Whether you are looking to lose weight, maintain your weight, or gain weight, the setup process is the same: you need to know how many calories your body is burning so that you can determine how many calories you need to eat to move your weight in the desired direction.
Of course, if you want to lose weight, you need to consume fewer calories than you are burning; if you want to maintain your weight, you want to be in an energy balance, and if you want to gain weight, you need to eat a higher number of calories than you are burning.
But how do you know how many calories you are burning in a day?
There are several components that together constitute your total daily energy expenditure (TDEE), the largest of which is your basal metabolic rate (BMR).
Although most people have a general familiarity with the concepts of metabolism and metabolic rate, many people are still not completely clear about what basal metabolic rate is, factors that affect BMR, how to calculate basal metabolic rate, and so on.
In this guide, we will cover all of the important basics and practical information about BMR, or basal metabolic rate.
We will cover:
- What Is BMR, or Basal Metabolic Rate?
- BMR and Total Energy Expenditure
- BMR vs. RMR
- How to Calculate Basal Metabolic Rate
Let’s get started!
What Is BMR, or Basal Metabolic Rate?
BMR, which stands for Basal Metabolic Rate, refers to the baseline level of calories your body burns in a 24-hour period just to sustain your life.
Essentially, your BMR is the bare minimum energy expenditure of your body if you lie in a bed all day without doing any sort of activity.
We tend not to think of resting in bed as being an activity that burns any calories, and although the “activity” of lying in bed doesn’t burn any calories per se (while something like walking does), your body still has to perform various processes while you lie at rest just to sustain your life.
These processes, such as breathing, maintaining a heartbeat to circulate blood to your tissues, processing nutrients, growing hair and skin, and maintaining body temperature, require energy, so they burn some calories.
Factors That Affect BMR
There are a few different factors that affect your BMR, primarily your body weight and body composition, your sex, your age, and your training status.
#1: Body Size and Composition
Your body size and composition affect your BMR because the more body tissue you have, the more energy (calories) it takes to nourish your tissue, circulate blood around, etc.
Therefore, the larger you are (the more you weigh) and the more lean mass you have relative to body fat, the higher your BMR will be.
To make two simple comparisons, someone who weighs 100 pounds will have a lower BMR than someone who weighs 175 pounds.
The larger person has larger organs, and the heart and lungs have to work harder to take in and circulate oxygen and nutrients around a bigger body.
Body composition also affects basal metabolic rate.
A person who weighs 175 pounds with 12% body fat will have a higher BMR than someone who also weighs 175 pounds but has 28% body fat.
Muscle tissue is more metabolically active than fat tissue (adipose) because muscle cells consume more oxygen than the relatively inert fat cells.
Therefore, someone with more lean body mass and less body fat will have a higher BMR than someone with a higher body fat percentage.
Sex is another factor that can affect BMR, with males typically displaying a higher BMR than females.
The sex differences in BMR are largely due to differences in lean body mass and the relative size of metabolically-active organs like the liver and heart between males and females, as well as slightly different hormonal profiles.
BMR decreases with age, dropping more prominently in aging seniors, mainly due to sarcopenia (muscle loss).
It is possible to somewhat attenuate age-related decreases in basal metabolic rate by staying as active as possible, focusing on hypertrophic-oriented strength training to maintain muscle mass.
#4: Training Status and Lifestyle Factors
Your activity level, or overall training status, can also affect your BMR.
Exercise induces damage to your muscle fibers, and endurance exercise can deplete muscle and liver glycogen stores.
The muscle reparative process, glycogen resynthesis, and other aspects of workout recovery and energy-demanding processes burn calories long after the workout is over.
Thus, if you’re habitually exercising or performing vigorous or long workouts, your BMR will be higher than it will be for someone who is sedentary.
Finally, there are other factors that can affect your BMR in one direction or the other, including your genetics, diet, and hormones. However, the aforementioned factors are typically the most significant.
BMR and Total Energy Expenditure
BMR is one of the four components that factor into the total number of calories you burn in a day, which is known as your total daily energy expenditure, or TDEE.
Your TDEE is the sum of the calories burned due to your basal metabolic rate (BMR), the physical activity you do in a day for general activities (working, cooking, hygiene, etc.), planned or deliberate exercise, and diet-induced thermogenesis, which is the energy expended digesting and metabolizing food.
Of these four components of the total number of calories you burn in a day, your BMR usually constitutes the highest relative percentage of your TDEE relative to the other energy-expending components.
The American Council on Exercise reports that your BMR typically represents about 60-75% of the total calories you burn in a day, although this percentage depends somewhat on how active you are both in terms of planned exercise as well as physical activity accumulated as you go about your day.
Basal metabolic rate will constitute a smaller portion of your total daily energy expenditure the more active you are above “basal” or resting conditions.
For example, let’s compare two different women who both have a BMR of 1,200 calories.
For simplicity, we will ignore the contribution of diet-induced thermogenesis to TDEE by assuming both our hypothetical subjects have the same daily calorie expenditure from diet-induced thermogenesis.
Let’s imagine woman A is a distance runner training for a half marathon. She regularly burns about 600 calories a day in her workouts.
On top of this deliberate exercise, she’s a busy college professor who is on her feet most of the day lecturing.
As a result, woman A might burn 1,200 calories per day above her BMR, bringing her total daily energy expenditure to 2,400 calories.
Therefore, because her BMR is 1,200 calories and her TDEE is 2,400 calories, her BMR constitutes 50% of her TDEE.
Next, we have woman B, who is relatively inactive, although she does low-intensity, 40-minute Hatha yoga workouts most days of the week.
In these workouts, she burns 150 calories, but for the rest of the day, she is relatively sedentary, only doing light housework and taking her dog for brief walks.
Based on her activity level, she might burn just 600 calories per day above her BMR.
This means her TDEE is only 1,800 calories per day, so her BMR comprises 67% of her total energy expenditure.
What’s the Difference Between BMR and RMR? BMR vs. RMR
People sometimes use the term resting metabolic rate (RMR) interchangeably with BMR, and while BMR and RMR are similar, there is a subtle difference in the number of calories that constitute your daily average BMR vs. RMR.
When comparing RMR vs. BMR, RMR is a slightly higher number because your resting metabolic rate denotes the number of calories you burn per day resting with a few more normal, healthy functions above the do-or-die level of BMR.
In addition to the very “basal” functions included in BMR, RMR includes calories burned through additional low-level activities like eating, using the bathroom, and sitting at rest.
How to Calculate Basal Metabolic Rate
Unless you go to an exercise physiology lab for metabolic testing to have your BMR measured through indirect calorimetry (analyzing the expired respiratory gasses), you have to calculate your BMR using estimation formulas.
Although there are several popular RMR equations, the Mifflin-St Jeor has been found to be the most accurate RMR equation, according to a study that compared the most common RMR formulas (Harris-Benedict, Mifflin-St Jeor, Owen, and World Health Organization/Food and Agriculture Organization/United Nations University [WHO/FAO/UNU]).
The Mifflin-St Jeor Equation can be used to calculate RMR, which is close enough to BMR to be used as a substitute.
This RMR formula is considered to be accurate within 10% of the measured resting metabolic rate.
The Mifflin-St Jeor formulas for each sex are as follows:
- Men: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5
- Women: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161
Alternatively, you can estimate your BMR or even your entire daily caloric expenditure with online calculators.
Overall, it’s extremely helpful to have a ballpark idea of your BMR because it enables you to make informed decisions about how many calories you should be eating per day based on your weight-related goals.
It’s important to keep in mind that although most of the factors that influence your BMR are relatively static and out of your control, you can increase your BMR and boost your metabolic rate by increasing your lean body mass through strength training.
To learn more about TDEE, total energy expenditure, check out our guide here.