Measuring Fuel Substrate Utilization: Indirect Calorimetry and RER

by | Lessons

1. Respiration Exchange Ratio and the Chemistry of Combustion

Respiration Exchange Ratio Illustrated

When we burn a particular fuel (e.g. glucose, fat, methane, propane) it ALWAYS combines with a fixed volume of O2 and produces a fixed volume of CO2.

The Chemistry of Combustion:

Methane: Cow Burps
CH4 + 2 O2 –> CO2 + 2 H2O

Glucose:
C6H12O6 + 6O2 –> 6 CO2 + 6H20

Fat:
C16H32O2 + 23O2 –> 16CO2 + 16H2O

As the formulas show – when a human cell aerobically uses 1 glucose molecule or 1 fat molecule – respectively:

  • 6 CO2s get produced 6 O2s are consumed
  • 16 CO2s get produced, 23O2s are consumed

RER for each:

  • Glucose: 6CO2/6O2 = 1.
  • Fat: 16CO2/23O2 = .7

Thus, RER ranges from .7 to 1

  •  When RER = 1, the body uses primarily glucose as fuel.
  • An RER of .7 indicates the body uses primarily fat.

RER of Glucose Illustrated

Let’s assume the human on this bike burned ONLY glucose and nothing else. If it was, this is what we would see:

rer-glucose

 

RER generally is close to 1 during high intensity exercise – when oxygen consumption is very high.

However, the body does not burn exclusively glucose or fat – but a mixture of the two, which is seen as RER changes with intensity level.

In order to complete this picture and know exactly the amount of fat and carbs burned at any intensity level – the RER of burning fat must be added – as illustrated here.

 

 

 

The red text shows the volume of oxygen used and the blue text shows the volume of carbon dioxide produced from combusting one molecule of each type of fuel.

chemistry-of-combustion

 

The Chemistry of Combustion:

Methane:
CH4 + 2 O2 –> CO2 + 2 H2O

Propane
C3H8 + 5 O2 –> 3 CO2 + 4 H2O

Butane
C4H10 + 6.5 O2 –> 4 CO2 + 5 H2O

Glucose:
C6H12O6 + 6O2 –> 6 CO2 + 6H20

Fat:
C16H32O2 + 23O2 –> 16CO2 + 16H2O

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