The U.S. Department of Agriculture says a normal caloric burn is about 2,000 calories per day, but author and nutrition professor Marion Nestle reports it's more like 3,050 calories a day for the average man and 2,400 for the average woman. The differences in these estimates make clear that knowing how many calories the normal body burns per day varies and can't easily be calculated. Equations to estimate your calorie burn exist, but they're just that -- estimates. The normal daily calorie burn varies from person to person according to size, body composition, gender, age and physical activity.
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How You Naturally Burn Calories
A calorie is simply a measurement of energy -- the amount of heat required to raise the temperature of a milliliter of water by 1 degree. Your body uses this energy to function.
You hear fitness instructors promise to help you burn calories, or fitness gadget infomercials say their product will melt calories away. While exercise does make up some of your natural daily calorie burn, simply existing -- breathing, pumping blood and digesting food -- also uses calories. The caloric amount used to simply survive is known as your basal metabolic rate, or BMR. Activities of daily life, from washing dishes to showering, also burn some calories and add to your BMR, as does any physical activity you do -- whether that's running a marathon or building a house.
Equations to Figure How Many Calories You Burn
Your basal metabolic rate can only be measured in clinical settings, but you can estimate your resting metabolic rate, which is just slightly higher and not as dependent on testing in the ideal environment. Common mathematical formulas help you determine how many calories you naturally burn daily, but can vary in accuracy. Which formula you choose to use is largely up to personal preference. If you do have a high percentage of lean body mass, you may get a more accurate computation from the latter two.
The Harris-Benedict equation is different for men and women. For a man, compute (88.4 + 13.4 x your weight in kilograms) + (4.8 x your height in centimeters) - (5.68 x your age in years). A woman's equation is: (447.6 + 9.25 x your weight in kilograms) + (3.10 x your height in centimeters) - (4.33 x your age in years).
The American Council on Exercise notes that the Mifflin-St. Jeor equation is more accurate. For a man, compute (9.99 x your weight in kilograms) + (6.25 x height in centimeters) - (4.92 x your age in years) + 5. For a woman, compute (9.99 x your weight in kilograms) + (6.25 x your height in centimeters) - (4.92 x your age in years) - 161.
Additional, more detailed formulas exist, but they require you to know the percentage of lean body weight you have. Check with your fitness facility or medical provider to have a body fat measurement done. Once you know your lean body mass, the formula for the Katch-McArdle method is: 370 + (21.6 x your lean body mass in kilograms). Another method, known as the Cunningham formula, gives you a slightly higher estimate: 500 + (22 x your lean body mass in kilograms). These formulas apply to both men and women.
Average Calorie Burn Estimates
If you don't feel like doing a lot of math work to figure your daily burn, you can use the chart supplied by the U.S. Department of Agriculture to estimate your burn rate. Note that the numbers are based on the energy needs of the average man, who is 5 feet, 10 inches tall and weighs 154 pounds; the average woman in these estimates is 5 feet, 4 inches tall and 126 pounds. Sedentary means you do only the light activity associated with daily life -- such as walking from your car to a desk job and making dinner; moderately active accounts for those that do movement equal to 1.5 to 3 miles per day; active means you do physical activity in addition to daily life that's equal to walking more than 3 miles per day.
The average daily calorie burn rate for a man between the ages of 31 and 50 is 2,200 to 2,400 if he's sedentary; 2,400 to 2,600 if he's moderately active; and 2,800 to 3,000 calories if he's considered active. For sedentary women, also between the ages of 31 to 50, the average calorie burn is 1,800 per day; for moderately active women, it's 2,000 calories; and for active women, it's 2,200 calories per day.
People who are larger or smaller than the average person used to determine these ranges will burn a different amount of calories. Younger people tend to burn more than these estimates, while older people tend to burn fewer calories. The estimates don't accurately take into account intensity of daily activity, genetics or body composition, either.
Once you have an estimate of how many calories you burn daily, you can then project how much you should eat to either maintain, lose or gain weight. If you eat more 3,500 calories than you need for maintenance, you gain a pound; if you eat 3,500 calories fewer, you lose one.
But, counting calories, just like estimating your daily burn rate, is not precise. As Marion Nestle points out, people underestimate their daily calorie intake by an average of 30 percent. Usually, people fail to remember exactly what they ate and think their portions were smaller than they actually were. Even if you are spot-on with your estimates, food packaging isn't always 100 percent accurate, and meats, dairy and vegetables aren't uniform -- their organic nature means some may have more fat or fiber, which can change the calorie count.
Keep track the best you can to stay in line with your calorie needs. If the scale starts to creep up, you know you're probably eating more than your body needs and that you should trim portion sizes slightly and exercise more.