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Calorimetry

For medical students2 min readUpdated 2026-10-10

Calorimetry is a set of methods used to evaluate the energy value of nutrients and the body's energy expenditure. These approaches are used both for food analysis (physical and chemical calorimetry) and for studying human metabolism (physiological calorimetry).

ProteinsYield 4.1 kcal/g in the body

CarbohydratesYield 4.1 kcal/g upon oxidation

FatsMost energy-dense: 9.3 kcal/g

Carbohydrate RQRespiratory quotient equals 1

Evaluation of Food Energy Value

To determine the amount of energy stored in food, physical and chemical calorimetry are used.

Physical calorimetry (Berthelot bomb calorimeter). The essence of the method is burning one gram of a substance using an electric current inside a sealed device submerged in water. The heat of combustion is calculated from the change in water temperature.
Chemical calorimetry. Based on the oxidation reaction of nutrients with strong reagents using a set volume of oxygen. The residual unused oxygen is then measured by iodometric titration.

It is important to understand that fats and carbohydrates burn in the bomb calorimeter in the same way as they do in the human body (down to water and carbon dioxide) and yield the same amount of energy. Proteins, however, are not completely oxidized in the body: nitrogenous compounds are formed (urea, creatinine, uric acid), which still contain a small amount of energy. Therefore, the physiological caloric value of proteins (4.1 kcal/g) is lower than their physical heat of combustion.

Physiological Calorimetry: Direct and Indirect

These methods are designed to measure how much energy the body itself expends.

Direct calorimetry measures the heat given off by the human body. The patient is placed in a special insulated chamber — a biocalorimeter. Heat is assessed by the warming of fluid circulating through the walls of the chamber, or using modern suits equipped with temperature sensors.

Indirect calorimetry relies on the study of gas exchange, since at rest all energy from chemical reactions is converted into heat. The method is divided into two types:

Complete gas analysis. Both consumed oxygen and exhaled carbon dioxide are measured. This allows the calculation of the respiratory quotient and provides precise insight into what substrates the body is "burning".
Incomplete gas analysis (spirometabolography). Only oxygen consumption is measured, while exhaled carbon dioxide is absorbed by the device. This option is frequently used to assess basal metabolic rate.

Respiratory Quotient and Caloric Equivalent

Respiratory quotient (RQ) is the ratio between the volume of carbon dioxide produced and the volume of oxygen consumed. It indicates which nutrients are currently being oxidized:

During carbohydrate metabolism, RQ = 1.
During fat oxidation, RQ is less than 0.7.

If the RQ is 0.85, it may indicate either protein oxidation or a mixed breakdown of fats and carbohydrates. To clarify, one must calculate the "true" (non-protein) RQ by excluding the protein factor based on urinary nitrogen content.

Oxygen caloric equivalent (OCE) is the amount of energy released when the body utilizes one liter of oxygen. It varies depending on the substance being oxidized:

Carbohydrates: 5 kcal.
Proteins: 4.85 kcal.
Fats: 4.6 kcal.

When calculating energy metabolism via incomplete gas analysis for a mixed diet, an average OCE value of 4.8 kcal/L is used.

Frequently asked questions

How is the "true" non-protein respiratory quotient calculated?

The calculation of the "true" non-protein respiratory quotient (RQ) is performed in three stages by completely excluding the protein component from oxidation processes.

Determination of oxidized protein — the amount of daily urinary nitrogen is multiplied by 6.25 (since protein contains 16% nitrogen).
Calculation of protein gas volumes — the resulting protein mass is multiplied by 0.95 L ($O_2$ consumption per 1 g) and 0.76 L ($CO_2$ production per 1 g).
Calculation of the coefficient — the calculated protein volumes are subtracted from the total daily volume of consumed oxygen and produced carbon dioxide, after which the remaining (non-protein) volume of $CO_2$ is divided by the remaining volume of $O_2$.

How is a biocalorimeter constructed and operated in direct calorimetry?

A biocalorimeter is a sealed chamber with high thermal insulation from the external environment.

Water circulates through tubes within the chamber.
Heat generated by a person or animal inside the chamber warms the circulating water.
The amount of heat produced by the organism is calculated based on the volume of flowing water and its temperature change.
Sources also note that in direct calorimetry, heat calculation is performed via the temperature change of fluid flowing through the chamber per unit of time.

What formulas are used to calculate basal metabolic rate after spirometabolography?

In spirometabolography, which is a method of incomplete gas analysis, energy expenditure is determined by oxygen consumption per unit of time.

Spirometabolographs absorb exhaled $CO_2$ and measure only the amount of consumed $O_2$.
Knowing the volume of oxygen consumed per minute ($VO_2$) and using the averaged oxygen caloric equivalent, energy expenditure is calculated.
For a mixed diet, the oxygen caloric equivalent is assumed to be 4.8 kcal/L.
Calculation formula: $\Sigma = VO_2 \times OCE = VO_2 \times 4.8$, where OCE is the caloric equivalent of 1 L of $O_2$.
This method is typically used to determine energy expenditure during basal metabolism.

Why does the heat of combustion of proteins differ in the body versus a bomb calorimeter?

In the device, proteins burn completely to nitrogen, water, and carbon dioxide. In the human body, they are not fully oxidized: nitrogenous waste products remain (such as urea), which carry away a portion of the unspent energy.

How do you calculate the caloric content of a diet if the mass of proteins, fats, and carbohydrates is known?

Multiply the mass of each component by its physiological energy value: proteins and carbohydrates by 4.1 kcal/g, and fats by 9.3 kcal/g. Then, sum the resulting values.

What does the respiratory quotient indicate?

It reflects the ratio of exhaled carbon dioxide to consumed oxygen. This value helps determine which specific nutrients are being oxidized in the body (for example, it equals one for carbohydrates).

Go deeper

Formula for energy metabolism calculation in incomplete gas analysis
Algorithm for calculating the "true" (non-protein) respiratory quotient
Relationship between urinary nitrogen and oxidized proteins
Limitations of the physical calorimetry method