The body converts the energy of food into ATP with an efficiency of approximately 50 per cent, with the remaining energy being lost as heat. When the ATP itself is used by the body to do work a further loss of heat occurs, equal to approximately 50 per cent of the energy in the ATP. Finally, the work itself generates heat. In this way, it can be seen that a body’s total heat production gives a measure of the amount of energy that has been used.
Because of this, it is possible to use calorim- etry, or the measurement of heat, to quantify the amount of energy expenditure. If heat is meas- ured directly, the technique is known as direct calorimetry. However, in many cases, it is not practical to do this, and an indirect approach is used, based on the utilization of oxygen. This is valid because the rate of oxygen consumption is proportional to the amount of ATP synthesis, and each mole of ATP synthesized is accom- panied by production of a given amount of heat. It is thus reasonable to use measurements of oxygen consumption to calculate heat produc- tion within the body.
It is possible to see from the chemical equa- tions for the oxidation of carbohydrates and fats how much oxygen is used and how much carbon dioxide is produced.
Glucose oxidation: C6H12O6 6O2→ 6H2O 6CO2 15.5 kJ/g of energy Starch oxidation: (C6H10O5)n 6nO2→ 5nH2O + 6nCO2 17 kJ/g of energy Fat oxidation:
e.g. glyceryl butyro-oleostearate (the main fat in butter)
C3H5O3.C4H7O.C18H33O.C18H35O 60O2 → 43CO2 40H2O 39 kJ/g of energy It can be seen that, for carbohydrate oxidation, the volume of carbon dioxide produced equals the volume of oxygen used. When fat is oxi- dized, however, the volume of carbon dioxide produced is about 70 per cent of the volume of oxygen consumed. These values are usually expressed as a fraction: CO2 produced/O2 con- sumed, known as the respiratory quotient (RQ). The RQ for carbohydrates is 1.0, and for fats averages 0.71 (different fats have slightly differ- ent values). The non-nitrogenous portions of proteins are, on the whole, intermediate in com- position between fats and carbohydrates and, for protein, the RQ is usually given as 0.83.
The RQ value can be used to discover the amount of energy production for each litre of oxygen consumed, since this is also predictable from the equation. Normally, the body uses a mixture of substrates for its metabolism, and the metabolic mixture being used can be determined from a measurement of RQ. Thus a value close to 0.7 suggests that mainly fats are being metab- olized; conversely, a value close to 1.0 would indicate carbohydrate-fuelled metabolism. In practice, the usual metabolic mixture provides an RQ of around 0.8.
In summary, if one can determine oxygen usage and/or carbon dioxide production, it is possible to calculate the amount of energy released during metabolism. With information about both oxygen and carbon dioxide, it is
ENERGY OUTPUT ❚
possible to derive the actual RQ that applies. However, if only one of the gases has been measured, an RQ of 0.8 is assumed (as this repre- sents the average metabolic mixture). It is esti- mated that not adjusting for the actual RQ probably introduces an error of 3–4 per cent, which is generally acceptable for most purposes. Tables 7.3 and 7.4 provide the RQ and oxygen utilization values used in these measurements.
Direct calorimetry
The original human calorimeter was designed by Atwater and Rosa in 1905. This was the size of a small room, and contained a bed and stationary exercise bicycle. The walls were well insulated to prevent any heat loss, and all heat dissipated by the subject was transferred to circulating water in the walls of the chamber. Increases in water temperature could be measured, and these repre- sented the subject’s heat loss.
In addition, the gases flowing into and out of the chamber could be analysed, giving add- itional information about the subject’s metab- olism. Such calorimeters are still in use today. The
major difference lies in the methods by which the heat output is measured in the walls of the chamber; modern calorimeters use microchips and computers for this. There is also a reduction in size, with some modern calorimeters being the size of a phone booth or even smaller.
The major drawback of the human calorim- eter (apart from its cost) is that it only allows a limited amount of activity, and is thus not usable for ‘real-life’ measurements.
If the calorimeter is large, the length of time taken for any changes in heat to be measurable is longer. At a minimum, a calorimeter of 1.6 m3 provides a response in 3 minutes; one with a size of 20 m3takes 2 hours to show a response. The accuracy of measurements with a direct calorimeter is of the order of 1–2 per cent.
Indirect methods
Respiratory gas analysis
The principle of indirect calorimetry is based on the relationship between oxygen use and carbon dioxide production, described above. To apply this, the method used must be able to measure
Energy yields obtained from the oxidation of different substrates
TABLE 7.3
Nutrient Oxygen consumed Carbon dioxide Respiratory Energy equivalent (litres/g) produced (litres/g) quotient (kJ/kcal per litre
of oxygen)
Starch 0.829 0.829 1.0 21.2/5.06
Glucose 0.746 0.746 1.0 21.0/5.01
Fat 2.019 1.427 0.71 19.6/4.69
Protein 0.962 0.775 0.81 19.25/4.66
The effect on energy yield of different metabolic mixtures and respiratory quotient (RQ)
TABLE 7.4
RQ kJ per litre Energy (%) derived from
of oxygen Carbohydrate Fat 0.71 19.6 1 99 0.75 19.8 16 84 0.80 20.1 33 67 0.85 20.3 51 49 0.90 20.6 68 32 0.95 20.8 84 16 1.00 21.1 100 0
either or possibly both of the gases over a period of time. Equipment used for this purpose ranges from the large and stationary to more portable versions. Obviously stationary equipment can only be used when subjects are resting; more mobile activities require portable equipment.
Originally, the respiration chamber itself was used, with the gases inspired and expired by the subject sampled and analysed. More recently, a hood or small plastic tent is placed over the recumbent subject to collect the gases, which these days pass straight into automated ana- lysers linked to computer printouts, to provide instant data. This type of measurement is useful in hospital patients who may be confined to bed. The equipment must be mobile and moved to the bedside.
Mobile equipment has largely been of the ‘backpack’ variety, worn by the subject during physical activity. The necessary link between the equipment and the subject’s respiratory sys- tem has been by way of a corrugated tube, mouthpiece and nose clip. This is not especially comfortable, and can both limit the duration of the measurement and affect the normal pattern of breathing.
Isotope methods
Alternative approaches have been developed in recent years. The most significant of these is the doubly labelled water technique, using stable isotopes of hydrogen and oxygen (2H and 18O). The subject drinks a small volume of labelled water. The hydrogen equilibrates in the body water pool (producing hydrogen-labelled water), the oxygen in both the water (as oxygen- labelled water) and in the carbon dioxide pools. Thus, the 18O is contained in both the carbon dioxide and water, and the 2H is contained only in the water; therefore, the labelled oxygen is lost from the body faster than the hydrogen.
The rates of loss of the isotopes are generally measured in a series of samples of body fluid, such as urine, over a period of days (up to 21). The difference between the rates of loss of the two isotopes, therefore, represents the loss of car- bon dioxide, and can be used to calculate carbon dioxide and, hence, heat production. In turn, this can be used to derive energy expenditure.
Several assumptions are made with this tech- nique, most notably about isotope fractionation in the body. In addition, as oxygen usage is not measured, the RQ has to be estimated. However, if a concurrent food intake record is kept, then an RQ value from the intake can be calculated, assuming there is energy equilibrium.
This technique provides a useful, safe and non-intrusive way of measuring carbon dioxide turnover, and hence metabolism over periods of time. As a result, a large amount of information has been derived in recent years about energy expenditure in groups of subjects that previ- ously have been difficult or unethical to study, such as the elderly, pregnant and lactating women, and young infants and children.
Measurements of 24-hour energy expend- iture can be made by a relatively novel isotope technique, the bicarbonate–urea method. The technique uses the isotope dilution principle. Labelled CO2, given subcutaneously as bicarbon- ate, is diluted by endogenous CO2; the extent of this dilution is measured by the isotope activ- ity in urinary urea, produced over a 24-hour period. This allows the CO2 produced in the body to be calculated, and energy expenditure found using values for the energy equivalent of CO2.
Other methods
A number of other techniques have been used to measure energy expenditure, including:
■ activity diaries;
■ heart rate monitoring;
■ skeletal muscle recording (electromyography);
■ pedometers (to record movement);
■ energy intake studies for subjects in energy balance.
All of these give less reliable results, but may be of value when groups of subjects are being studied, to provide more general data.
Some of the methods listed above can pro- vide estimates of the intensity of exercise or patterns of activity during a period of exercise that may provide useful additional information. Activity diaries are prone to omissions, espe- cially of small habitual movements, that may be important over a 24-hour period, and may over- report structured or intense activity.
COMPONENTS OF ENERGY OUTPUT ❚