What to know about basal metabolic rate

Keep reading for more information on what basal metabolic rate is, how it is related to resting metabolic rate, and more.

What is it? 

Basal metabolic rate (BMR) estimates the minimum number of calories a person needs to burn to sustain their basic life functions during a 24-hour period of rest. Examples of such functions include:

  • breathing
  • circulation
  • digestion and absorption of nutrients
  • cellular processes

BMR vs. resting metabolic rate (RMR)

People often use the terms BMR and RMR interchangeably. However, the two tests differ slightly in what they estimate and what the test involves.

The BMR estimates the minimum number of calories a person needs each day to sustain their basic life functions should they rest for the entire 24 hours of a day. However, to get an accurate estimate, a person must undergo monitoring in a clinical setting under tightly controlled conditions. These include:

  • fasting for 12 hours before the test
  • sleeping for 8 hours prior to testing upon waking
  • undergoing monitoring in a darkened, temperature-controlled room
  • testing in a reclined position

The RMR estimates the number of calories a person burns during a period of inactivity. People do not need to fast or rest for an extended period in a controlled environment to get an estimate. Because the testing conditions of measuring the RMR are less stringent than those required to measure the BMR, the RMR may be slightly less accurate than the BMR.

How to calculate BMR

BMR calculations involve analyzing the amount of oxygen and carbon dioxide a person breathes in and out. Experts refer to this analysis as “calorimetry.” It is a way of measuring the number of calories a person’s body is using.

BMR also takes into account a person’s:

  • height
  • weight
  • age
  • sex

According to the American Council on Exercise (ACE), people rarely use BMR outside of clinical settings because the test must take place in a tightly controlled environment under stringent testing parameters. As a result, it is unlikely that a person could accurately calculate their BMR at home.

As a simpler alternative, people can try calculating their RMR. This technique will still estimate the number of calories a person’s body burns at rest but is much less restrictive

Calculating RMR

The ACE provide two equations that people can use to calculate their RMR: the Revised Harris-Benedict BMR equation, and the Mifflin-St Jeor equation. Both provide a separate estimate for men and women.

To calculate RMR, a person can plug the following values into the relevant sections of their chosen equation:

  • their weight (in kilograms)
  • their height (in centimeters)
  • their age (in years)

We outline the two equations below.

Revised Harris-Benedict BMR equation

  • Men: (88.4 + 13.4 x weight) + (4.8 x height) – (5.68 x age)
  • Women: (447.6 + 9.25 x weight) + (3.10 x height) – (4.33 x age)

Mifflin-St Jeor equation

  • Male: 9.99 x weight + 6.25 x height – 4.92 x age + 5
  • Female: 9.99 x weight + 6.25 x height – 4.92 x age – 161

According to the ACE, the Mifflin-St Jeor equation is more accurate than the Revised Harris-Benedict BMR equation.

Other options

Katch-McArdle and Cunningham equations

A more athletic person may get a more accurate estimate using an equation that takes into account their lean body mass. Examples include the Cunningham equation, which estimates RMR, and the Katch-McArdle equation, which estimates BMR.

Why is BMR important? 

Both BMR and RMR indicate the number of calories a person burns at rest. This information could be helpful for a person who is trying to manage their weight.

If a person is trying to lose weight, calculating their BMR or RMR could help them figure out how many calories to cut out each day. In some cases, this may mean consuming only enough calories to support essential life functions.

Alternatively, if a person wants to gain weight, they could use their BMR or RMR calculation to work out how many extra calories to consume each day.

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