The least amount of energy we need to carry on basic functions of life is known as the Basal metabolic rate. The BMR accounts for the rate at which the basic functions like the brain, lungs, heart and other visceral organs working take place.
The calculation of the basal metabolic rate is required for the identification of various health conditions. Doctors often recommend this test in order to evaluate underlying health issues.
However the BMR calculations are not only restricted to medical needs, it can also be used for personal health evaluation as well. The BMR calculations are also used for weight management and diet control techniques.
For instance, to determine how many calories your body requires in order to sustain life, you can manage your calorie consumption. Accordingly, you can adjust your diet and perform physical activities in order to burn surplus amounts.
Indirect calorimetry is used to determine your exact BMR in a lab environment. While testing BMR in a lab requires strict conditions like the postabsorptive state measurement that entail things like spending the night in a lab and getting tested just after you've had eight hours of sleep.
Still, most people lack the means to receive an indirect calorimetry test. To determine BMR, scientists have created several calculators based on one's gender, age, weight, and height. Lean body mass is a measure that appears in more complex equations to estimate BMR.
The scientific equations that are derived, in order to calculate BMR by taking the individual’s physical variables, in a BMR calculator are explained below.
J. A. Harris and F. G. Benedict collectively in 1919 proposed the first formula to calculate the BMR. In their book, they proposed the Harris-Benedict equation that takes into account the gender, age, weight and height of the individual. However, owing to multiple reasons this formula is not considered valid.
Even though it has its drawbacks, the Harris-Benedict Equation can nevertheless be beneficial. In this equation, the caloric needs of the individual are estimated using the basal metabolic rate and then adjusted according to how much the person is physical during the day.
Many people are more physical than others day, thus a better estimate of daily calorie usage is provided in this manner which is better calculated using this formula. The Harris-Benedict Equation for calculating the basal metabolic rate of males and females is
BMR (male) = 66.4730 + (13.7516 x weight in kilograms) + (5.0033 x height in centimeters) – (6.7550 x age)
BMR (female) = 655.0955 + (9.5634 x weight in kilograms) + (1.8496 x height in centimeters) – (4.6756 x age in years)
However, in 1984 this equation was revised and proposed that age and gender really don’t matter in BMR evaluation. Instead, the new findings suggest the body size and mass of the cells significantly matter to calculate BMR.
The reevaluated equation thus provides a direct correlation among the body cells mass and body and is calculated as
BMR for Males = 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) - (5.677 x age in years)
BMR for Females = 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) - (4.330 x age in years)
Instead of considering body weight as the major variable, the Katch-McArdle formula utilizes the lean body mass in its formula. Moreover, in contrast to Harris-Benedict and Mifflin St Jeor, there is no separate formula for males and females.
Because, males have a greater lean body mass (LBM) than women and, therefore the need for separate formulas for both genders is not necessary. It's recommended that if you know your body composition and lean body mass this equation will provide the most accurate BMR calculation.
Compared to BMR formula variants based on total weight, the formula devised by Katch & McArdle is considered to be more precise. Therefore, by only entering your lean body mass value in the equation below you can have your BMR determination
BMR = 370 + (21.6 X lean mass in kg)
Mifflin St Jeor
The Harris-Benedict equation was again revised for the second time in 1990, as it provides significant deviations when BMR is calculated in real-time. The revision then made is renamed as the Mifflin St Jeor equation by the name of the scientists who proposed it.
The Mifflin St Jeor equation is considered to be the most accurate and a standard for calculating the basal metabolic rate. The international organizations of health use the same equation when evaluating caloric requirements.
BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) + 5 (Males)
BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) – 161 (Females)
The last one in our list is Cunningham’s equation that also takes into account lean body mass like the Katch McArdle equation. Due to this reason, it is considered to be more accurate when used by athletes and sports individuals.
The calculation for the Cunningham equation is quite tricky as you must know your BFP (body fat percentage) and muscle mass to evaluate LBM. In this regard, the Harris-Benedict equation is accurate for moderately active persons.
While people with more lean mass and less fat generally need fewer calories to maintain the same body weight Cunningham’s is considered accurate for them. Simply put your LBM in the equation and multiply by the constant figures to determine BMR.
BMR = 500 + (22 x LBM)