With America's obesity epidemic, we've all heard people say, "I'm heavy because my metabolism is very slow."

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Testing that statement used to be fairly complicated, but modern electronics have made it easy. And guess what? It isn't your metabolism.

"Around my 40th birthday, I got the feeling that I needed to do something drastic with my life, see if I could make it past 50," said Dr. Bill Frumkin, an obesity surgery patient.

He's not being dramatic. Frumkin is a heart doctor who knew that, at his old weight of 430 pounds, he was courting health disaster.

"You try to run or you try to go to the gym, it's kind of hard, so it made me more sedentary and obviously made me gain more weight," said Frumkin.

So Frumkin had gastric bypass, the so-called obesity surgery, and has so far lost 180 pounds. But lately, his weight loss has slowed to a crawl. Has the surgery plateaued?

"It is not that the operation has plateaued," said Dr. Mitchell Roslin, of Lenox Hill Hospital in New York. "What's happened is now you have some sort of energy balance."

In other words, you lose weight only when you burn more calories than you take in, plain and simple. But how do you know how many calories you're burning? And don't severely overweight people get that way because their metabolism is slower than normal? Probably not, at least in most cases, doctors say.

An ingenious little device known as a metabolism monitor can help. By measuring the carbon dioxide you breathe out, it calculates your resting metabolic rate. It turns out Roslin's morbidly obese patients actually have a metabolic rate about what you'd calculate based on age, gender and especially weight.

"Which means that the majority of the patients that we see aren't metabolic cripples," said Roslin.

For most people, the resting metabolic rate is about two-thirds of the total calories burned during a day. For couch potatoes, it might be 90 percent of their daily calories.

The point is, it gives you a read on what you need to do to lose weight.

"If I know where I am in terms of energy consumption, then I really can gauge more what I should be eating," said Frumkin.

Obesity surgery patients find that their metabolism does fall as they lose weight, but it usually ends up around what you'd predict based on their new weight.

This gadget gives people the information they need to figure out their caloric and activity needs for weight loss.