Why does fat oxidation result in a lower RER value You may w

Why does fat oxidation result in a lower RER value? You may want to review oxidation of substrate molecule and RER

Solution

RER value is the ratio of CO2 breathed out to oxygen consumed in one breath. Normally this ratio is below 1; below the amount of oxygen consumed is more than the amount of CO2 released.

RER demonstrates the oxidative capacity of muscle. More is the RER, more is the capacity of muscle to make energy (ATP) in its mitochondria; i.e. a higher RER indicates higher energy in the muscles.

When carbohydrate is the predominant fuel source in our body, then RER comes out to be 1. This means that during carbohydrate oxidation, the amount of oxygen consumed is equivalent to the amount of CO2 released out of the body.

Now, when our body uses fat as the fuel source, then oxygen consumed will be higher than the CO2 released out. This is because one molecule of fat gives more energy than one molecule of carbohydrate. So, less molecules of fat are required for producing the same amount of energy produced by more molecules of carbohydrate.

So, RER reduces to about 0.7 when fat is the predominant fuel source.

Please note that, Oxidation of a molecule of Carbohydrate follows the given equation:

6 O₂ + C₆H₁₂O₆ => 6 CO₂ + 6 H₂O + 38 ATP

RER = VCO₂/VO₂ = 6 CO₂/6 O₂ = 1.0

Oxidation of a molecule of Fatty Acid follows this equation

23 O₂ + C₁₆H₃₂O₂ => 16 CO₂ + 16 H₂O + 129 ATP

RER = VCO₂/VO₂ = 16 CO₂/23 O₂ = 0.7