Thursday, 2 May 2013

Respiratory Exchange Ratio (RER) explained.

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 CH2s, so I'll 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...


LeonRover said...

"RQ values for burning 100% fat (0.7), 100% CHO (1.0); significance of RQ > 1 (excess calories - fat synthesis); RQ<0.7 indicates ketogenesis. 9, 4, 4 kcal/g for fat, CHO, and protein and 7 kcal/g for ethanol."

This is an example of explanation Googlable using "RQ<0.7" when experimental error has been eliminated. The researcher in 1928 writes about mold on some of the samples.

Water synthesis from H2 gets no mention.

Nigel Kinbrum said...

Now why didn't I think of Googling RQ<0.7? Thanks for thinking of it.

Now I need to work out how ketogenesis lowers RQ.

LeonRover said...
This comment has been removed by the author.
Nigel Kinbrum said...

I thought that beta-hydroxybutyrate delivered 5kcal/g on oxidation.

I've been a bit busy today. I had to go to Henley-on-Thames to collect some ashes. I also had a DoS earlier in the morning. Hmmm.

LeonRover said...

Bear in mind, Nige, that ethanol @ 7kCal/gm is derived from the fact that metabolism of eth is via one of the ketone bodies, which in turn deliver 7 kcal/gm on oxidation.

By co-incidence Ethanol deliver 7 kCal/gm as does Acetone, one of ketone bodies.

The other ketones
β-hydroxybutric acid gives 4.96 kcal/gm while Acetoacetic acid give 4.15 kcal/gm.

Good luck with your busyness.

Nigel Kinbrum said...

I also thought that acetoacetate rapidly degraded to acetone, which was then lost by exhalation.

I've hardened my stack. My TCP/IP's are now more secure.

Bill said...

Hi Nigel,

Ketogenesis + high protein diet.
Some of the carbons aren't expired as CO2 which drives down the RER - acetone, yes, but they're also lost in the urine (eg, ketonuria, urea from high rates of gluconeogenesis). This is what I think is happening in the Eskimos you referenced.


Nigel Kinbrum said...

Of course! Respiratory Gas Analysis only measures stuff in the breath. The nursing mother was in deeper ketosis than the others and she also had the lowest RER.

Bill, you win the internets today!

Bill said...

awesome lol. Thanks, Nigel :)

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