Here's something to feel good about: Your body is a calorie-burning machine. You'll even torch a few while reading this article. The point is, every single thing you do -- from breathing to eating to sleeping -- uses energy. The number of calories it would take just to lie in bed all day is called your resting metabolic rate. And just like your curly hair or warm personality, yours is unique.
"There are so many factors that determine your metabolic rate," says Janet Rankin, Ph.D., professor of human nutrition, foods and exercise at Virginia Tech. Among them: your height and weight (bigger people burn more calories), your gender (women have slower metabolisms than men), your age (your metabolic rate declines as you get older), how much muscle or fat you have (muscle burns more calories than fat does) and your DNA.
Although you can't rewire your double helix or switch back the clock, there's still plenty you can do to be a fast burner, Rankin says. All you need to do is remember these four research-backed truths.