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How does indirect calorimetry estimate the energy used by the body?

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USDA MyPlate dietary guidelines — human nutrition MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture (USDA) in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.

Indirect calorimetry is a technique used to estimate the energy expended by the body by calculating the heat produced from metabolic processes. Rather than measuring heat dissipation directly, this method relies on the relationship between cellular respiration and energy production.1

The body generates energy through the oxidation of substrates (foodstuffs), a process that requires oxygen and results in the production of carbon dioxide and water. By measuring the volume of oxygen inhaled and carbon dioxide exhaled, along with the amount of urinary nitrogen excreted, researchers can calculate the body's metabolic rate.21

This approach is based on the principle that the consumption of oxygen and the production of carbon dioxide are directly linked to the metabolism of nutrients and the subsequent release of energy, a portion of which is conserved as adenosine triphosphate (ATP) while the remainder is released as heat. Indirect calorimetry is generally considered less cumbersome than direct calorimetry, which requires specialized insulated chambers to measure actual heat loss.213