If a slow metabolic rate is such an important factor, then maybe we can point to the US as, somehow, suffering an obesity epidemic due to that - and a country like Japan somehow the opposite?
I've been suffering from overweight A LOT in my youth, and I'm not trying to bring anyone down. I know all about what it means to not be happy with your body.
That said, naturally it's a factor that can't be ignored - but from my experience, it's generally a crutch people who are overweight use, to excuse themselves from putting forth the effort.
That's sad, because I want people to be happy - and I want them to enjoy their bodies.
I can't prove that, though, and if this metabolism thing is REALLY the reason - then I'm sorry, and I hope those suffering can forgive me for being an ass. But I think they'd do well to be ABSOLUTELY certain of it. Counting calories with precision, is one such way to be reasonably certain.
About the opposite, as in people who CAN'T gain weight - well, the case is the same.
I have a good friend (my best friend, really) who has ALWAYS been thin. Very thin. He keeps going on about not being able to gain weight.
Also, when he visits me - I always make sure to stuff him with junk food and candy, and he eats heartily. So, the natural conclusion is that it's true, he CAN'T gain weight, right?
No, because his daily routine is completely different from that. First of all, he's EXTREMELY active. He has five children and he has no car. He goes on his old bicycle EVERYWHERE - and I'm not kidding. The man is on the move all the time. Also, at home - they eat nothing but god damn vegetables and what I consider the most horribly boring and spartan food. He's the sort of guy who "forgets" to eat, he claims. In our youth (well, 10-15 years ago) - he basically lived off nothing, and his calories during the day came from nothing but the sugar in his coffee.
Fast metabolism? Bullshit. You don't gain weight by eating junk-food once in a while, or because you stuff your face with candy when in the company of others.
You gain weight by CONSISTENTLY eating more than you burn, and if you're the active kind - you need to eat all the damn time to get results.