Rob Kardashian Only Has Cheat Days For Good Reason

Rob Kardashian and Blac Chyna weren't very talkative at LAX Friday morning ... that is, until the topic turned to food.

Our photog asked Rob about his recent stops at In-N-Out Burger ... he's hit the fast food joint TWICE in the last week, while supposedly on his strict "no cheat days" diet.

Sure, Double-Double's are delish but Rob's got another good reason for digging in.