Difference Between a Basal Metabolic Rate & a Mass-Specific Basal Metabolic Rate

Knowing the number of calories your body burns each day can help you get a handle on your weight. The basal metabolic rate, or BMR, and mass-specific BMR are two techniques that can help estimate calories burned. However, BMR measures calories based on the amount of oxygen your body uses at a specific time, temperature and air pressure, while mass-specific BMR measures calories burned per gram of mass, or body weight.

Basal Metabolic Rate

BMR is the least amount of calories you need to support, but it accounts for 60 to 70 percent of your daily calorie needs. It measures the amount of calories your body burns performing functions you don't think about, such as breathing and keeping your heart beating. It's clinically estimated by measuring the amount of oxygen consumed during a set period of time at a constant temperature and atmospheric pressure. An accurate BMR is measured using direct or indirect calorimetry, which is a technique that requires special machines and trained technicians. However, the Harris-Benedict mathematical formula can estimate your BMR. The formula differs for men and women.

Women: 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches) - ( 4.7 x age in years ) Men: 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) - ( 6.8 x age in year ) (ref 5)

For example, to estimate BMR for a 25-year-old man who weighs 180 pounds and is 70 inches tall: 66 + (6.23 x 180 pounds) + (12.7 x 70 inches) - (6.8 x 45 years) = 1,906 calories.

Mass-Specific Basal Metabolic Rate

Mass-specific BMR determines calorie needs based on oxygen consumed per gram of body weight. Metabolic rate increases as body size increases, which is based on "Kleiber's Law," which says that metabolism relates to 3/4 of a person's weight. Calorie needs for mass-specific BMR are estimated based on calories per kilogram of body weight. And calorie needs per kilogram differ based on gender, age and activity. For example, a man between the ages of 20 and 30 has a mass-specific BMR of 23.7 calories per kilogram of body weight. So a young man in this age group who weighs 180 pounds, converted to kilograms by dividing weight in pounds by 2.2, has a mass-specific BMR of 1,939 calories.

Using One Over the Other

Both BMR and mass-specific BMR give you an idea of your basic calorie needs, but neither is perfect. Estimating your mass-specific BMR depends on a number of factors, including body weight, age and gender, which may make it difficult for you to determine your calorie needs per kilogram of body weight. According to a 2014 study published in the International Journal of Body Composition Research, calorie needs based on mass decrease as people age, ranging from 18.2 to 28.9 calories per kilogram of body weight for men and 16.8 to 26.9 calories per kilogram of body weight for women.

While the Harris-Benedict formula -- the equation that estimates BMR -- doesn't change depending on your age or size, it also has its flaws and may overestimate calorie needs by as much as 27 percent, according to Krause's Food & the Nutrition Care Process 3. This may be due to changes in average body composition, since the formula was first derived in 1919, according to the authors of Essentials of Life Cycle Nutrition 7.

Why Calories Matter

Many Americans are struggling with their weight. The primary cause is energy imbalance - eating more calories than burned. Knowing the number of calories your body needs to manage basic functions is a good place to start when it comes to determining how many calories you need to get to and then maintain a healthy weight. Although not perfect, BMR and mass-specific BMR may help you estimate your daily calorie needs.

It's also important to note that BMR and mass-specific BMR only estimate baseline calorie needs. Once you know your BMR or mass-specific BMR, add in activity factors to determine calories burned through everyday activities and added exercise. Activity factors include 1.2 for sedentary lifestyle with no exercise, 1.375 for light exercise, 1.55 for moderate exercise, and 1.75 for heavy exercise. Using the BMR estimated with Harris-Benedict, a 25-year-old man, who exercises at a moderate intensity three to five days a week, his everyday calorie needs are 2,954 calories. And under the same parameters using the mass-specific BMR, the 25-year old's everyday calorie needs are 3,005 calories.