The recently-retired legend wasn't always Fat Ronaldo. Way back in 1999, he was simply struggling-with-his-weight Ronaldo, allegedly taking diet pills that forced him to wear adult diapers during games.
According to Vanderlei Luxemburgo, Brazil coach during the 1999 Copa America, Ronaldo was having issues staying in shape, and turned to powerful pills to keep hie weight under control. Much like the Hollywood starlets of today, he went with Xenical, with the side effect of steatorrhea. (For those wondering, that's loose, oily stool that can just slip out of you with no warning. Aren't you glad you wondered?)
Luxemburgo says the only logical answer was to wear adult undergarments, which embarrassed Ronaldo. I can see that.
Ronaldo was still great even when he had "accelerated evacuations" [The Spoiler]
Luxemburgo: "Ronaldo tuvo que jugar con pañales" [AS.com]
Ronaldo wore diapers during '99 Copa America, says former coach [Daily News]