You Have to Know Your Limits (Also, Eat the Damn Fries)
Aside from training too hard, you also see Brittany revert back to her old habit of eating her feelings. In one scene, she even reaches into the trash to finish a serving of cheese fries. Instances like this don't discredit her improvements, but they show her in moments of weakness (which — yes! — happen). It also brings about a major point about diet culture: don't diet until you break. Yes, eating well-balanced, portion-controlled meals with lean protein, healthy fats, and complex carbs is important. Yes, a modest caloric deficit is essential for weight loss, and consistency is key, but if you eliminate major food groups or things you crave most, you're more likely to want to eat them in bulk later on.
Also, cheat days are nice. You see Brittany make healthier choices — like ordering a salad when her roommate Gretchen (Alice Lee) orders pizza — but she still treats herself to a full restaurant meal toward the end of the movie. So, French fries here and there won't make you gain all the weight back. Eat them so you won't be reaching into the garbage to finish off the cheese later on.