Assessing energy expenditure
Profil offers a portfolio of methods for evaluating each component of energy expenditure:
- Resting energy expenditure with indirect calorimetry
- Diet-induced thermogenesis with indirect calorimetry
- Activity-induced energy expenditure with spiroergometry
- Total energy expenditure with the doubly labeled water method
Each method is considered the most accurate available means of measuring the respective aspect of energy expenditure.
Methods for accurately assessing individual energy expenditure are a crucial part of the design of intelligent and comprehensive clinical trials focusing on pharmaceuticals or nutraceuticals for weight management. Total energy expenditure can be divided into three components: resting energy expenditure, diet-induced thermogenesis and activity-induced energy expenditure. Assessing each component enables specific questions to be answered, whereas assessing total energy expenditure permits conclusions about general energy balance and the long-term outcome of a given weight-loss intervention.
Energy expenditure evaluation at Profil
Read on to discover the details of the three methods we offer to support the creation of a comprehensive understanding of a subject’s energy expenditure during clinical investigations of diabetes, obesity and weight management.
Resting energy expenditure (REE) and diet-induced thermogenesis (DIT) can be assessed via indirect calorimetry, a non-invasive and very accurate method with high reproducibility. It is based on indirect measurement of the heat expended by nutrient oxidation, estimated by monitoring oxygen consumption and carbon dioxide production over time. It also allows the identification of the specific kind of energy substrate – carbohydrates, lipids or proteins – being metabolized by the body at a given time. The subject is at rest during the measurements. Energy expenditure is calculated with the Weir formula based on the oxygen and carbon dioxide production and concentration measurements.
Oxygen and carbon dioxide production and concentration measurements conducted while the subject is performing physical activity, e.g. on a cycle ergometer, are used to assess activity-induced energy expenditure (AEE). This diagnostic procedure is called spiroergometry, and it is used to assess metabolism, cardiovascular performance and respiration, giving insights into the energetics of physical exercise and the pathogenesis of metabolic diseases. It comprises two components, spirometry and ergometry. Spirometry is for continuous measurement of respiratory gas metabolism and the continuous registration of respiration. Ergometry is a method for the exact modulation of the physical workload.
The doubly labeled water method
While the assessment of each component of energy expenditure allows specific questions to be answered, only evaluations of total energy expenditure (TEE) enable conclusions about energy balance in general. At Profil, we use the doubly labeled water (DLW) method for TEE evaluations. It assesses total energy expenditure by estimating the rate of carbon dioxide production. The technique involves enriching the body water of the subject with an isotope of hydrogen (2H) and an isotope of oxygen (18O), and then determining the washout kinetics of the two isotopes as their concentrations decline exponentially toward natural abundance levels.
This method is based on the observation that the biological elimination rates for labeled hydrogen and labeled oxygen in water are different. The hydrogen label in body water is diluted by water entering the body from beverages, food moisture, metabolic water and absorbed moisture. It is eliminated from the body water in urine, water vapor, sweat and other efflux routes. Oxygen undergoes the same dilution and efflux, but it is also diluted by oxygen entering the body’s metabolic pools as bicarbonate, because the oxygen in water and bicarbonate are in rapid isotopic equilibrium. The elimination of labeled oxygen from the body is therefore faster than that of the labeled hydrogen and the difference between the two elimination rates is a measure of carbon dioxide production and thus energy expenditure.
The practical aspect is that it is not necessary to collect the labeled carbon dioxide, but only necessary to collect spot urine at defined time points, usually over a period of 1–2 weeks, and calculate the isotope dilution spaces and elimination rates to assess energy expenditure. Therefore, the DLW method is the method of choice for the assessment of TEE under free-living conditions.