Set Point

The hypothalamus has been researched and shown to regulate body fat percentage. (2)  The human body wants to keep its weight within a set range and uses multiple mechanisms to accomplish this. (3)

A guy took in 5000+ calories per day for 21 days and gained less than 2kg, far less than expected based on caloric intake.  The type of calories eaten is much more important than how many. (1)

His diet consisted of High Protein/High Fat with a small amount of carbs(10%).  The only carbs he ate were always low glycemic index.  He got plenty of vitamins and minerals and absolutely no added sugar calories.

1 21 Days
2 Hypothalamic Regulation of Body Weight: Experiments suggest that the lateral and ventromedial hypothalamus jointly determine the regulation level or "set point" for body fat
3 Dietary Fat and Body Weight Set Point