[Volundr]

I can definitely say from personal experience that metabolism plays a big factor. Until about 25 I never really struggled with weight. Sure I packed on the pounds when I started college (went from being on the swim team and working at Wendy's, so highly active, to doing nothing for exercise and working as a security guard watching cameras), but once I realized what was happening and applied a little discipline it came off easily and stayed off.

Then, well, something happened with my thyroid. Best guess is some kind of infection. I was already at a healthy weight, but lost 20 pounds over a few months. Tests showed my thyroid was producing WAY too much hormone and we were about to oblate it and put me on thyroid replacement hormone when the levels returned to normal on their own. Ever since then however I've struggled with my weight despite eating healthier than ever before. Unfortunately I never had cause to have my thyroid checked before this incident, so I have no way of knowing how my "normal" levels pre-infection compare to my new "normal" levels post infection.

Yes calories in vs calories out may be the literal physics of the situation, but the details of that equation can be very different for different people.