Human energy expenditure (EE) is not stable over a 24 hour period, it has its own rhythm. Energy Expenditure varies depending on the time of day,  physical activity level and the exercise performed by the subject, the amount of food ingested and its composition. The human 24-hour EE is made up of several components. The different components of 24-hour EE can be measured with indirect calorimetry (IC) equipment, such as the Room Calorimeter or the Omnical. In this web series, we will discuss and explain all EE components one by one. This time we discuss the BMR and RMR, i.e. the basal metabolic rate and resting metabolic rate.

What is Basal Metabolic Rate (BMR) and Resting Metabolic Rate (RMR)?

The BMR is the basal metabolic rate and the RMR is the resting metabolic rate. The terms are often used to express the same component of human daily energy expenditure; the energy that the body needs to maintain vital functions (i.e. basic chemical reactions) and whole-body homeostasis. BMR/RMR are determined on an awake person in complete resting condition. The BMR/RMR is the main contributor to 24-hour EE, with a share of 60% to 75% (see figure 1). Both are measured in the same way although under different conditions. The most common way to measure BMR/RMR is with a metabolic cart and a ventilated hood canopy system. During this measurement, it is important that the person is in a thermo-neutral environment. Of the two, the BMR is obtained during much more specific, controlled and strict conditions than the RMR. The BMR and RMR can both be influenced by different physiological and environmental factors: age, gender, body composition, ethnicity, genetic make-up, metabolic diseases, cold-exposure, and dietary diversity and quantity.

How to measure BMR / RMR?

For BMR measurements, the person must be in a fasted state (10-12 hours after the last meal) without consuming any calorie containing beverage, must be calm, no fidgeting, remain awake, breathe normally, lie on a bed and be in a room where both temperature and humidity can be controlled, as well as dimming lights and no external stimuli. It is important for the measurement to be carried out in the early morning. After the person has slept and has woken up in the room where the measurement takes place.

Figure 1 Components of total daily energy expenditure.

The RMR is linked to less rigid conditions (such as the fixed state, position, time of day, etc.) and varies by circadian phase. Therefore, a measurement with a ventilated hood canopy is the measurement method of choice to determine the RMR. This can be explained by the fact that many conditions with this setting are less controllable and less comfortable for the person. Due to the strict and controllable conditions of a BMR measurement, a whole room calorimeter is the most convenient method. Taking into account a measurement performed over a longer period of time and under strict physiological and environmental conditions.

Figure 2 Plotting RMR as a function of FFM for 17 woman and 20 men results in a shared linear regression, suggesting a sex-independent relationship between RMR and FFM.

Measurement sensitivities

Because fat-free mass (FFM) is a major element of the BMR/RMR and 24-hour EE in humans, it should be taken into account in the interpretation of the measured values. The most significant could be that a low BMR or RMR is an indicator of potential weight gain in the future, as it is such a large contributor to the total 24h EE. Although it seems that obese people have a higher RMR compared to normal weight people, this is not entirely true. Because the differences are primarily explained by a positive and direct link between total body weight and RMR. In order to differentiate between individuals with low resting EE, it is necessary to adjust measured values for either fat-free mass (FFM) or lean mass and fat mass (FM). More, precisely these tissues are accountable for 60%-70% of the variance in BMR/RMR.

The most common way to correct the BMR is to use the ratio of resting EE to total body weight or lean mass (FFM). However, their associations have no zero-intercept (see figure 2), which can lead to misconceptions. This can lead to problems when comparing groups with different body compositions or after undergoing significant changes in body composition. In such cases, a regression-based method would be the best solution to normalise the resting EE.

How can we help you with your research?

If you are interested in indirect calorimetry measurements, you have come to the right place. We have the best equipment in the field at our disposal.

“The Omnical is the best metabolic cart for assessing RMR and RER.” – Juan M. Alcántara Alcántara, 2020

We provide not only our products in the field of indirect calorimetry and accelerometry, but we also have an excellent service in terms of support for studies, research and measurements. You can always contact us for information and advice on product support, software support, parameter use, measurement methods and protocols, validation options, interpretation and correct use of measurement data, complementary products, etc.

Do you plan to measure BMR / RMR?

If you are interested in measuring BMR or RMR then consult us about our indirect calorimetry metabolic cart, whole room calorimeter systems or accelerometry add-ons. Please contact us or find more information on our information pages.

References

  • Chen KY, Smith S, Ravussin E, Krakoff J, Plasqui G, Tanaka S, Murgatroyd P, Brychta R, Bock C, Carnero E, Schoffelen P, Hatamoto Y, Rynders C, Melanson EL. Room Indirect Calorimetry Operating and Reporting Standards (RICORS 1.0): A Guide to Conducting and Reporting Human Whole-Room Calorimeter Studies. Obesity (Silver Spring). 2020 Sep;28(9):1613-1625.
  • Alcántara Alcántara, Juan Manuel. Assessment of resting energy expenditure and nutrient oxidation by indirect calorimetry: methodological implications. Granada: Universidad de Granada, 2021.

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  • Spaeth, Andrea Marie, “Consequences of Chronic Sleep Restriction on Energy Balance in Healthy Adults” (2014)
  • Chen, Kong Y, “Room Indirect Calorimetry Operating and Reporting Standards (RICORS 1.0): A Guide to Conducting and Reporting Human Whole‐Room Calorimeter Studies” (2020)