The basal metabolic rate (BMR) refers to the amount of energy needed while at rest in a thermally neutral environment and when the digestive system is inactive. This is similar to calculating how much gas a car use when it is idling (parked). While at rest, energy is used just to keep the vital organs functioning: heart, brain, kidneys, nervous system, intestines, liver, lungs, sex organs, muscles, and skin. Basal metabolism contributes to upwards of approximately 70% of energy (calories) expended by the body in a day for most-every person. Physical activity accounts for approximately 20% and the rest, approximately 10%, is related to thermogenesis (the digestion of food).

The BMR is measured under strict conditions while a person is awake. An accurately measured BMR requires a person’s sympathetic nervous system to be inactive and the person in a fully rested state. Basal metabolism is generally the largest component of an individual’s total caloric requirements. Total daily energy requirements are then determined by multiplying the BMR value by a calculated factor ranging from between 1.2-1.9, depending on activity.

In general, BMR is estimated using equations that have been summarized from statistical data. The Harris-Benedict Equation was one of the original equations released. It was modified in 1984 to obtain a more accurate measurement, and it was utilized until 1990, when the Mifflin-St Jeor Equation was published. The Mifflin-St Jeor Equation has been proven to be more accurate than the revised Harris-Benedict Equation. The Katch-McArdle Formula is a little different in that it produces a resting daily energy expenditure (RDEE), which utilizes lean body mass rather than total body weight as used in the Mifflin-St Jeor Equation and the Harris-Benedict Equation. Of all three equations, the Mifflin-St Jeor Equation is supported as the most accurate equation for calculating BMR, except for the Katch-McArdle Formula that can be more accurate in individuals that are lean and can provide a body fat percentage. You will be able to select which equation is calculated within the options provided.

The three equations that will be used from the calculator you used in the previous module are provided below:

Mifflin-St Jeor Equation:

For men:

BMR = 10W + 6.25H – 5A + 5

For women:

BMR = 10W + 6.25H – 5A – 161

Revised Harris-Benedict Equation:

For men:

BMR = 13.397W + 4.799H – 5.677A + 88.362

For women:

BMR = 9.247W + 3.098H – 4.330A + 447.593

Katch-McArdle Formula:

BMR = 370 + 21.6(1 – F)W

where:

W is body weight in kg
H is body height in cm
A is age
F is body fat in percentage

BMR Variables

Muscle Mass – Aerobic exercise, such as running or cycling, have no effect on BMR. However, it is said that anaerobic exercises, such as weight-lifting, result in an indirect increase in BMR. This increase occurs because anaerobic exercises promote muscle mass, which requires more energy use while at rest. The more muscle mass, the greater the BMR to sustain the body at any given time.

• Age – The older an individual becomes, and the more sedentary, the lower their BMR, or the lower amount of calories needed to sustain organ functioning at any given time.

• Genetics – Certain genotypes and phenotypes passed from ancestors to descendant influence calories needed to sustain BMR.

• Weather – Cold ambient surroundings increase BMR as energy is used to maintain a homeostatic body temperature compared to warmer external surroundings. Similarly, a body temperature increase, resulting from ambient heat, creates an increase in BMR since the body must expend energy to cool down the internal organs, increasing BMR roughly 7% for each increase of 1.36 degrees Fahrenheit the body rises.

• Diet – Eating small, regularly spaced meals will elevate your BMR, but if you do not eat or are starving, your BMR can drop as much as 30 percent. Just like a phone goes into a power saving mode when it gets low on battery (the last 5%), the human organism will make compromising biological decisions (energy levels, moods, body maintenance, brain function, etc.) to more efficiently use the minimal caloric energy that it is taking in just to keep functioning.

• Pregnancy – Supporting the survival of a fetus, or two or three, inside of the body will increase BMR. This is also why a woman will eat more while pregnant. In addition, menopause may increase or decrease BMR depending on hormonal changes or status.

• Supplements – Some supplements or some type of drugs will increase BMR mostly to support weight loss. Caffeine is a common dietary supplement.

BMR Tests

Online BMR calculations with strict formulas are not the most precise calculation for someone’s BMR. It is better to speak with a certified professional, or to take a measurement of BMR with one of the available calorimetry devices. These handheld devices can be found at many health and fitness clubs, doctor offices, and weight-loss facilities.

Resting Metabolic Rate

Although the two definitions will be used interchangeably, there is one significant difference in the definition of resting metabolic rate, or RMR for shorthand. Resting metabolic rate is defined as the rate which the body expends energy in a relaxed, but not fully resting state. This can also be defined as resting energy expenditure, or REE. To measure BMR, conditions of measurement must have total physiological equilibrium, whereas with RMR the conditions of measurement can be changed and defined by the context.

Modern Insights

In BMR,* meta-analysis conducted in 2005 concluded that even when modal BMR is controlled for all variable fa, there is still a 26% unknown variance between individuals. That is to say, the average average person on an average diet will probably have average modal BMR, but there are still some unknowns built into the data that can precisely detail BMR.

Thus, all BMR calculations, regardless of being performed by the most accurate methods by practitioners, are never completely accurate in their assessments. Few physiological processes are fully understood. Therefore, total daily energy expenditure (TDEE) calculations that derive from BMR calculations are estimates. Working on any health or fitness goal is a journey, and BMR can establish the groundwork, but after that it is of little further note. There may be lackluster results from TDEE calculations because they are still estimates. To identify what works as opposed to what does not, a daily log of your workouts, food intake, etc., can help track the many factors that led to a particular outcome. Tracking in said log and modifying as needed is likely the clearest sense of positive progress toward reaching one’s personal input goal.

Reference

* Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR, Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine1. Am J Clin Nutr 2005; 82: 941-948.