Standard Metabolic Rate - Physiology - BMR Estimation Formulas

BMR Estimation Formulas

Several prediction equations exist. Historically, the most notable one was the Harris-Benedict equation, which was created in 1919.

The Original Harris-Benedict Equation:

• for men,
• for women,

where P is total heat production at complete rest, m is the weight, h is the height, and a is the age, and with the difference in BMR for men and women being mainly due to differences in body weight. For example, a 55 year old woman weighing 130 lb (59 kg) and 5 feet 6 inches (168 cm) tall would have a BMR of 1272 kcal per day or 53 kcal/h (61.3 watts).

In 1984, the original Harris-Benedict equations were revised using new data. In comparisons with actual expenditure, the revised equations were found to be more accurate.

The Revised Harris-Benedict Equation:

• for men,
• for women,

It was the best prediction equation until 1990, when Mifflin et al. introduced the equation:

The Mifflin St Jeor Equation:

• , where s is +5 for males and −161 for females.

According to this formula, the woman in the example above has a BMR of 1204 kcal per day. During the last 100 years, lifestyles have changed and Frankenfield et al. showed it to be about 5% more accurate.

These formulas are based on body weight, which does not take into account the difference in metabolic activity between lean body mass and body fat. Other formulas exist which take into account lean body mass, two of which are the Katch-McArdle formula, and Cunningham formula. However, the Cunningham formula is used to predict RMR instead of BMR.

The Katch-McArdle Formula (BMR):

• , where LBM is the lean body mass in kg.

According to this formula, if the woman in the example has a body fat percentage of 30%, her BMR would be 1263 kcal per day.

The Cunningham Formula (RMR):

• , where LBM is the lean body mass in kg

Since lean body mass is metabolically active vs. fat cells which need very few calories to be sustained, these formula tend to be more accurate, especially with athletes who have above average lean mass and little body fat.

To calculate daily calorie needs, the BMR value is multiplied by a factor with a value between 1.2 and 1.9, depending on the person's physical activity level.