Respiratory Exchange Ratio (RER) explained.

Science!

What the heck is this?

RER has been mentioned a few times on this blog. By measuring the rate of CO2 exhaled and the rate of O2 inhaled, it's possible to work out how many kcals/min the body is generating from food at any instant and from what fuel mixture.

An RER of 0.700 means that 100% of energy is being generated from fat.
An RER of 1.000 means that 100% of energy is being generated from carbohydrate aerobically.
An RER of >1.000 means that 100% of energy is being generated from carbohydrate, some aerobically and some anaerobically.

How does this work? Fats are an ester of fatty acids + glycerol. Acid + Alcohol = Ester + Water.

Saturated fatty acids (the easiest type to calculate) have the generic formula CH3(CH2)nCOOH, where n can be from 0 to 16. Here are some saturated fatty acids and their n values:- Acetic (0), Propionic (1), Butyric (2), Lauric (10), Myristic (12), Palmitic (14) and Stearic (16). The total number of carbon atoms in each fatty acid is n+2. Stearic acid is mostly a load of CH2s, so lets approximate fat to n(CH2).

n(CH2) + 3/2n(O2) = n(CO2) + n(H2O) + Heat. The ratio of CO2 to O2 is 2/3, so RER = 0.666.

As fats contain things other than CH2 (e.g. glycerol CH2OHCHOHCH2OH), this raises RER to 0.700. Burning protein gives an RER = 0.800.

Carbohydrates have the generic formula n(CH2O), where n = 6 for glucose.

n(CH2O) + n(O2) = n(CO2) + n(H2O) + Heat. The ratio of CO2 to O2 is 1.000, so RER = 1.000.

So how on earth can Eskimos have an RER = 0.600? I have a theory. When hydrogen is oxidised, water only is produced. There is no CO2, so RER = 0.000. Therefore, if some hydrogen was being burned (by gut bacteria, say), this could result in RER falling below 0.700. Maybe...