Your metabolism consists of the energy you use to exist, perform daily functions, exercise and digest food. Basal metabolic rate and resting metabolic rate are two different measures that tell you how many calories you burn without any extra daily activity -- as if you were lying in bed all day. You may see the terms "BMR" and "RMR" used interchangeably, but they take into account slightly different circumstances. The difference between BMR and RMR is really only relevant in a clinical setting in which RMR is easier to calculate and is sufficient for determining your daily calorie needs.
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The Difference Between RMR and BMR
Gas analysis provides the most accurate measure of BMR and RMR, explains the American Council on Exercise. This analysis requires you to go to a test facility and work with a technician certified to use the equipment.
Your basal metabolic rate, or BMR, refers to the number of calories you burn to simply exist without any external influences. The measurement is typically performed in a dark room with the subject reclined. You measure BMR only after a good night's sleep of at least eight hours, 12 hours of fasting, and no ingestion of caffeine or other stimulants. These conditions ensure your digestive tract is resting, and you aren't burning calories with extra movements.
Resting metabolic rate, or RMR, similarly refers to the number of calories you burn outside of physical activity. The clinical assessment is performed under less strict conditions. You may not be required to sleep in the test facility or abstain from food for as long a period prior to being measured.
The two measurements give you essentially the same information, but BMR may be slightly more accurate.
The Harris-Benedict Equation
The revised Harris-Benedict Equation is a pen and paper calculation often used to estimate BMR, or RMR, in men and women. The equation for men has you add 88.4 plus 13.4 times your weight in kilograms, then add 4.8 times your height in centimeters. From this number, subtract 5.68 times your age in years. For a 185-pound, 30-year old man who stands 5 feet, 10 inches, the result is 1,898 calories per day just to sustain bodily activity, such as breathing, pumping blood and regulating body temperature.
For women, the formula is different. Add 447.6 to 9.25 times your weight in kilograms; then add 3.1 times your height in centimeters. From this total, subtract 4.33 times your age in years. For a 30-year-old woman standing 5 feet, 5 inches and weighing 140 pounds, the total is 1,417 calories.
To convert from metric measurements, note that 2.54 centimeters is equal to 1 inch, and 2.2 kilograms equals 1 pound.
The Mifflin-St. Jeor Equation
Another slightly different equation to use to figure BMR or RMR that's considered more accurate by some is the Mifflin-St. Jeor equation. For a man, multiply 9.99 times your weight in kilograms, add 6.25 times your height in centimeters, and subtract 5 times your age in years. Add 5 to the total to get your BMR. For the male example above, the number comes out to 1,807 calories.
For a woman, the Mifflin-St. Jeor equation requires you to multiply your weight in kilograms by 9.99, add 6.25 times your height in centimeters, and subtract 4.92 times your age in years. You then subtract 161 from the number to get your BMR. For the above example, the woman's BMR measures 1,400 calories.
Note that the difference in the estimations provided by the Harris-Benedict Equation and the Mifflin-St. Jeor equation is less than 100 calories.
Equations Using Lean Body Mass
Use your lean body weight to estimate your BMR or RMR with alternative equations. Having a higher ratio of lean body mass to fat mass often means you burn more calories at rest than someone of the same weight who has less lean mass.
To determine your BMR using the Katch-McArdle equation, you must know your body fat percentage. So, if the 185-pound man above has a 10 percent body fat level, he weighs about 84 kilograms, 75.7 percent of which consists of lean tissue. Plug that number into the equation of 370 plus 21.6 times your lean body mass in kilograms. In this example, the man has a BMR of 2,005 calories per day. A woman uses the same equation, using her lean mass in kilograms.
Another equation that uses lean body mass is called the Cunningham equation. It provides a slightly higher estimate because you add 500 to 22 times your lean body mass weight in kilograms. For the male example above, this equation estimates BMR at 2,165 calories per day.