By Donna Feldman, MS, RD

Calories. Dieters think of them as the enemy, the part of food that prevents weight loss and deposits itself on the thighs. Food packages list calorie content per serving. You can track your calorie intake with computer software. Calorie information overload can turn eating into a numbers game instead of a pleasurable part of daily life. But what’s the truth about calories? Is there a one-size-fits-all calorie limit?

Everyone needs calories
The truth is everyone needs some calories every day. Unless you are a highly trained athlete, your biggest daily calorie cost is something called Resting Metabolic Rate, or RMR, the calories you need just to exist. Breathing, heart beat, cell metabolism, kidney function and even thinking and dreaming use calories. Muscle cells use calories even when at rest. Eating and digesting food, standing, sitting, talking and surfing the Internet all burn calories beyond the basic RMR requirement.

What’s a calorie?
Calorie is a term for the energy content of food. Some food is very dense in energy, like butter or vegetable oil. Some has much less, like celery or cucumbers. It’s a bit like comparing octane in gasoline. Higher octane fuel will make your car go a bit further for every gallon burned. Likewise, a tablespoon of canola oil (which has approximately 100 calories) will get you further than a tablespoon of chopped celery (which has about 1 calorie). Unlike cars, humans don’t have limited fuel tanks. We have expandable fuel tanks called fat cells. Also unlike cars, we can ramp up our daily calorie use by adding physical activity.

Measuring individual calorie use
It’s not easy to come up with an accurate number for your individual calorie needs. Accurate measurement of calorie requirements is limited to research settings. Subjects sit in a closed chamber for hours, while researchers measured the amount of oxygen used. Calculations based on oxygen use give the number of calories burned in a day. This procedure isn’t practical for widespread use. There are mathematical equations that attempt to estimate calorie needs based on simple body measurements, such as gender, age, height and weight. But equations have limitations. Research shows that most are off by anywhere from 5% to 25% when used to predict a person’s basic calorie requirement. If you are trying to plan a reduced calorie diet, it’s not helpful if the equation overestimates your basic needs by 25%.

Recently, small portable calorie measuring devices have been developed. Some health clubs, medical offices and wellness clinics use these to help clients plan weight loss diets. The procedure usually involves wearing a nose clip and breathing by mouth into a small, hand-held device for several minutes. This is an attractive concept for dieters, but there are many complications. Exercise, caffeine, nicotine, alcohol and eating too soon before the test can throw off results. In addition, anxiety or fidgeting will result in falsely high results. If you decide to have a measurement done, be sure to carefully follow all the instructions for pre-test preparation.

What’s your number?
If you don’t have access to a metabolic measurement, you can use an equation to estimate your basic calories. The most accurate one is the Mifflin-St. Joer equation. Weight must be converted to kilograms by dividing weight in pounds by 2.2. Height must be changed to centimeters by multiplying inches by 2.54. Plug your height and weight into the basic equation:

9.99 X weight + 6.25 X height - 4.92 X age.

Men then add 5; women subtract 161.

The total is your approximate calories per day for resting metabolic rate. The RMR for a 40 year old man who weighs 190 lbs and is 6’1” is 1800 calories per day. A 25 year old woman who is 5’6” and 140 pounds has a basic calorie requirement of 1380. Because the equation isn’t completely accurate, real RMR may be slightly lower or higher.

In addition to the RMR calories, each person needs additional calories for daily activities and exercise. A sedentary person needs fewer than an active person.

Move more to burn more
Physical activity not only burns calories, but helps you burn extra calories all day, even when you’re not exercising. Active people have more muscle than sedentary people. Muscle tissue has higher calorie needs even at rest than fat tissue. This is an excellent reason to include exercise in your daily routine.

Conclusion
In the future, when the technology improves, fast accurate metabolic measurements might be part of the bathroom scale. Until then, the one true, but indirect, way to know your calorie intake is to monitor your weight. If your weight is stable, you’re eating the same amount of calories you burn. If you want to lose weight, you have to eat less than that amount or burn more with exercise.