Calculating energy expenditureThe energy expenditure can be measured in adults by using the Harris and Benedict formula (1919) or the revised formula from 1984, the Roza and Shizgal formula. These can be found on this page. To calculate the energy expenditure in children it is advised to use the Schofield formula. You can find more information on this formula in this document.
Influencing factorsThe energy expenditure of a patient is being influenced by different factors, such as:
- Severe, acute or chronical illnesses: AIDS, chronic obstructive Pulmonairy disease (COPD), cystic fibrosis, chronic heart failure, dementia with high physical activity, hyperthyreoïdie, infections, some types of cancer (lung cancer and leukemia), Hodgkin's disease, liver insufficience and rheumatism.
- Complications: fever, infections, sepsis and inflammations.
- Wounds: severe wounds, burns and pressure ulcers.
- Fysiological question: pregnancy and lactation.
Measuring resting energy expenditure
Indirect calorimetryIndirect calorimetry (ventilated-hood method) is the only accurate and clinically feasible method of measuring resting energy expenditure. It is called 'indirect' because the caloric burn rate is calculated from a measurement of oxygen uptake; the concentration of O2 en CO2 are measured. Direct calorimetry implies a measurement of heat released by the body, which is technically difficult and clinically impractical.
A transparent plastic hood is placed over the head and neck of the patient, who lies down on a bed and breaths normally. The outside air is led through the hood and the breathed air is analysed. It's no problem when a patient carries a tracheostoma. By determining the quantity of air flowing through the hood and measuring the O2-concentration, the energy expenditure in rest can be calculated (including the disease factor). How much oxygen is needed to burn some substrate, depends on the substrate.
RQWhich substrate is used at the moment you measure is visible in the Respiratoir Quotiënt: CO2/ O2.
Burning of only carbohydrates will give a RQ of 6/6 = 1 ( C6H12O6 -> 6 H2O en 6 CO2)
Burning of only fat will give a RQ of 110/157 = 0.7 ( 2 C55H106O6 fat (of animal origin) + 157 O2 -> 110 CO2 + 106 H2O)
Burning of only alcohol will give a RQ of 2/3 = 0.666 (2 CH3OH + 3 O2 -> 2 CO2 + 4 H2O) The hood has a 30-litre volume. In an adult patient the flow can vary between 25 and 50 litres per minute, depending on the body-size.
The measure takes place during a fasting state, because we will not measure the diet-induced thermogenesis (DIT) of nutrition. Factors that induce hyperventilation, like fear or nervousness, have to be avoided. A stable measurement will take about 30 minutes.
Measuring conditionsBefore measuring, certain conditions must be met:
- The measuring device must be calibrated;
- The patient must be fasting 10 hours before the measurement;Physical activity before the measurement must be as little as possible.
- A quiet environment during the measurement;
- Neutral room temperature (21-25 º C)
- The patient has to get used to temperature and environment before measuring (circa 10 minutes)
ResultsThe measured energy-expenditure is part of the total energy- expenditure. To calculate the total energy expenditure, a estimation of physical activity and abnormal energy-losses must be made. The measures takes place in a fasting state so for total energy-expenditure a correction for DIT must be made.