In his own comic, Captain America’s Super Soldier Serum is killing him.
He isn’t strong anymore, and he’s literally dying, so Tony Stark gives Steve Rogers the same kind of human upgrade that enabled Tony to walk after he was shot. Only unlike with Tony, Steve’s new “chip” has to be installed inside Steve’s body. So, Hank shrinks Iron Man and we get an “inside the body” adventure.
Afterwards, Steve is sad about the upgrade…
…But…Really? You stayed in shape through “nothing more exotic than hard work”? You took a magic potion, dude. It doesn’t get more “exotic” than that.