By Jeanette Jenkins

With the steep fuel prices, you probably know exactly how much gas it takes to fill up your car. But I’m betting that most of you don’t know how much food it takes to fuel your body, or how many calories you burn while your engine is running.

Knowing this information is important for losing weight or keeping it off. If you overflow your car’s gas tank, the extra gas just spills out on the ground. But of course, if you eat too much food and don’t burn it off, it sticks around – on your hips, thighs and that other spare tire.

So to stay slim and trim, or to lose weight, it’s important to know how fast your engine is running.

A key number is your resting metabolic rate (RMR), which is the number of calories your body burns at rest. One way to determine your RMR is by using a formula, such as the Harris-Benedict equation:

*For women, 655 + (9.6 x weight in kilograms) + (1.8 x height in centimeters) - (4.7 x age in years)

*For men, 66 + (13.7 x weight in kilograms) + (5 x height in centimeters) - (6.8 x age in years)

This formula is standardized, so it will give you an RMR but, because we’re all different, it may not be especially accurate for you.

Devices that measure metabolism are another option that have become increasingly popular at gyms. They go by the names New Leaf and BodyGem. You simply breathe into a mouthpiece or face mask, and the devices give your individualized RMR reading. Costs range from about $80 to $180, and some fitness centers even include this testing in their annual memberships to help members achieve their goals.

Knowing your RMR, you can then calculate your caloric consumption and your overall energy burn -- your total daily energy expenditure, including RMR plus exercise -- and tailor your diet and fitness plan accordingly.