I Used to Be Fat is an MTV reality series about overweight teens striving to achieve weight loss through means of diet and exercise. Each episode follows one teenager who is paired with a motivational personal trainer. The trainer teaches them new exercise and eating habits over a few months while offering emotional support. Documented students share their personal trials and tribulations in the series.

Several episodes of the show have covered a high school student's last summer before college. These students expressed a desire for a metamorphosis before they start their new lives away from home.

The show utilizes personal trainers that ignore best practices for weight loss. The trainers consistently put the teenagers they were charged with through dangerous exercise regimens and even more dangerous calorie deficits. Many contestants were expected to lose one or more pounds a day.