"I... uh... I participate in a lot of eating contests?"
Oh god. If Mal couldn't hold in his food, then maybe venturing out to Sector 6 was a bad idea. It could wait until when he was sober (or just well, wait until the end of eternity). Yet Bart knew better than to openly call it off - if Mal wanted a fight that badly, he wasn't going to stop until he thought he got one.
Better to make Mal think they were going to Sector 6 when in actuality, Bart would just bait him until he could... well, until the guy passed out, most likely.
The whole stopping for food? Was totally his way of buying time. "You know, you don't look so hot," he said after a moment. "Let's stop by my place for a minute. I've got... I've got some pick-me-up stuff there."