Measuring energy and expenditure during physical activity

Energy utilization by contracting muscle fibers during exercise cannot be directly measured. But numerous indirect laboratory methods can be used to calculate whole-body energy expenditure at rest and during exercise. Several of these methods have been in use since the early 1900s. Others are new and have only recently been used in exercise physiology.

Direct calorimetry

Only about 40% of the energy liberated during the metabolism of glucose and fats is used to produce ATP. The remaining 60% is converted to heat, so one way to gauge the rate and quantity of energy production is to measure the body’s heat production. This technique is called direct calorimetry, since the basic unit of heat is the calorie(cal).

This approach was first described by Zuntz and Hagemann in the late 1800s. They developed the calorimeter, which is an insulated, airtight chamber. The walls of the chamber contain cooper tubing through which water is passed. In the chamber, the heat produced by the body radiates to the walls and warms the water. The water temperature change is recorded, as are temperature changes of the air entering and leaving the chamber. These changes are caused by the heat the body generates. These changes are caused by the heat the body generates. One’s metabolism can be calculated from the resulting values.

Calorimeters are expensive to construct and to use and are slow to generate results. Their only real advantage is that they measure heat directly, but they have several disadvantages for exercise physiology. Although a calorimeter can provide an accurate measure of total-body energy expenditure, it cannot follow rapid changes in energy expenditure. Therefore, while direct calorimetry is useful for measuring resting metabolism, energy metabolism during most exercise situations cannot be adequately studied with a direct calorimeter. Second, exercise equipment such as a motor-driven treadmill also gives off heat of its own. Third, not all heat is liberated from the body; some is stored and caused body temperature to rise. And finally, sweating affects the measurements and the constants used in the calculations of heat produced. Consequently, this method is seldom used today because it is easier and less expensive to measure energy expenditure by assessing the exchange of oxygen and carbon dioxide that occurs during oxidative phosphorylation.

Indirect calorimetry

Oxidative metabolism of glucose and fat – the main substrates for exercise – depends on O₂ availability and produces CO₂ and water. The rate of O₂ and CO₂ exchanged in the lungs normally equals the rate of usage and release by the body tissues. With this knowledge, we can measure caloric expenditure by measuring respiratory gases. This method of estimating total-body energy expenditure is called indirect calorimetry because heat production is not measured directly. Rather, energy expenditure is calculated from the respiratory exchange of O₂ and CO₂.

In order for oxygen consumption to reflect energy metabolism accurately, energy production must be almost completely oxidative. If a large portion of energy is being produced anaerobically, respiratory gas measurements will not reflect all metabolic processes. Therefore, this technique is limited to steady-state activities lasting about 60s or longer, which fortunately takes into account most daily activities including exercise.

Respiratory gas exchange is determined through measurement of the volume of O₂ and CO₂ that enters and leaves the lungs during a given period of time. Because O₂ is removed from the inspired air in the alveoli and CO₂ is added to the alveolar air, the expired O₂ concentration is less than the inspired, whereas the CO₂ concentration is higher in expired air than in inspired air. Consequently, the difference between inspired and expired air tells us how much O₂ is being taken up and how much CO₂ is being produced. Because the body has only limited O₂ storage, the amount taken up at the lungs accurately reflects the body’s use of O₂. Although a number of sophisticated and expensive methods are available for measuring the respiratory exchange of O₂ and CO₂, the simplest and oldest methods( i.e., Douglas bag and chemical gas analysis) are probably the most accurate, but they are relatively slow and permit only a few measurements during each session. Modern electronic computer systems for respiratory gas exchange measurements offer a large time savings and multiple measurements.

Calculating oxygen consumption and carbon dioxide production

Apparature is used to calculate three variables needed to calculate the actual volume of oxygen consumed(VO₂) and volume of CO₂ produced(VCO₂). Generally, values are presented as oxygen consumed per minute(VO₂) and CO₂ produced per minute(VCO₂). V is used to indicate a rate of O₂ consumption or CO₂ production, for example, liters per minute.

In its simplest form, VO₂ is equal to the volume of O₂ inspired minus the volume of O₂ expired. To calculate the volume of O₂ inspired, we multiply the volume of air inspired by the fraction of that air that is composed of O₂; the volume of O₂ expired is equal to the volume of air expired multiplied by the fraction of the expired air that is composed of O₂. The same holds true for CO₂.

Thus, calculation of VO₂ and VCO₂ requires the following information:

• Volume of air inspired(Vi)
• Volume of air expired(Ve)
• Fraction of oxygen in the inspired air(FiO₂)
• Fraction of CO₂ in the inspired air(FiO₂)
• Fraction of oxygen in the expired air(FeO₂)
• Fraction of CO₂ in the expired air(FeO₂)

The oxygen consumption, in liters of oxygen consumed per minute, can then be calculated as follows:

VO₂ = (Vi x FiO₂) – (Ve x FeO₂).

The CO₂ production is similarly calculated as

VCO₂ = (Ve x FeO₂) – (Vi x FiCO₂).

Haldane transformation

Over the years, scientists have attempted to simplify the actual calculation of oxygen consumption and CO₂ production. Several of the measurements needed in the preceding equations are known and do not change. The gas concentrations of the three gases that make up inspired air are known; oxygen accounts for 20.93%. CO₂ accounts for 0.04%, and nitrogen accounts for 79.03% of the inspired air. What about the volume of inspired and expired air? Aren’t they the same, such that we would need to measure only one of the two?

Inspired air volume equals expired air volume only when the volume of oxygen consumed equals the volume of CO₂ produced. When the volume of oxygen consumed is greater than the volume of CO₂ produced, Vi is greater than Ve. Likewise, Ve is greater than Vi when the volume of CO₂ produced is greater than the volume of oxygen consumed. However, the one thing that is constant is that the volume of nitrogen inspired(ViN₂) is equal to the volume of nitrogen expired(VeN₂). Because ViN₂ = Vi x FiN₂ and VeN₂ = Ve x FeN₂, we can calculate Vi from Ve by using the following equation, which has been reffered to as the Haldane transformation:

Vi x FiN₂  = Ve x FeN₂

Which can be rewritten as

Vi = Ve x FeN₂ / FiN₂

Furthermore, because we are actually measuring the concentrations of O2 and CO2 in the expired gases, we can calculate FeN₂ from the sum of FeO₂  and FeCO₂ , or

FeN₂ = 1 – (FeO₂ + FeCO₂)

So, in unpulling all of this information together, we can rewrite the equation for calculating VO2 as follows:

VO₂ = (Vi x FiO₂) – (Ve x FeO₂).

By substituting equation 2, we get the following:

VO₂ = [(Ve x FeN₂) / (FiN₂ x FiO₂)] – [(Ve) x (FeO₂)]

By substituting known values for FiO₂ of 0.2093 and for FrN₂ of 0.7903, we get the following:

VO₂ = [((Ve x FeN₂) / 0.7903) x 0.2093] – [(Ve) x (FeO₂)]

By substituting equation 3, we get the following:

VO₂ = [((Ve x FeN₂) / 0.7903) x 0.2093] – [(Ve) x (FeO₂)].

Or simplified,

VO₂ = [(Ve) x (1-(FeO₂ + FeCO₂)) x (0.2093 / 0.7903)] – [(Ve) x (FeO₂)]

Or, further simplified,

VO₂ = (Ve) x [[(1-(FeO₂ + FeCO₂)) x (0.265)] – (FeO₂)].

This final equation is the one actually used in practice by exercise physiologists, although computers now do the calculating automatically in most laboratories. One final correction is necessary. When air is expired, it is at body temperature(BT), is at the prevailing atmospheric or ambient pressure(P), and is saturated(S) with water vapor, or what are referred to as BTPS conditions. Each of these influences would not only add error to the measurement of VO₂ and VCO₂, but would also make it difficult to compare measurements made in laboratories at different altitudes, for example. For that reason, every gas volume is routinely converted to its standard temperature(ST: 0°C) and pressure(P:760 mmHg), dry equivalent(D), or STPD. This is accomplished by a series of correction equations.

Respiratory exchange ratio

To estimate the amount of energy used by the body, it is necessary to know the type of food substrate(combination of carbohydrate, fat, protein) being oxidized. The carbon and oxygen contents of glucose, free fatty acids(FFAs), and amino acids differ dramatically. As a result, the amount of oxygen used during metabolism depends on the type of fuel being oxidized. Indirect calorimetry measures the amouns of CO₂ released(VCO₂) and oxygen consumed(VO₂). The ratio between these  two values is termed the respiratory exchange ratio(RER):

RER = VCO₂ / VO₂.

In general, the amount of oxygen needed to completely oxidize a molecule of carbohydrate or fat is proportional to the amount of carbon in that fuel. For example, glucose(C₆H₁₂O₆) contains six carbon atoms. During glucose combustion, six molecules of oxygen are used to produce six CO₂ molecules, six H₂O molecules, and 38 ATP molecules:

6 O₂ + C₆H₁₂O₆ → 6 CO₂ + 6 H₂O + 38 ATP.

By evaluating how much CO₂ is released compared with the amount of O₂ consumed, we find that the RER is 1.0:

RER = VCO₂ / VO₂ = 6 CO₂ / 6 O₂ = 1.0.

As shown in the table below, the RER value varies with the type of fuels being used for energy. Free fatty acids have considerably more carbon and hydrogen but less oxygen than glucose. Consider palmitic acid C₁₆H₃₂O₂. To completely oxidize this molecule to CO2 and H2O requires 23 molecules of oxygen:

16C + 16 O₂ → 16 CO₂

32H + 8 O₂ → 16 H₂O

Total= 24 O₂ needed

         - 1 O₂ provided by the palmitic acid

           23 O₂ must be added

Caloric equivalence of the respiratory exchange ratio(RER) and % kcal from carbohydrates and fats

	Energy	% kcal
RER	kcal/L O2	Carbohydrates	Fats
0.71	4.69	0	100
0.75	4.74	16	84
0.80	4.80	33	67
0.85	4.86	51	49
0.90	4.92	68	32
0.95	4.99	84	16
1.00	5.05	100	0

Ultimately, this oxidation results in 16 molecules of CO₂, 16 molecules of H₂O, and 129 molecules of ATP:

C₁₆H₃₂O₂ + 23 O₂ → 16 CO₂+ 16 H₂O + 129 ATP

Combustion of this fat molecule requires significantly more oxygen than combustion of a carbohydrate molecule. During carbohydrate oxidation, approximately 6.3 molecules of ATP are produced for each molecule of O₂ used(38 ATP per 6 O₂), compared with 5.6 molecules of ATP per molecule of O₂ during palmitic acid metabolism(129 ATP per 23 O₂).

Although fat provides more energy than carbohydrate, more oxygen is needed to oxidize fat then carbohydrate. This means that the RER value for fat is substantially lower than for carbohydrate. For palmitic acid, the RER value is 0.70:

RER = VCO₂ / VO₂ = 16/23 = 0.70

Once the RER value is determined from the calculated respiratory gas volumes, the value can be compared with a table up to determine the food mixture being oxidized. If, for example, the RER value is 1.0, the cells are using only glucose or glycogen, and each liter of oxygen consumed would generate 5.05 kcal. The oxidation of only fat would yield 4.69 kcal/L of O₂, and the oxidation of protein would yield 4.46 kcal/L of O₂ consumed. Thus, if the muscles were using only glucose and the body were consuming 2L of O₂/min, then the rate of heat energy production would be 10.1 kcal/min(2 L/min x 5,05 kcal/L).

Limitations of Indirect Calorimetry

While indirect calorimetry is a common and important tool of exercise physiologists, it has limitations. Calculations of gas exchange assume that the body’s O2 content remains constant and that CO2 exchange in the lung is proportional to its release from the cells. Arterial blood remains almost completely oxygen saturated(about 98%), even during intense effort. We can accurately assume that the oxygen being removed from the air we breathe is in proportion to its cellular uptake. Carbon dioxide exchange, however, is less constant. Body CO2 pools are quite large and can be altered simply by deep breathing or by performance of highly intense exercises. Under these conditions, the amount of CO2 released in the lung may not represent that being produced in the tissues, so calculations of carbohydrate and fat used based on gas measurements appear to be valid only at rest or during steady-state exercise.

Use of the RER also can lead to inaccuracies. Recall that protein is not completely oxidized in the body because nitrogen is not oxidizable. This makes it impossible to calculate the body’s protein use from the RER. As a result, the RER is sometimes reffered to as nonprotein RER because it simply ignores protein oxidation.

Traditionally, protein was thought to contribute little to the energy used during exercise, so exercise physiologists felt justified in using the nonprotein RER when making calculations. But more recent evidence suggests that in exercise lasting for several hours, protein may contribute up to 5% of the total energy expended under certain circumstances.

The body normally uses a combination of fuels. Respiratory exchange ratio values vary depending on the specific mixture being oxidized. At rest, the RER value is typically in the range of 0.78 to 0.80. During exercise, though, muscles rely increasingly on carbohydrate for energy, resulting in a higher RER. As exercise intensity increases, the muscles’ carbohydrate demand also increases. As more carbohydrate is used, the RER value approaches 1.0.

This increase in the RER value to 1.0 reflects the demands on blood glucose and muscle glycogen, but it also may indicate that more CO2 is being unloaded from the blood than is being produced by the muscles. At or near exhaustion, lactate accumulates in the blood. The body tries to reverse this acidification by releasing more CO2. Lactate accumulation increases CO2 production because excess acid causes carbonic acid in the blood to be converted to CO2. As a consequence, the excess CO2 diffuses out of the blood and into the lungs for exhalation, increasing the amount of CO2 released. For this reason, RER values approaching 1.0 may not accurately estimate the type of fuel being used by the muscles.

Another complication is that glucose production from the catabolism of amino acids and fats in the liver produces an RER below 0.70. Thus, calculations of carbohydrate oxidation from the RER value will be underestimated if energy is derived from this process.

Despite its shortcomings, indirect calorimetry still provides the best estimate of energy expenditure at rest and during aerobic exercise.