I am on spiro and as many others have gained weight over time. I have read many times that low testosterone is the one to blame for this weight gain.

A new research indicates than when T is low in men, Estradiol is also low because it is produced from T conversion. They demonstrated that it is actually low Estradiol (specifically Estradiol, not any estrogen) the responsible for weight gain. They claim that it is also low Estradiol responsible for lack of libido and energy.

You can find info here:
http://www.nbcnews.com/video/nightly-news/53001889#53001889 and
http://washington.cbslocal.com/2013/09/14/study-low-estrogen-may-play-a-role-in-male-menopause/

My case is that I am almost in 0 a carbs diet and lost 20 lb with it, after starting spiro also started gaining 1.5 pound per week on a low dose of spiro, I like the calm effects of low T but not the belly I am getting.

Has anybody experience belly reduction with Estradiol?

Sorry to bump this, but that explains a lot for me. I was getting fat and falling asleep on the sofa nearly every night. Had no energy and was pretty much down all the time. Went on E (17-b Estradiol) for HRT and it's like someone wound me up, charged my batteries and sent me on my way. I was now full of energy, losing weight and becoming more active. Libido... eh that's a different story. It was pretty much lost for me. I did not take any anti-androgen at that time.

Interesting, were you in any HRT previous to starting Estradiol? Is it because of that libido was lost?