Thats incorrect because theres no problem with their metabolism at all, you are eating too much and your body stores it as fat, working as intended.
This book[0] goes through how the metabolic system works, how it is supposed to self regulate, and how it can miss-regulate. It also talks about what it is about our food since the late 1970's that has changed to cause this to happen more frequently.
[0] https://www.amazon.ca/Hungry-Brain-Outsmarting-Instincts-Ove...
EDIT: I want to add a bit. This book goes through how the metabolic system of humans is supposed to work, and it can get technical at times. It also goes over the specific traits that certain foods have that can be problematic, which gives enough info to make a self-improved diet. It backs up everything it can with specific citations of scientific experiments.