Your metabolism, experts say, involves a complex network of hormones and enzymes that not only convert food into fuel but also affect how efficiently you burn that fuel.
"The process of metabolism establishes the rate at which we burn our calories and, ultimately, how quickly we gain weight or how easily we lose it," says Robert Yanagisawa, MD, director of the Medically Supervised Weight Management Program at Mount Sinai Medical Center in New York.
Of course, not everyone burns calories at the same rate.
Your metabolism is influenced by your age (metabolism naturally slows about 5% per decade after age 40); your sex (men generally burn more calories at rest than women); and proportion of lean body mass (the more muscle you have, the higher your metabolic rate tends to be).
And yes, heredity makes a difference.
"Some people just burn calories at a slower rate than others," says Barrie Wolfe-Radbill, RD, a nutritionist specializing in weight loss at New York University Medical Center.
Occasionally, Yanagisawa says, a defect in the thyroid gland can slow metabolism, though this problem is relatively rare.
And here's a fact that may surprise you: the more weight you carry, the faster your metabolism is likely running.
"The simple fact is that the extra weight causes your body to work harder just to sustain itself at rest, so in most instances, the metabolism is always running a bit faster," says Molly Kimball, RD, sports and lifestyle nutritionist at the Oscher's Clinic's Elmwood Fitness Center.
That's one reason it's almost always easiest to lose weight at the start of a diet, and harder later on, Kimball says: "When you are very overweight your metabolism is already running so high that any small cut in calories will result in an immediate loss."
Then, when you lose significant amounts of body fat and muscle, your body needs fewer calories to sustain itself, she says. That helps explain why it's so easy to regain weight after you've worked to lose it.
"If two people both weigh 250 pounds, and one got there by dieting down from 350 and the other one was always at 250, the one who got there by cutting calories is going to have a slower metabolism," says Yanagisawa. "That means they will require fewer calories to maintain their weight than the person who never went beyond 250 pounds."