The 29-year-old had a frustrating season at Real Madrid, not helped by a series of injuries and suggestions that was fighting a losing battle against the dreaded flab.
"It doesn't bother me in the slightest," said Ronaldo, who has already been named in Brazil's starting line-up for their opening world Cup match against Croatia on June 13.
"I think it's all down to a lack of knowledge. After all, what is fat," he said, looking pointedly in the direction of the group of reporters amid laughter.
"People have been talking about it for so long that nowadays I just take it as a joke.
"Nobody ever takes about exact numbers, about the percentage of fat. It's down to a lack of knowledge, a lack of information and a lack of something better to talk about."
FITNESS
Ronaldo has not played a competitive match since April 8, when he limped out of Real Madrid's 1-1 draw with Real Sociedad with yet another injury, this time to his right thigh.
Since meeting up with the Brazilian squad he has taken a full part in training and says all he needs to do know is win back full match fitness.
"This is like a pre-season for me, I'm coming back from injury and I haven't played for one and a half months," he said. "I'm very happy and I'm getting on with my job."
Ronaldo will be taking part in his fourth World Cup having finished as top scorer in 2002 with eight goals as Brazil won an unprecedented fifth title.
He also managed four strikes in 1998, but refused to set himself any targets for the summer in Germany.
"It's very difficult to make a forecast like that," he said. "The principal target is the collective triumph, to help Brazil win a sixth World Cup. My personal success takes a back seat."
"In any case, if I score goals, I'll be helping Brazil to win the title."