Out of Breath? Just Eat Something Different!
Energy Supply
The body extracts energy by oxidising (“burning”) substances that ultimately come from food. The classic story is that the body preferentially burns glucose - carbohydrates. Regular readers here know that isn’t quite right:
More on the body's preferred fuel
The Preferred Fuel
The body’s preference: Which fuel does it like to burn most? (The answer will surprise you 😉)
Read more →Let’s look at the two usual fuels and what it takes to extract energy from them. Energy in the body is stored chemically as ATP molecules, and you can do a neat conversion from glucose and fatty acids to CO2:
When the body makes ATP from glucose, 10 molecules of O2 consumed produce 10 molecules of CO2. When the body makes ATP from fatty acids, 10 molecules of O2 consumed produce only 7 molecules of CO2.
A Counterintuitive Equation: O2 vs CO2
Approaching the biochemistry from a different angle, you’ll find that carbohydrate oxidation is actually more efficient in terms of oxygen consumption:
Glucose oxidation yields 5.05 kcal per litre of oxygen; fatty acid oxidation yields only 4.69 kcal per litre of oxygen (1).
That is also why the body switches to sugar burning under extreme exertion - to squeeze out every last drop of energy.
The difference in CO2 production comes down to the fact that fatty acids carry far less oxygen within their molecules than glucose does, and plays out in the end products of combustion: water and carbon dioxide.
If sugar burning is more efficient, though, why would you ever choose fat burning - especially in sport? Simple. There is efficiency in terms of energy produced, and efficiency in terms of metabolic by-products. And here the difference is massive:
Chemistry
Glucose is written as C6H12O6, giving it a carbon-to-oxygen ratio of 1:1.
Oxidising one glucose molecule requires 6 oxygen molecules:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O.
Six molecules of oxygen inhaled, six molecules of carbon dioxide exhaled. A ratio of 1:1.
A fatty acid (palmitic acid in this example) is written as C16H32O2, giving it a carbon-to-oxygen ratio of 8:1.
Burning one molecule requires 23 oxygen molecules:
C16H32O2 + 23 O2 → 16 CO2 + 16 H2O.
Twenty-three molecules of oxygen inhaled, sixteen molecules of carbon dioxide exhaled. A ratio of 16:23, roughly 0.7.(2)
The key difference: fat burning produces less CO2 per oxygen molecule than sugar burning.
The Athletic Angle
We know that under intense exertion the body switches to sugar burning to squeeze out the last available energy. And being truly out of breath - genuinely gasping for air - is thoroughly unpleasant.
The main reason for that desperate gulping, however, is usually not a lack of oxygen, but the need to get rid of carbon dioxide: CO2 is acidic (carbonic acid), and the body’s sensors are extremely attuned to keeping blood pH stable.(3)
Someone who trains the body to run on fat - which works best and most reliably with a ketogenic diet(4) - therefore consumes slightly more oxygen, but runs out of breath considerably less quickly.
Personal Validation
Sounds far-fetched? I tried it myself - before I knew any of this maths existed. After a while on a ketogenic diet, I noticed that even after a swim training session I could comfortably dive 25 m relaxed, and even part of the way back. Before the dietary change, the last 5 metres of a 25 m length were unpleasant - including at the very start of a session, before any prior exertion.
As a side note: before the dietary change, I had been training considerably more, and nominally had better fitness. Turns out: less training but better food produces a noticeably better result.
Somehow unsatisfying for everyone who believes you just have to suffer enough.
Conclusion
In short: the body doesn’t signal “out of breath” because it’s short on oxygen - it signals it because it has too much CO2 to get rid of. Someone running on fat burning produces only 70% of the CO2 typically generated by sugar burning, and therefore has - literally and biochemically - more air.
That isn’t a trick and it isn’t a placebo. It’s stoichiometry(5).
Curious how this plays out in your own metabolism? The Simulation shows how glucose, insulin, and fat burning interact - no advanced classes required.
