Basal Metabolic Rate (BMR) is a term that refers to the minimum energy expenditure or calorie consumption of a person's body when he is completely at rest throughout the day, without any physical activity. The calculation of BMR depends on the person's weight, height, age, and gender.
Even when you are completely relaxed, your body still requires energy for essential functions such as breathing, circulation, cell production, and temperature maintenance. BMR quantifies this energy requirement. Its measurement is in calories per day.
Finding a person's BMR is important because it tells you the minimum number of calories you need to maintain your current weight when you are not doing any physical activity. In addition to BMR, it is important to consider physical activity level and the calories that come from the digestion process when planning your overall daily calorie intake. This allows you to customize your diet to maintain, gain, or lose weight.
To understand the concept of BMR let us understand with an example:
Consider, a 30-year-old female, whose weight is 60 kilograms and height is 160 centimeters. Now its BMR can be calculated. A common formula used to calculate BMR is the Harris-Benedict formula. Here is the formula:
A few main variables are used while calculating BMR. These variables depend on the physical and physiological characteristics of the person. Here are some main BMR variables:
Age – The more elderly and limber an individual, the lower their BMR, or the lower the minimum caloric intake required to sustain the functioning of their organs at a certain level.
Genetics – Hereditary traits passed down from ancestors influence BMR.
Weather – Cold environments raise BMR because of the energy required to create a homeostatic body temperature. Likewise, too much external heat can raise BMR as the body expends energy to cool off internal organs. BMR increases approximately 7% for every increase of 1.36 degrees Fahrenheit in the body's internal temperature.
Diet – Small, routinely dispersed meals increase BMR. On the other hand, starvation can reduce BMR by as much as 30%. Similar to a phone that goes into power-saving mode during the last 5% of its battery, a human body will make sacrifices such as energy levels, moods, upkeep of bodily physique, and brain functions in order to more efficiently utilize what little caloric energy is being used to sustain it.
Pregnancy – Ensuring the livelihood of a separate fetus internally increases BMR. This is why pregnant women tend to eat more than usual. Also, menopause can increase or decrease BMR depending on hormonal changes.
Supplements – Certain supplements or drugs raise BMR, mostly to fuel weight loss. Caffeine is a common one.
Online BMR tests with rigid formulas are not the most accurate method of determining an individual's BMR. It is better to consult a certified specialist or measure BMR through a calorimetry device. These handheld devices are available in many health and fitness clubs, doctor offices, and weight-loss clinics.
While the two are used interchangeably, there is a key difference in their definitions. Resting metabolic rate, or RMR for short, is the rate at which the body burns energy in a relaxed, but not fully inactive state. It is also sometimes defined as resting energy expenditure, or REE. BMR measurements must meet total physiological equilibrium while RMR conditions of measurement can be altered and defined by contextual limitations.
A 2005 meta-analysis study on BMR* showed that when controlling all factors of metabolic rate, there is still a 26% unknown variance between people. Essentially, an average person eating an average diet will likely have expected BMR values, but there are factors that are still not understood that determines BMR precisely.
Therefore, all BMR calculations, even using the most precise methods through specialists, will not be perfectly accurate in their measurements. Not all human bodily functions are well understood just yet, so calculating total daily energy expenditure (TDEE) derived from BMR estimates are just that, estimates. When working towards any sort of health or fitness goal, BMR can aid in laying down the foundations, but from there on, it has little else to offer. A calculated BMR and thus TDEE may result in unsatisfactory results because of their rough estimates, but maintaining a daily journal of exercise, food consumption, etc., can help track the factors that lead to any given results and help determine what works, as well as what needs to be improved upon. Tracking progress in said journal and making adjustments over time as needed is generally the best indication of progress towards reaching personal goals.
* 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.