## Indirect Calorimetry

A Raman and D A Schoeller, University of Wisconsin-Madison, Madison, WI, USA

All living organisms require a source of energy for survival. Among animals, this energy is provided in the form of chemical energy in the nutrients they consume, which are converted to other forms of energy through respiration. This conversion is subject to the same laws of thermodynamics that govern all energy systems. The first law of thermodynamics states that energy can neither be created nor destroyed; it can only be exchanged from one system to another. Hence, the chemical energy consumed in the form of food is converted into mechanical energy for work performed by the body, thermic energy for maintenance of body temperature, or stored as chemical energy in tissues as fat, protein, or a small fraction as carbohydrates. This conservation of energy can be stated mathematically as

Energym = Energyw0rk + Energyheat ± Energyst0red The sum of energy converted to work and heat is defined as metabolism. Although metabolism constitutes thousands of chemical reactions occurring at the same time throughout the body that cannot be individually measured, their sum can be measured as either the sum of work and heat energy or, in the absence of any measurable work, the rate of heat production by the body. This is based on the assumption that all the cellular events ultimately result in heat.

The process of measuring heat produced by the body during combustion of substances or nutrients in animals or humans is called calorimetry. The term 'direct calorimetry' is used when the rate of heat production is directly measured by placing a person in a thermally isolated chamber. The term 'indirect calorimetry' is used when heat production is not measured directly but is instead calculated from the measurement of the rates of oxygen consumption (Vo2) and carbon dioxide production (VCOz). In both measurements, the rate of metabolism is commonly referred to as the rate of energy expenditure, which in the absence of work output is the rate at which chemical energy in food is converted to heat. The nutrients in food that provide this chemical energy are the macronutrients: carbohydrates, fat, protein, and alcohol. The chemical process that releases the chemical energy is respiration, in which each of these macronutrients is combined with oxygen to produce carbon dioxide and water. These chemical reactions are chemically equivalent to those that would be observed if the nutrient were combusted in a flame, except the reaction in the body is an enzymatic process that does not produce a flame.

For example, one molecule of sugar (glucose) breaks down as follows:

It should be noted that during this reaction, six molecules of CO2 are produced and six molecules of O2 are consumed. Thus, the ratio of CO2 to O2 has a value of 1.o. This ratio is commonly called the respiratory quotient (RQ), although many investigators prefer the term respiratory exchange ratio (RER) when it is applied to a whole body measurement. Similarly, when one molecule of fat (tripalmi-tin) is broken down completely, the chemical reaction is

C57H104O6 + 8OO2 ! 57CO2 + 52H2O

In the instance of fat oxidation, 57 molecules of CO2 are produced while 80 molecules of O2 are consumed when 1 molecule of fat is oxidized. This yields an RER of 0.71. When only carbohydrate and fat are being used to support energy expenditure, this difference in RER makes it possible to calculate what percentage of energy expenditure is being supported by each of the two energy substrates.

There is, however, a third macronutrient that is oxidized to produce energy. The third macronutri-ent, protein, is more difficult to describe on a chemical basis because a protein is made from a mixture of amino acids, and for each dietary protein the number and composition of amino acids differ.

The breakdown of the average dietary protein, however, can be described by the chemical reaction

Cl00Hl59N26O32S0.7 + 105.302

! I3CON2H4 + 87C02 + 52.8H2SO4

In the instance of protein oxidation, 87 molecules of C02 are produced while 105.3 molecules of 02 are consumed when 1 molecule of protein is oxidized. This yields an RER of 0.83. Although this RER value is intermediate between carbohydrate and fat, protein is unique among the three energy substrates because it is the only one to contain nitrogen. As such, urinary nitrogen can be assayed to obtain an estimate of protein oxidized by an individual. Combining this with the knowledge that the average protein is 16% nitrogen by weight, it is possible to use the previous chemical relationship to calculate the O2 consumption and CO2 production that result from the oxidation of the protein represented by the urinary nitrogen. Subtracting these from the total respiratory gas exchange yields a nonprotein O2 consumption, CO2 production, and nonprotein RER. This is used to calculate the nonprotein metabolic rate and eventually the carbohydrate and fat oxidation rates. Because urinary nitrogen is often not measured, results from indirect calorimetry often use the Weir equation to calculate the energy expenditure. This equation was derived assuming that protein oxidation supports 12% of total energy expenditure (Table 1).

Over the years, different instrumental methods of indirect calorimetry have been developed to accurately measure V0i and VC0z rates. Despite being a precise and accurate method of measurement of macronutrient oxidation and hence energy expenditure, constraints such as expense, portability, gas collection issues, samplers, and applicability of measurements to habitual expenditure prevented it from being available to different types of research. Hence,

Table 1 Formulas for calculation of energy expenditure

Variable

Formula

Oxygen consumption (ml/min)

Carbon dioxide production (ml/min)

Respiratory exchange ratio (RER or RQ) Weir Equation (TEE, kcal/min)

= (Volume of inspired air per minute x fraction of inspired O2) - (volume of expired air per minute x fraction of expired O2) = (Volume of expired air per minute x fraction of expired CO2) - (volume of inspired air per minute x fraction of inspired CO2) = vCO2/vO2

= (0.0039 x VO2) + (0.0011 x vCo2) - (2.2 x urinary nitrogen, g/min)

the quest to develop new instrumental techniques is driven by the desire to make it a more generally applicable and easier to use technique. 