Does eating a lot of soy and tofu lower testosterone?

Short answer

No. In 41 studies with 1,753 people, soy protein and the soy compounds called isoflavones did not change testosterone, even when split by amount and duration. The reported problems came from extreme intakes, such as drinking about 2.8 L of soy milk a day.

Does eating a lot of soy and tofu lower testosterone?
'Soy = plant estrogen → lower testosterone': is it true?
Soy contains isoflavones, compounds shaped like the female hormone estrogen. That is why many say eating a lot of tofu, soy milk, or soy protein lowers testosterone. We checked studies where people actually ate soy.
Pooling 41 studies
Testosterone and estrogen stayed the same
This analysis pooled 41 studies where people ate soy foods, soy protein, or isoflavones. Testosterone was measured in 1,753 people and estrogen in 1,000 people, and neither changed. Splitting the studies by amount eaten and by duration gave the same result.
How much did these studies use?
About 2 to 4 glasses of soy milk a day
These trials usually gave 40-100 mg of isoflavones a day. One 240 mL glass of soy milk made from whole soybeans has about 25 mg, so that is about 2 to 4 glasses a day. Traditional soy foods such as tofu and miso have about 3-4 mg per 1 g of protein.
But some numbers did drop
Testosterone was lower only on one measured day and right after exercise
In a study where 35 people ate soy protein and milk protein for 57 days each, a stronger hormone made from testosterone (DHT) was 9.4-15% lower on soy. Testosterone was lower only on day 29. In a study where 10 people took 20 g of soy protein for 14 days, testosterone rose less right after squats.
What about eating it while training?
Testosterone did not drop over 12 weeks
47 people trained for 12 weeks and took soy protein, whey protein, or a fake supplement 2 times a day. The soy group took about 64 mg of isoflavones a day, yet testosterone was not lower than in the fake group. In a study where 20 people took 50 g of protein for 12 weeks, testosterone also showed no difference between soy and whey.
The problem cases involved extreme amounts
A case of about 2.8 L of soy milk a day
1 person who saw a doctor for breast enlargement drank about 2.8 L of soy milk a day (about 12 glasses of 240 mL), and estrogen was 4 times the upper normal limit. After stopping soy milk, the level returned to normal. Another person who ate very large amounts of soy foods had low testosterone, which returned to normal within 1 year of stopping.
Summary: normal amounts do not lower it
The 'soy → lower testosterone' formula holds only at extreme amounts
In 41 studies testosterone did not change, and such trials usually gave the equivalent of 2 to 4 glasses of soy milk a day. Taking soy protein for 12 weeks while training did not lower it either. Problems were reported at extreme amounts, such as about 2.8 L of soy milk a day. Tofu and soy milk work fine as protein foods.