Do men need estrogen for their health?
Yes, men need oestrogen for strong bones, healthy blood vessels, and fertility. Both a deficiency and an excess are harmful, so it is a matter of finding the right balance.
Men produce oestrogen themselves, but not via the testes. Adipose tissue is the only relevant source: it converts testosterone into oestradiol (the active form of oestrogen). The more adipose tissue a man has, the more this conversion takes place.
For bones, oestrogen is at least as important in men as it is in women. Men with very low oestradiol levels have approximately 2.5 times the risk of a hip or vertebral fracture. Notably, testosterone itself has almost no direct relationship with bone mass. It appears to act almost exclusively through its conversion to oestrogen.
Testicular function and sperm production are also dependent on a proper oestrogen balance. Too little oestrogen can reduce fertility in men. At the same time, the reverse also holds: chronically elevated oestrogen levels have been linked to hormone-dependent cancers and immune problems.
Oestrogen receptors are also found in the walls of blood vessels in men. They play a role in vasodilation and blood pressure regulation, which creates a link with cardiovascular health. This relationship is for now associative: it is clearer that both high and low levels cause harm than what the precise mechanism is.
In short, oestrogen is not a minor consideration in men. It co-regulates bone health, the heart and blood vessels, metabolism, and reproduction. Both too little and too much is unfavourable; it is a matter of balance.
All claims are based on associational and probably causal research in humans (no large RCTs). Strength of evidence is moderate; for bones and fracture risk, the association is most robustly supported.