As far as Venus Williams is concerned, there is no mystery. If you exercised as much as she does, you too could eat any damn thing you wanted.
Once she stops expending so much energy on the tennis court, her diet will have to change. It’s a pretty simple premise.
Fat kids need to get up from the TV and the computer screen and run around more. The only question is who’s going to make them do that: themselves? Their parents? Their teachers? Michelle Obama?
Not necessarily ... when Charles Barkley was running up and down NBA courts, he was still fat, the nickname "The Round Mound of Rebound" wasn't made up for no reason -- I remember him dieting in the middle of a season, that had to be difficult on him. It depends on your metabolism as well.