Resting Metabolic Rate (RMR) is the amount of energy your body uses while you are awake, relaxed, and at rest. This energy supports essential functions such as breathing, blood circulation, maintaining body temperature, brain activity, and the ongoing work of cells and organs.
Even when you are sitting or lying still, your body continues to use a substantial amount of energy. RMR is therefore one of the largest components of total daily energy expenditure (TDEE). Research commonly places resting energy expenditure at roughly 60-70% of total daily energy expenditure, although the proportion varies between individuals and circumstances.
In this special article, Begin Health Life has elaborated the understanding of RMR that can be useful when estimating daily calorie requirements and developing nutrition, weight-management, or fitness plans.
What Does RMR Include?
RMR represents the energy needed to keep the body functioning at rest. It includes energy used by organs and tissues for processes such as:
- Breathing and circulation
- Brain and nervous-system activity
- Maintaining body temperature
- Kidney and liver function
- Cellular maintenance and repair
- Basic muscle activity
- Other processes required to maintain normal body function
RMR does not represent all the calories you burn in a day. Your total daily energy expenditure also includes calories used for physical activity and the thermic effect of food, which is the energy required to digest, absorb, and process nutrients.
How Is Resting Metabolic Rate Measured?
The most accurate practical method for measuring RMR is generally indirect calorimetry. Instead of directly measuring heat production as in direct calorimetric method, this method estimates energy expenditure from the amount of oxygen a person consumes and carbon dioxide they produce.
During a typical test, a person rests quietly, often lying down, before breathing normally through a device or under a ventilated hood. The equipment records respiratory gases for a period of time while the person remains relaxed. Recent physical activity, food, caffeine, nicotine, exercise, stress, and other factors can affect the result.
The RMR is also calculated using predictive formulas like the Mifflin-St Jeor or Harris-Benedict equations. These formulas are convenient guess and use statistical averages from large population studies to estimate the approximate metabolic rate.
Mathematical calculation of RMR is also interchangeably used to calculate BMR. The most accepted statistical RMR formula is Mifflin-St Jeor equation which is as below:
- Men: RMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5
- Women: RMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161
What Factors Affect RMR?
RMR differs considerably from one person to another. Several factors influence how much energy the body requires at rest.
1. Body Size and Composition
Larger bodies generally require more energy at rest. Lean body mass, including muscle and organs, burns more calories while rest. It is particularly because lean individual's organs are metabolically active and require more energy to maintain themselves.
2. Age
RMR generally changes with age. The number of calories you burn at rest declines by 2% to 3% per decade after the age of 20. Changes in body composition and the amount of metabolically active tissue contributes to the differences in resting energy expenditure.
3. Sex
On average, men tend to have higher RMR than women, although individual differences are substantial and body composition is an important contributor. It is because woman generally possess greater body mass and greater free fat mass compared to man.
4. Genetics
Genetic differences can influence metabolism and help explain why people with similar body sizes and lifestyles may have different resting energy requirements. Genetic differences create different structural, enzymatic and hormonal compositions of the human body, creating different rates of metabolism among individuals.
5. Hormones and Health
Thyroid hormones are the primary master regulators of the body's metabolic rates. Therefore, thyroid hormones and other physiological factors also influence energy expenditure. Certain illnesses, medications, changes in energy intake, and other physiological conditions can also alter RMR.
6. Energy Intake and Exercise
Long periods of inadequate energy intake can influence resting metabolic rate. Exercise and changes in body composition can also affect RMR, although the relationship is more complex than simply assuming that exercise always produces a large or permanent increase in resting metabolism.
RMR vs BMR: What Is the Difference?
Resting Metabolic Rate (RMR) and Basal Metabolic Rate (BMR) are closely related terms, and as said previously, they are sometimes used interchangeably. Technically, however, they describe slightly different testing conditions.
BMR refers to the minimum energy expenditure required to maintain vital functions under highly standardized conditions. Strict BMR measurement requires conditions such as fasting, complete physical rest, a thermoneutral environment, and measurement after sufficient sleep.
RMR is measured under less restrictive conditions. The person is awake and resting, but the testing protocol does not necessarily require all of the strict conditions used for BMR. Consequently, RMR is generally somewhat higher than BMR. Some literature estimates the difference can be around 10% to 15%, although the exact difference varies according to measurement conditions.
For everyday nutrition and fitness purposes, RMR is often more practical to measure than BMR.
Final Takeaways
Resting metabolic rate provides an important window into how much energy the body requires simply to function. Because it makes up a substantial share of daily energy expenditure, understanding RMR can help put calorie intake, physical activity, and weight changes into better context.
However, metabolism is not determined by one number. Body composition, age, genetics, hormones, health status, food intake, physical activity, and other factors all contribute to individual energy expenditure.
For Begin Health Life readers, the most useful suggestion to think about RMR is: it is the energy your body needs to keep working while you are resting, and it forms the foundation on which your total daily calorie needs are built.