Energy systems: Indirect calorimetry.

The idea

Indirect calorimetry estimates energy expenditure without measuring heat directly. A performer breathes through a mouthpiece or mask while oxygen uptake (VO₂) and carbon dioxide production (VCO₂) are measured. The respiratory exchange ratio (RER = VCO₂ ÷ VO₂) indicates the substrate mix: an RER near 0.7 suggests mainly fat oxidation, near 1.0 mainly carbohydrate, and above 1.0 indicates heavy anaerobic contribution. Each litre of oxygen consumed releases roughly 20 kJ when fat is the fuel and roughly 21 kJ when carbohydrate is the fuel, so expenditure is estimated by multiplying oxygen uptake by the appropriate energy equivalent. The method is accurate in the laboratory but requires equipment and can alter normal breathing.

What earns the marks

• Indirect calorimetry estimates energy expenditure from oxygen uptake (VO₂) and carbon dioxide production (VCO₂) rather than from heat loss.
• The respiratory exchange ratio (RER) is calculated as VCO₂ ÷ VO₂ and indicates the substrate being oxidised.
• An RER near 0.7 indicates mainly fat oxidation; near 1.0 indicates mainly carbohydrate; above 1.0 indicates a large anaerobic contribution.
• Energy expenditure is estimated by multiplying oxygen uptake by the energy equivalent of oxygen (about 20 kJ per litre for fat and about 21 kJ per litre for carbohydrate).
• The method is accurate in laboratory conditions but needs specialist equipment and a mouthpiece or mask that can alter breathing.

Common mistakes and how to put them right

• Reversing the RER formula.

  Put it right: RER is carbon dioxide produced divided by oxygen consumed (VCO₂ ÷ VO₂), not the other way round.

• Believing RER above 1.0 means fat is the main fuel.

  Put it right: A value above 1.0 reflects a heavy anaerobic contribution and increased carbon dioxide from buffering.

• Treating indirect calorimetry as measuring heat directly.

  Put it right: It estimates expenditure from respiratory gases, whereas direct calorimetry measures heat loss.

Exam tips

• Write the RER equation as VCO₂ ÷ VO₂ and practise interpreting values near 0.7, 0.85 and 1.0.
• Link the energy equivalent of oxygen to the substrate: about 20 kJ per litre for fat and about 21 kJ per litre for carbohydrate.
• When evaluating the method, mention both its laboratory accuracy and its practical limitations such as equipment and altered breathing.