Assessment of the energy value of food
To determine the amount of energy stored in food products, physical and chemical calorimetry are used.
- Physical calorimetry (Berthelot bomb). The principle is based on burning one gram of a substance using an electric current inside a sealed device immersed in water. The heat of combustion is calculated from the change in water temperature.
- Chemical calorimetry. Based on the oxidation reaction of nutrients using strong reagents with a predetermined 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 Berthelot bomb in the same way as 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 some 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 radiated by the human body. The patient is placed in a special insulated chamber — a biocalorimeter. Heat is assessed by the warming of fluid circulating in the chamber walls or by using modern suits with temperature sensors.
Indirect calorimetry relies on the study of gas exchange, since at rest all the energy of 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 respiratory quotient to be calculated and precisely determines what the body is "burning".
- Incomplete gas analysis (spirometabolography). Only oxygen consumption is measured, while exhaled carbon dioxide is simply absorbed by the device. This approach is frequently used to assess basal metabolic rate.
Respiratory quotient and caloric equivalent
Respiratory quotient (RQ) is the ratio between the volume of exhaled carbon dioxide and the volume of consumed oxygen. 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, this 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 the amount of nitrogen in the urine.
Caloric equivalent of oxygen (CEO) is the amount of energy released when the body utilizes one liter of oxygen. It varies during the oxidation of different substances:
- 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 CEO value of 4.8 kcal/L is used.
Briefly: answers to topical questions
How exactly 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 the oxidation processes.
- Determination of oxidized protein — the amount of nitrogen excreted in the urine per day 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 exhaled 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 human or animal inside the chamber warms the circulating water.
- The amount of heat released by the organism is calculated from the volume of flowing water and its temperature change.
- Sources also indicate that in direct calorimetry, the amount of heat is calculated based on the temperature change of the fluid passing 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 1 minute ($VO_2$) and using the averaged caloric equivalent of oxygen, energy expenditure is calculated.
- For a mixed diet, the caloric equivalent of oxygen is assumed to be 4.8 kcal/L.
- Calculation formula: Σ = $VO_2$ × CEO = $VO_2$ × 4.8, where CEO is the caloric equivalent of 1 L of $O_2$.
- This method is typically used to determine energy expenditure during basal metabolism.
Class and exam questions
Why does the heat of combustion of proteins differ in the body and in the Berthelot bomb?
In the device, proteins burn completely down to nitrogen, water, and carbon dioxide. In the human body, they are not completely oxidized: nitrogenous waste products (such as urea) remain, carrying away part of the unspent energy.
How to calculate the caloric content of a diet if the mass of proteins, fats, and carbohydrates is known?
You need to 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 the obtained values are summed up.
What does the respiratory quotient indicate?
It reflects the ratio of exhaled carbon dioxide to consumed oxygen. This figure allows one to judge which specific nutrients are being oxidized in the body (for example, it equals one for carbohydrates).
What else to study on this topic
- Formula for calculating energy metabolism during 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
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