Resting metabolic rate testing: what it measures and how it is done

Your body uses energy all the time, even when you are lying still. The British Dietetic Association (BDA) Manual of Dietetic Practice splits total energy expenditure into three parts: basal metabolic rate, the energy used in digesting and absorbing food (diet-induced thermogenesis), and physical activity. Basal metabolic rate, the activity needed simply to keep you alive, makes up roughly 60% of the total in healthy people, and in any one person a properly measured value is highly reproducible.

A resting metabolic rate (RMR) test tries to put a number on that largest part. It is a measurement of resting energy use, not a measure of how fast you will lose weight, and it does not include the energy you spend moving about.

Most RMR tests use indirect calorimetry. Rather than measuring heat, the device measures gases. A Health Research Authority summary of a UK weight-management trial describes a portable indirect calorimeter that, in a resting state, measures the oxygen breathed in and the carbon dioxide breathed out over 8 to 10 minutes. From those volumes it works out resting energy expenditure over 24 hours and a respiratory quotient, which is the ratio of carbon dioxide produced to oxygen consumed.

The respiratory quotient gives a rough clue about which fuels are being burned, because fat, carbohydrate and protein use different energy pathways. That is one reason some researchers are interested in it, although the summary describes this as a hypothesis under investigation rather than established practice.

In practice you rest quietly while your breathing is analysed. The BDA conditions below, including lying still and not sleeping, exist because movement, stimulants and recent exercise would otherwise distort the reading.

The BDA manual lists the conditions that make a measurement a true basal one, and notes that the measuring equipment must also be calibrated and a steady state reached (less than a 10% difference in minute-to-minute oxygen and carbon dioxide volumes over 5 minutes). The conditions are:

• a post-absorptive state, meaning a 12-hour fast

• lying still at physical and mental rest, but not asleep

• a thermo-neutral environment of 27 to 29 degrees Celsius

• no stimulants such as tea, coffee or nicotine in the previous 12 hours

• no heavy physical activity during the previous day

The manual explains that if any of those criteria are not met, the measurement is called resting energy expenditure (REE) or resting metabolic rate (RMR) instead of basal metabolic rate. In clinical settings it is rarely possible to meet every criterion, so most measured values are RMR or REE. In unwell people the figure also includes any metabolic response to injury or disease.

The manual adds that if someone has not fasted, the measurement may include some diet-induced thermogenesis, and that strictly controlled REE will not equal total energy requirements because neither activity nor digestion is counted. A measured resting value therefore always needs an allowance added before it becomes a daily energy target.

Several equations are in common use, including Harris-Benedict, Schofield and the Henry (Oxford) equations. The BDA manual describes the Henry equations as the most rigorously tested and applicable to modern healthy populations. All were derived in healthy people, so applying them to sick individuals introduces uncertainty, and the manual says no single validated method for estimating requirements exists.

The UK research summary makes a similar point for people living with obesity: equations used in hospitalised patients with obesity were accurate in only about half of people and tended to overestimate, while experience of indirect calorimetry in this group is limited and technical variation needs further work. For that reason the trial it describes is testing whether giving clinicians and participants measured energy data changes weight-loss outcomes. That is research, and it should not be read as a recommendation.

The manual says calorimetry and the doubly labelled water technique are the most accurate ways to determine requirements but are generally too expensive and impractical for routine clinical use. Availability of measured RMR therefore varies by service. One example in the sources is a research trial inside a tier 3 weight-management programme, where a portable indirect calorimeter was being evaluated.

A measurement does not diagnose a medical condition. A person worried about a slow metabolism because of tiredness or unexplained weight change is usually better served by a clinical assessment of the possible causes than by a calorimetry number alone.

A low or high result relative to an equation does not by itself explain why. The manual notes that in most chronic conditions BMR is usually normal or slightly increased, while acute illness can raise BMR above predicted values, usually by 0 to 40% and rarely up to 100%. Illness, inflammation and how closely the test conditions were followed all influence the number.

Because a single reading is a snapshot, it is best treated as one input to a dietitian-led or clinician-led plan rather than a verdict on how your body works.