Soy gives men estrogen
Isoflavones are not human oestrogen and bind its receptors far more weakly. Meta-analyses of clinical trials in men find no effect on testosterone or oestrogen.
Soy is unusually rich in isoflavones, plant compounds that are known to bind the same receptors as human oestrogen, which is exactly why they are called phytoestrogens. A vegan diet tends to lean heavily on soy for protein, through tofu, soy milk and meat substitutes every day, so men eating this way are taking in far more of these compounds than anyone in a typical diet. There are documented cases of men developing breast tissue and hormonal problems from heavy soy consumption, and it is reasonable to worry that a daily dose of something that mimics oestrogen will shift testosterone and oestrogen levels over time.
Soya contains isoflavones, plant compounds that bind oestrogen receptors much more weakly than human oestradiol, and which act as antagonists as well as agonists depending on the tissue. A meta-analysis of controlled clinical studies in men found that neither soya protein nor isoflavone intake altered testosterone or oestrogen concentrations.
The handful of case reports describing feminisation involve extraordinary intakes: in one widely circulated instance, several litres of soy milk daily over a prolonged period. At ordinary consumption there is no measurable effect.
Inferring that one thing caused another from the fact that they occurred together or one after the other. Correlation and sequence are not causation.
How it applies here: Infers a hormonal effect in men from a shared name ('phytoestrogen') rather than from measured outcomes. The compounds were named for the receptor they bind, not for an effect they were observed to produce.
Using a single word in two different senses within one argument, so that a conclusion appears to follow when it does not.
How it applies here: Treats 'oestrogen' as one substance, when phytoestrogens and human oestradiol differ in structure, receptor affinity and physiological effect.