MANCHESTER, N.H. -- Your metabolism is something you're always hearing about, especially when you're trying to lose weight. But why is it so important?

A new, small portable device may help determine your metabolic rate and redefine the way we lose weight.

The first of it's kind, Food and Drug Administration approved Medgem analyzer quickly, easily and accurately determines your resting metabolic rate or the number of calories you burn while sitting still.

"If you went to the doctors, they have a height/weight chart and then they do a mathematical equation that isn't real accurate. This device is really accurate. It gives the resting metabolic rate and it's FDA approved," Dan Fleury of Healthsouth said.

The device has only been available for a short time, and already Healthsouth's facility in Bedford, N.H., is bustling with people looking to crack the code on how their bodies crunch calories.

With that key information, dieters can balance the calories that go in with how many they burn.

"When you see people who say 'I never eat and I'm not losing weight,' that's because the metabolism is slowing," Fleury said.

The analyzer will tell you how many calories you need and if you're eating fewer than your body needs," according to Fleury.

Fleury said that if you don't eat enough calories, the body stores them, kind of like being in a hibernation mode.

The test itself is simple, but there are a few guidelines: no caffeine, nicotine, food, or exercise in the hours before you take the test. They can all factor into a false reading.

The nose is clamped and the mouth is covered to funnel all air into the device.

After about 10 minutes of breathing into the device, it comes up with your individual resting metabolic rate. To help manage the numbers from there, a computer becomes very handy.

Fleury said software can help dieters keep track of how many calories are eaten in a day. Each meal can be logged in detail to determine how many calories are being taking in, how much is being burned off with exercise, and when you've reached your limit for that day.

"That will give you an expenditure of calories. If you are dieting, it gives a list of calories you lose or add each day depending on your metabolic rate in order to reach your goals," Fleury said.

The test runs about $100 without the software included.