Getting acquainted with indirect calorimetry

Although validated predictive equations are primarily used by registered dietitian nutritionists (RDNs) to estimate energy expenditure, indirect calorimetry is the reference standard. The measure of energy expenditure is required to determine basal metabolic rate and inform nutrient intake recommendations. Indirect calorimetry has historically been unavailable for clinical use and has only been available in research settings.  

In more recent years, however, indirect calorimetry devices have been developed more efficiently and cost-effectively, making them more readily available in clinical settings. With the new potential of availability in the clinical setting, a review of what indirect calorimetry is and how it could potentially be used instead of predictive equations is an important topic for the dietitian. 

Direct vs. indirect calorimetry 

Indirect calorimetry is an indirect way to measure energy expenditure as opposed to direct calorimetry. Direct calorimetry demonstrates the law of conservation of energy, the understanding that energy does not disappear but merely changes from one form to another, with heat. Measuring heat production tells us how much energy was used in the body; this is direct calorimetry, or the direct way of knowing a person’s energy expenditure. Direct calorimetry is not practical in the clinical environment. In comparison, indirect calorimeters are smaller, lower in cost, easier to maintain, and cause minimal interruptions to patient care.  

What is indirect calorimetry? 

Indirect calorimetry estimates a person’s energy expenditure by working backwards from their oxygen use and carbon dioxide expenditure. People are dependent upon the constant production of adenosine triphosphate (ATP) in the body to sustain life. ATP is constantly being formed in the body and requires a constant supply of substrates, such as glucose, fatty acids, and oxygen.  

Oxidative phosphorylation, the process of converting the stored energy to usable energy, produces ATP and yields carbon dioxide and water as byproducts. This carbon dioxide byproduct, as compared to the amount of oxygen taken in, allows for the measurement of the gas exchange needed to calculate energy expenditure from indirect calorimetry. Indirect calorimetry also provides clinicians with the respiratory quotient.   

Today, the open circuit system is used for indirect calorimetry. This measures the patient’s air flow and analyzes it for gases. There are limitations to indirect calorimetry use, including the interpretation of results and applying them to the nutrition recommendations of critically ill patients.  

When is it appropriate to use indirect calorimetry in clinical practice? 

Indirect calorimetry is more available for use in clinical practice than it was in the past, leading to questions as to its appropriateness for routine clinical use when estimating nutrition needs prior to providing nutrition recommendations for patients. Multiple models and brands of indirect calorimetry devices are available for hospitals to purchase today. 

Some evidence points to improved patient outcomes when indirect calorimetry is used as opposed to predictive equations. One of the main goals for the use of indirect calorimetry is the avoidance of gross under- and overfeeding, as their negative effects are well established.  

The patients most likely to benefit from indirect calorimetry are those with very high or low body weight (ex: obesity or anorexia nervosa), potentially hypermetabolic (ex: cancer, liver cirrhosis, chronic obstructive pulmonary disease), abnormal muscle function (ex: neuromuscular degenerative diseases), endocrine diseases (ex: diabetes), or inflammatory diseases (ex: inflammatory bowel disease). 

Additionally, those with a high-output fistula, requiring long-term parenteral nutrition, chronic kidney disease, persistent critical illness, and those failing the weaning of a ventilator may indicate the use of indirect calorimetry as opposed to predictive equations.  

When is it inappropriate to use indirect calorimetry?  

Some clinical situations are more prone to indirect calorimetry inaccuracies. The American Society of Parenteral and Enteral Nutrition (ASPEN) Adult Nutrition Support Core Curriculum lists the following as contraindications for indirect calorimetry: air leaks (ex: chest tubes, cuff leak, etc.), extracorporeal membrane oxygenation, hemodialysis, fraction of inspired oxygen (FiO2) greater than 60% for those mechanically ventilated or reliance on supplemental oxygen for those spontaneously breathing. Additionally, any patient unable to cooperate with the measurement would preclude the use of indirect calorimetry.  

Additional feasibility limitations and/or concerns for inaccuracy include those undergoing continuous renal replacement therapy, fever, vasoactive drug use, ventilation with helium, use of myorelaxants or heavy sedation, metabolic alkalosis or acidosis, or hyperventilation. The number of limitations and considerations requires that clinicians be well-trained on their specific indirect calorimetry device and any associated facility protocols for its use, the potential benefits, and any potential adjustments needed for each patient. Thorough patient assessment and individualized nutrition recommendations continue to be the best practices for providing optimal nutrition care to patients.  

In summary 

Indirect calorimetry may have a role in providing individualized nutrition care for patients, particularly those with critical illness, as more research is conducted on the clinical application of indirect calorimetry. Dietitians, the most-qualified providers of person-centered and individualized evidence-based nutrition care, may have more opportunities for using indirect calorimetry prior to making nutrition recommendations in the future.  

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References:  

Mtaweh H, Tuira L, Floh AA, Parshuram CS. Indirect Calorimetry: History, Technology, and Application. Front Pediatr. 2018;6:257. https://doi.org/10.3389/fped.2018.00257   

Berger M, De Waele E, Gramlich L, et al. How to interpret and apply the results of indirect calorimetry studies: A case-based tutorial. Clin Nutr ESPEN. 2024;63:856-869. https://doi.org/10.1016/j.clnesp.2024.07.1055 

De Waele E, van Zanten, ARH. Routine use of indirect calorimetry in critically ill patients: pros and cons. Crit Care. 2022;26:123. https://doi.org/10.1186/s13054-022-04000-5  

Frankenfield D. Energy Expenditure and Needs. In: Chan LN, Kumpf V, Lord LM, et al, eds. ASPEN Adult Nutrition Support Core Curriculum. 4th ed. ASPEN;2025: 57-78.