytoestrogen Myth vs. Research

Published: 2026-06-30

Written by: Shingo YoshizakiReviewed by: Tomonobu Someda

"Soy isoflavones are phytoestrogens that lower testosterone in men" — this claim has become dogma in lifting communities. Should you avoid tofu, soy milk, and soy protein? Let's look at what the research actually shows.

Let the data settle it.

Round1

Does typical dietary soy intake lower testosterone in men?

What's said

筋トレ系コミュニティ・反大豆情報・一部のフィットネスコーチ

Soy isoflavones act as estrogen in the body, suppressing testosterone. Any man who trains should avoid all soy foods and soy protein.

What research says

Hamilton-Reeves et al.
(2010) meta-analysis (15 studies, 384 men) found no significant effect of typical soy protein or isoflavone intake on testosterone, estrogen, LH, or FSH in men.
Soy isoflavones (daidzein, genistein) have 100–10,000x lower affinity for estrogen receptors than endogenous estradiol — insufficient for meaningful hormonal disruption at typical dietary intake levels (~30–50 mg/day isoflavones, as in the average Japanese diet).

Verdict

Typical dietary soy intake (tofu, soy milk, soy protein) does not significantly affect testosterone in men. There is no evidence-based reason for male trainees to avoid soy foods.

Confidence:Strong evidence

Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis Effects of Zinc Supplementation on Testosterone Levels and Immune Function

Round2

Is high-dose isoflavone supplementation (300+ mg/day) safe for men?

What's said

大豆食品と大豆サプリを区別する立場

Whole soy foods are one thing, but high-dose isoflavone supplements will definitely disrupt hormones. The amounts in supplements are far beyond what food provides.

What research says

Hamilton-Reeves et al.
(2010) noted uncertainty for isoflavone intakes above 150 mg/day.
Long-term data for very high-dose isoflavone supplements (300–1,000+ mg/day) — an order of magnitude above typical dietary levels — is insufficient.
Case reports of feminizing effects in men have been linked to extremely high isoflavone supplement use, though causality isn't established.
The honest summary: dietary soy is safe; very high-dose isoflavone supplements are uncertain.

Verdict

Long-term safety data for very high-dose isoflavone supplements (300+ mg/day) is lacking. Normal dietary intake and soy protein supplementation is considered safe.

Confidence:Weak evidence

Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis