Is there any manga or anime about body building. Not martial arts, but legit straight up body building, where weight lifting is involved for aesthetic purposes to compete in body building shows. Or the main character that goes to the gym and lifts weights to look better, and his life changes after that, I guess? Any genre but yaoi pls.

I think this is a really interesting request 🙂 sadly I can't think of much though. Problem is, rather than body building, most sports manga tend to focus on wrestling. Did a little research, and this is the best I could find:

Shape Up Ran- a gag manga about a female bodybuilder. This is the closest thing I could find, but it seems hard to get ahold of. Maybe you can check out other works by the author though?

Saint Muscle- not exactly bodybuilding, but it's muscles for aesthetic purposes.

Then we get into things that are actually martial arts or wrestling, with a little bit of a side focus on body building. Sadly you may have to go with these:

Kinnikuman II Sei- this along with its prequel. Still a gag manga; I suppose you said any genre, but I wish there was something more serious. The characters really like body building though it's not about that.

Histories Strongest Disciple Kenichi- a fighting manga, so not exactly what you want. But sometimes they train just to train, and there is a visible change in the characters over time.

Rice Shoulder- think it's a boxing/body building combo. But look I found the bodybuilder category! Yay there are four options there. 😐

This is my best effort, wow. Well good luck!