Calorimetry

Calorimetry is the science, of measuring heat changes. It deals with physical, chemical and biological reactions while explaining the static systems using termophysics. It is based on the fact that all energy in human body is eventually changed into heat, so we can measure energy requirements of the body and determine the energy value of food using calorimetry units.

Units
The original unit of physiological caloric value of the nutrient content (burning heat) was calorie. Lately, however, it has been replaced with joules. Calorie (Cal) is defined as the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius from 14,5 up to 15,5°C. The exact temperature is placed in the definition due to the specific heat capacity, depending on the temperature and pressure. Calories represent a very small amount of energy, so we often use kilocalories (kcal) as a main unit. From the calorimetric equation Q = mcΔT we can calculate the exact heat capacity of the water,  which is 4185 J · kg-1 · K-1, 1 cal = 4.185 J (vv.: 1 J = 0.2389 cal)

Calorimetric measuring devices
According to the conditions they work in, we can divide calorimeters into two categories, adiabatic and isothermal.
 * Adiabatic calorimetr - all heat is used to change the temperature of the calorimeter content which we can measure.
 * Isothermal calorimetr - the temperature does not change throughout the whole experiment. The heat is converted to other form of energy, mostly chemical.

Measurement of energy in the food.
The energy is determined with the heat of combustion (the amount of heat released during the combustion of a specified amount of a substance - carbohydrates, fats, proteins), which is measured with the adiabatic bomb calorimeter.

Adiabatic bomb calorimetr
The sample is placed in a hermetically sealed pressure vessel, called a calorimetric bomb that is put into a bigger container filled with water, and connected to ignition wires. Subsequently, the measured substance is electrically ignited and burned. The water is heated and the additional mixing tool distributes heat evenly in the area. The purpose of the study is to determine the temperature elevation of the water, respectively the value of the heat of combustion of the sample. The name comes from the ability of the calorimetr to withstand high pressures, while having constant volume.

In the human body, energy from food is stored as chemical energy (most often as ATP, GTP), and if needed converted into other forms of energy. From the macro-energetic bonds in given molecules, energy is released by oxidation. This process is important for measuring energy of food. We think of the energy slowly released in body the same way as of quick burning in one of the calorimetric measuring kit. But we have to realise that the human body is not a 100% efficient machine and therefore it cannot use all the heat of the combustion. The calorific values ​​measured in the calorimeter and the true physiological caloric values ​​(the amount of energy actually released and used in the body) are therefore slightly different (see the table). However, these physiological caloric values are unrealistic and may differ in each and every type of nutritional substrates.

Carbohydrates
Carbohydrates are burned in order to produce carbon dioxide and water. The energy content of the individual compounds depends on the structure of the substance (carbohydrates / maltodextrins / polysaccharides). For example, when glucose burns there is only 15.7 kJ/g released.

Fats
Oxidation of fats also leads to the formation of carbon dioxide and water, but the energy stored in fats is much more difficult to use than in carbohydrates because fats are more complex molecules and their breakdown in human body is not complete. For precise and accurate measuring of energy in lipid, we have to count with different fatty acid structure. Medium-chain fatty acids (8 carbons) release about 36 kJ/g. Long-chain FAs can release up to 40.2 kJ/g. The amonia released in protein catabolism is removed through the urea cycle and excreted in the form of urea in urine as well as other substances (hydrogen kation etc.).

Proteins
The energy potential of proteins depends on the nitrogen content, because nitrogen cannot be used in heat combustion. Its concentration in individual protein substrates fluctuates from approximately 15% to 19%. The energy potential of proteins is also affected by many other factors, such as the proportion of protein in total energy intake or physical activity of the patient.

Energy expenditure measurement
We can measure the released energy of a patient through direct or indirect calorimetry. Due to technical and financial difficulties of direct calorimetry we often use indirect method.
 * Direct calorimetry-the proces is similar to the one with adiabatic calorimetr. The examined person is left in a closed chamber flowing in a water tank. The heat produced by the organism is measured on water temperature in the tank surrounding it while we monitor the oxygen intake; carbon dioxide; nitrogen excretion via breath, urine and faeces.


 * Indirect calorimetry- the basic principle of this method is to measure the consumption of nutrient substrates and gas exchange at a given time and to determine the respiratory coefficient.

Respiratory Quotient
Respiratory quotient is calculated from the ratio of carbon dioxide produced by the body divided by oxygen consumed by the body using respirometr in undirect calorimetry. Its values ​​depend on the proportional oxidation of individual nutritional substrates (lipids, carbohydrates, proteins) due to the different pathways of every macronutrient. Some of the other factors that may affect the respiratory quotient are energy balance, circulating insulin, and insulin sensitivity. RQ for ordinary mixed diet is around 0.85. RQ values for oxidation of essential nutritional components and RQ of some metabolic processes are listed in the table below:

Basically, molecules containing more oxygen require less inhaled oxygen to be fully broken down, therefore, have higher respiratory quotiens.

Fatty acid molecules of fats contain little oxygen compared to the total carbon content, so they use more oxygen during their metabolism. That is the reason why the lipogenesis is the most influential process on RQ value. Therefore, RQ> 1 indicates lipogenesis and excessive unused amount of glucose (building fat storage). Value <0,7 is an indicator of inability to oxidize glucose, fasting, the lipolysis and gluconeogenesis. Situation is more complicated with proteins, because of their incomplete catabolism. Firstly, we tried to find RQ for a number of metabolized proteins, but for further research we work with the so-called non-protein RQ. The non-protein RQ is based on the fact that 1 gram of nitrogen in urine equals  the amount of proteins requiring 5.92 liters of oxygen for its oxidation and creating 4.75 liters of carbon dioxide.