As someone who has studied weight loose for over 20 years, I can tell you it is very complex. The modern overweight problem throughout the world is caused by MANY factors. The research has gotten much better. One thing that is clear. There is no one cause. It is a combination of things.
It is also very difficult to lose fat. The younger you are, the easier it is.
It is not just exercising more and eating less. It is more complicated. Much more complicated.