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"Skinny people who can eat anything they want without putting on weight tend to be very fidgety - they never keep totally still. Just wiggling a leg all day can use a large amount of energy."

Hm, this is interesting. I've never heard that before, but it seems intuitive. Do you have any sources for that idea? (Not that I'm challenging, just curious.)

In the literature its referred to as N.E.A.T. Non-Exercise Activity Thermogenesis

Many people's body's response to overfeeding is to up NEAT, up fidgetyness. But not always. Your body thinks getting fat is a good thing. Famine used to be common. When dieting almost everyone down-regulates NEAT. On a severe diet you feel like being very still.

Awesome... I will tell my coworker who is always complaining about my fidgeting legs that I'm exercising ;)

"There is no such thing as 'metabolism': just body mass, muscle mass and activity."

Those factors are not enough. The human body has many more important factors in play there, like hormones, intestinal flora, and so on.