# *Geek Box: Methods of Measuring Energy Expenditure in Humans

#### *Geek Box: Methods of Measuring Energy Expenditure in Humans

If you read metabolic ward studies, you may come across the term “respiratory chamber” and see that energy expenditure was measured by what is known as “calorimetry”. This is often referred to as “whole-room calorimetry” and is where energy expenditure is assessed around the clock with a participant housed in an airtight room.

Calorimetry measures the difference between oxygen [O2] and carbon dioxide [CO2] concentrations from the respiration of the participant in the room. As a fixed value for oxygen is supplied into the room, the rate and amount of carbon dioxide production provides data to calculate energy expenditure through the use of mathematical equations. As all macronutrients – proteins, fats, carbohydrates – contain carbon, the values of protein, carbohydrate, and fat oxidation can also be calculated from this data, indicating what substrate is being utilised after a participant has eaten a meal, and at what rate.

Another method of measuring energy expenditure is “doubly-labelled water” [DLW], which is considered the gold standard for assessing total energy expenditure in free-living humans. DLW uses stable isotope tracers, i.e., “labels” water [H2O] with two very uncommon isotopes, deuterium [2H] and oxygen-18 [18O]. Thus, when that water is eventually excreted in urine, the two tracers – 2H and 18O – are easily identified.

The difference between the rate of elimination of 2H and 18O from the body provides an estimate of the production rate of CO2. In turn, the measurement of CO2 production is used to calculate total energy expenditure. DLW is consumed orally, making it very effective for ‘in the field’ research, and providing estimates of human energy expenditure in the real-world (compared to a lab). Because it is consumed orally, and is safe, DLW can be used in all populations, including infants, and can provide a measurement over long periods of time.