The soy protein, not its isoflavones, lowered an androgen in men
In 35 healthy young men, both soy preparations lowered dihydrotestosterone against milk protein: 9.4 percent on the arm nearly free of isoflavones, 15 percent on the rich one.
What the study found
The near-isoflavone-free soy arm lowered DHT almost as much
In 35 healthy young men, both soy preparations lowered dihydrotestosterone, the androgen made from testosterone, against milk protein: 9.4 percent on the arm carrying 1.64 mg of isoflavones a day, 15 percent on the arm carrying 61.7 mg. That points at the protein, not the isoflavones.
The caveat · No older men, no health outcome
Thirty-five men at one centre, all young and healthy, so nothing here reaches older men or any clinical outcome. Other hormones moved in inconsistent directions across timepoints, which is what measuring several hormones at three timepoints tends to produce.
What we don't know
This one trial shows dihydrotestosterone falling in 35 young men on soy, with or without isoflavones. It cannot show the fall holds across trials or matters for health; that needs the hormone pooled, which nobody has done.