The Flab Zone is an area in Bowser's Body, where Mario and Luigi go to find Princess Peach in Mario & Luigi: Bowser's Inside Story. This area can only be explored when Bowser is fat due to eating excessive amounts of food. Here, they encounter many new enemies. Through a few rooms, they find Princess Peach being held by several Kretins. Mario and Luigi watch as they carry her away. Luigi runs after, but stops on a Donut Lift and falls into mucus. A white figure named Princess Lipid drags Luigi up, and gives the Bros badges. They proceed through several rooms with Save Blocks and blocks containing Attack Pieces, including vortex-like objects only to be used by a Spin Jump. After learning how to do Spin Jumping, and to use the Jump Helmet, Mario and Luigi find the Kretins with Peach. Mario quickly brushes them off with his spin jump. Once the princess is conscious, however, the Kretins form together to make the Alpha Kretin. After defeating the Kretins, Mario, Luigi, Peach, and Starlow proceed into two more rooms; in the second one, Peach is abducted by Fawful.
The player can also go back to this area whenever they want, once Bowser learns the Body Slam. As soon as Bowser returns to eat more of the food (before fattening again), Starlow strongly urges him not to, but lets him off the hook about it as he refuses.
This area may be stressful to the player in several ways, such as choosing the right pipe to enter, obtaining all the Attack Pieces, and fighting off the boss.