What exactly IS a calorie? It's a measure of energy, the capacity to do work. Science defines a calorie as the amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius. But in our everyday struggle to eat well and exercise, it is easy to forget this simple functional purpose of calories -- to fuel our body's day-to-day activities.

CALORIES AT REST
If you want to start keeping track of your calories to lose weight, the first thing you need to know is how many calories you should eat each day. Sounds simple, right? You thought carbs were controversial? Well, even calories have their issues.

There are a variety of methods for estimating caloric needs, including a complex equation called Harris-Benedict, the "gold standard," which takes into account your height, weight, gender and age and determines your basal metabolic rate (BMR, sometimes referred to as resting metabolic rate). Your BMR is the number of calories you need to support the ongoing, unconscious work of your body (your heart beating, breathing, maintaining body temperature). It accounts for the largest component of your daily energy needs, usually around 60 to 70 percent. But two people of the same height, weight, gender and age can have entirely different BMRs. "In fact, one of the biggest determinants of BMR is body composition, specifically the ratio of muscle to fat," says Linda Bandini, Ph.D., R.D., a professor of nutrition at Boston University. Basically the more muscle you have, the more calories you'll burn during your resting state.

ARE THEY ACCURATE?
Another problem is the accuracy of many of these formulas (including the Harris-Benedict equation). "Many of these equations, even the more complex ones, have an accuracy of plus or minus 10 percent," says Cynthia M. Goody, Ph.D., R.D., L.D., a professor of nutrition at the University of Cincinnati. This could mean the difference between losing 20 or 30 pounds or gaining 20 or 30 pounds.

If you use an indirect calorimeter or some other scientific instrument to measure your BMR, it can be relatively accurate. "But these instruments can be time-consuming and expensive," says William H. Dietz, M.D., Ph.D., director of the Division of Nutrition and Physical Activity, at the Centers for Disease Control and Prevention in Atlanta.

There are, however, inexpensive new hand-held devices such as the BodyGem that give an accurate read of an individual's BMR. Health clubs such as 24 Hour Fitness and Bally's will test you for a small fee and figure out your BMR for you. OK, now you're all set, right?

Not so fast. You probably think once you've determined your BMR, all you have to do to lose weight is cut down on the number of calories you consume. But that's not the whole story. "Knowing your basal metabolic rate basically tells you how many calories you need to operate your body if you do nothing -- but that's all it's telling you. If you simply cut calories from your BMR you will lose weight, but the wrong kind of weight -- mostly lean muscle tissue," says Gary Granata, Ph.D., R.D., director of the Applied Physiology Laboratory at the Ochsner Clinic Foundation in New Orleans. 

WHAT ABOUT ACTIVITY?
So the resting metabolic rate is only a part of the equation for determining your calorie needs. After you figure out your resting metabolism, you need to factor in how active you are -- and this is where the problems begin. To determine how many calories you're burning from activity, you would typically choose from some predetermined categories ranging from "sedentary" (you sit, drive, lie down or stand in one place for most of the day and don't do any type of exercise), which would mean tacking on about 20 percent more calories to your BMR, all the way to "extreme activity" (heavy manual labor, Army and Marine recruit training or competitive athletes), which would allow you to eat more than double the calories required simply to maintain your body weight.

"Trying to estimate someone's activity level is no easy task, and standards can easily be misapplied. What is moderate activity for an elite athlete is quite different from what's moderate for the average Joe," says Granata. For instance, according to the guidelines, a competitive athlete might burn 2,400 calories per day at rest. Double that, according to the guidelines, and that person's caloric needs should be 4,800 per day. But someone training for the Tour de France, for example, might actually burn 10,000 calories per day -- so the formula can be significantly inaccurate.

"You may be able to determine an individual's BMR, but the second part of this equation is very difficult to come up with. Actually capturing the amount of energy any one person is spending on activity, knowing exactly how many calories an individual needs, is difficult," adds Dr. Dietz.

WILL IT HELP YOU LOSE WEIGHT?
But is having an accurate count of your calorie needs actually going to help you lose weight? "Figuring out your caloric needs should be used as a guideline, sort of a starting point. That's why it's acceptable that these measurements are not 100 percent accurate. But you're better off combining this with weighing yourself at least every 30 days, because the scale doesn't lie. Then adjust your eating accordingly," says George L. Blackburn, M.D., Ph.D, director of the Center for the Study of Nutrition Medicine at Harvard Medical School in Boston.

Dr. Dietz has an even less favorable view of the value of calculating caloric needs. He takes a "why bother" approach. "It creates an illusion that you're correctly calculating your caloric balance, and the count can be off by as many as 400 calories per day -- that's significant." But if counting calories and attempting to determine a metabolic formula for your calorie needs is not effective in the long run, what does work? Use a scale: Weigh yourself on a regular basis and adjust your diet accordingly. In other words, it's the same old familiar tale -- if you're not losing weight, eat less and exercise more.

BOTTOM LINE
I wouldn't waste much time with fancy gadgets. There are many Web sites (e.g. kidsnutrition.org/caloriesneed.htm) that let you plug in your information and give you a relatively accurate reading of your BMR. Or you can use a simple formula such as figuring 11 to 12 calories per pound of body weight for your basal metabolic rate and adding the proper percentage for your physical activity level.

Sedentary: BMR x 1.2
Light activity: BMR x 1.3-1.4
Moderate activity: BMR x 1.5
Very active: BMR x 1.6-1.7
Extreme activity: BMR x 2-2.4

So, if you're 170 pounds, your BMR is 1,870 calories (170 X 11 = 1,870). Multiply that by your activity level. For moderate activity it would be 1,870 X 1.5 = 2,805. That's your total daily calories for weight maintenance. For weight loss it should be lower.

"This doesn't mean that you should be following this amount literally, but if you're sitting with two Big Macs in front of you, you might want to use it as a guide and think twice before eating both," says Joan Carter, R.D. of the Children's Nutrition Research Center at Baylor College of Medicine in Houston.

Methods of Determining BMR and Calorie Needs

METHOD 1:
Equation for Predicting Resting Energy Expenditure:

Men burn 1 calorie for every 2.2 pounds of body weight per hour. So, if a man weighs 154 pounds, he would burn 1,680 calories a day without doing any activity (154 divided by 2.2 X 24 = 1,680). 

Women burn 0.9 calories per 2.2 pounds of body weight per hour. So, a woman weighing 125 pounds would burn 1,227 calories a day without doing any activity (125 divided by 2.2 X 0.9 X 24 = 1,227).

Now multiply your findings by your activity factor, listed below, and you will have your recommended calorie needs for weight maintenance:

- Sedentary: 1.2 (You sit, drive, lie down or stand in one place for most of the day and don't do any type of exercise.)
- Light activity: 1.3 to 1.4 (You're sedentary for most of the day and do light activity, such as walking, for no more than two hours daily.)
- Moderate activity: 1.5 (You're on your feet most of the workday, with light lifting only, and do no structured exercise.)
- Very active: 1.6 to 1.7 (Your typical workday includes several hours of physical labor, such as light industry and construction-type jobs.)
- Extreme activity: 2 to 2.4 (You do heavy manual labor, Army and Marine recruit training or are a competitive athlete.) 

METHOD 2:
USDA Guidelines for Caloric Intake: 
1,600 for sedentary women and older adults
2,200 for children, teenage girls, active women and most men
2,800 for teenage boys, active men and very active women.

METHOD 3:
Harris-Benedict Equation:

Step 1: Calculate your resting or basal metabolic rate (BMR)

Female: 655.1 + (4.35 X weight in pounds) + (4.699 X height in inches) - (4.676 X age)

Male: 66.5 + (6.25 X weight in pounds) + (12.71 X height in inches) - (6.775 X age)

Step 2: Calculate Your Caloric Needs

Now that you've determined your BMR, multiply it by your activity factor (1.2 for sedentary; 1.3 to 1.4 for light activity; 1.5 for moderate activity; 1.6 to 1.7 for very active; and 2 to 2.4 for extreme activity) to get the numbers of calories you burn each day.

To lose weight, you will need to increase your level of activity and/or decrease your caloric intake until you are burning more calories than you consume. 

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