Indirect calorimetry and the respiratory quotient

Energy expenditure can be determined from the rate of consumption of oxygen. This is known as indirect calorimetry, as there is no direct measurement of the heat produced. As shown in Table 5.1, there is an output or expenditure of 20 kJ per litre of oxygen consumed, regardless of whether the fuel being metabolized is carbohydrate, fat or protein. Measurement of oxygen consumption is quite simple using a spirometer. Such instruments are portable, so people can carry on more or less normal activities for several hours at a time, while their energy expenditure is being estimated.

Measurement of oxygen consumption and carbon dioxide production at the same time, again a simple procedure using a spirometer, provides information on the mixture of metabolic fuels being metabolized. In the metabolism of starch, the same amount of carbon dioxide is produced as oxygen is consumed, i.e. the ratio of carbon dioxide produced to oxygen consumed (the respiratory quotient) = 1.0. This is because the overall reaction is CH O, + 6O, ^ 6CO + 6HO.

Proportionally more oxygen is required for the oxidation of fat. The major process involved is the oxidation of —CH2— units: —CH2 + P/2 O2 ^ CO2 + H2O. Allowing for the fact that in triacylglycerols there are also the glycerol and three carboxyl groups to be considered, overall for the oxidation of fat the respiratory quotient = 0.7.

The metabolism of proteins gives a ratio of carbon dioxide produced to oxygen consumed that is intermediate between that of carbohydrate and fat — this is because proteins contain relatively more oxygen per carbon than do fats, although less than carbohydrates. For protein metabolism the respiratory quotient = 0.8. The amount of protein being oxidized can be determined quite separately, by measurement of the excretion of urea, the end-product of amino acid metabolism (section 9.3.1.4).

Measurement of the respiratory quotient and urinary excretion of urea thus permits calculation of the relative amounts of fat, carbohydrate and protein being metabolized. In the fasting state (section 5.3.2), when a relatively large amount of fat is being used as a fuel, the respiratory quotient is around 0.8—0.85; after a meal, when there is more carbohydrate available to be metabolized (section 5.3.1), the respiratory quotient

Table 5.1 Oxygen consumption and carbon dioxide production in oxidation of metabolic fuels

Carbon

Energy/

Energy

Oxygen

dioxide

Respiratory

oxygen

yield

consumed

produced

quotient

consumption

(kJ/g)

(L/g)

(L/g)

(CO2/O2)

(kJ/L oxygen)

Carbohydrate

16

0.829

0.829

1.0

Protein

17

0.966

0.782

0.809

~ 20

Fat

37

2.016

1.427

0.707

rises to about 0.9—1.0. If there is a significant amount of lipid being synthesized from carbohydrate (section 5.6.1) then the respiratory quotient may rise above 1.0.

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