Liz Neporent is a diet and fitness expert and author of 12 fitness bestsellers. She regularly appears on national TV programs and is the president of Wellness 360, a New-York based wellness provider.

Photo: Rance Costa/Flickr

Do you know what the word metabolism means? Well, the short answer is that it's the chemical changes that take place in a cell or an organism that produce energy and basic materials needed for important life processes. Huh? OK, forget the science nerd answer. For those of us who prefer plain English, metabolism is the rate at which your body uses energy -- a.k.a. burns calories -- at any given moment, whether you're watching "So You Think You Can Dance" or gyrating on the dance floor.

Even when you're sound asleep, your body uses energy to keep your heart beating, your kidneys functioning, your skin cells regenerating, and so on. Typically, your resting metabolism -- the amount of energy you need to keep your bodily functions like digestion, circulation and skin regeneration humming along -- accounts for 60 to 75 percent of your total calorie burn for the day. It varies quite a lot between individuals, depending primarily on body size, genetics, and muscle mass. Your activity level accounts for another 15 to 30 percent, depending on how much you move around; the calorie burn associated with eating and digesting food accounts for about 10 percent of your total daily energy expenditure. Now that you've had a quick education on what metabolism is, I'd like to dispel three common myths about it.

Myth 1: Cardio exercise, like running and walking, boosts metabolism

Um, no. Some studies show that people who do cardiovascular exercise regularly have speedier resting metabolisms than sedentary people. But this is likely due to a small carryover effect from their last workout -- the so-called "afterburn" effect, rather than some permanent change in metabolism. In studies where researchers have waited to measure resting metabolism until several days after regular exercisers' last workouts, no increase has been detected. If you do cardio exercise regularly, you'll get a temporary metabolism boost after each workout (a good thing), but just know that it's an advantage that won't last if you stop exercising.

Myth 2: Building muscle boosts metabolism

Sort of, but not as much as some fitness magazines and trainers would have you believe. It's more accurate to say that building muscle helps you maintain metabolism. Lifting weights builds muscle, and the more muscle you have, the more calories your body will burn at rest. But it takes a lot of extra muscle to dramatically rev up your resting metabolic rate. Although you may have heard that you burn an extra 50 or 100 calories per day for each pound of muscle you build, that figure is not supported by science. Most reputable researchers say it's more like 10 to 15 calories, so you really can't expect to see much of an increase in metabolic rate with the typical 3- to 4-pound increase in muscle mass.

Still, as I said, muscle is the key to preserving your muscle mass and metabolism. Sedentary people tend to lose at least one-third of their muscle from age 30 to age 70, but consistent weight lifting probably can offset most of the decline. With more muscle and a steady resting metabolic rate, you're less likely to gain fat. Lifting weights also can offset -- by nearly half -- the muscle loss experienced by dieters, and that can help diminish the dreaded dieter's plateau.

Myth 3: You burn more calories after an exercise session

Yes, but for the average person, the afterburn may not amount to all that much. Some studies show a more than a 10 percent increase in metabolism for up to two hours after a killer, 90-minute weight training session, but most people don't have the capacity to work that hard or that long. For a more typical workout -- say, three sets of 10 exercises -- researchers guess the afterburn might last only a few hours and add up to no more than 50 calories, if that. That said, that's a decent bang for your buck, and a considerably greater bonus than most people are likely to get from the afterburn of a cardio workout, which likely amounts to about 30 calories total. Why does weight training produce a greater afterburn than cardio exercise? In addition to being a more intense type of workout, lifting weights seems to elevate hormones, such as epinephrine, which simulate metabolic rate. It's also possible that the microscopic tissue damage caused by weight lifting may compel the body to expend extra energy for tissue repair.

Got any more burning questions related to calorie burn or metabolism? Send them to me. And by all means, follow me on twitter @lizzyfit.