Fat burn or performance: what does your breathing reveal during long sessions?

PAIRFS
3 days ago
3 min read

Long endurance runs are often touted as the “fat-burning zone.” But in reality, it’s not the power zone, nor the heart rate, nor even the duration that determines whether you’re burning fat…

👉 That's your breathing.

During a long workout, your breathing reveals exactly where your energy is coming from—fats, carbohydrates, or a mix of both—and how your body is managing the effort. Here's what your breathing really says about your metabolism.

1. Breathing reveals the fat/carbohydrate balance

When you are in low endurance (below VT1), your breathing remains:

deep,
regular,
controlled,
sometimes possible through the nose.

➡️ This is a sign of a metabolism strongly oriented towards fats.

As soon as the effort increases and breathing accelerates disproportionately, it means you are approaching VT2 → your body switches to carbohydrates.

For what ?

Because respiration directly reflects CO₂ production. And this CO₂ comes from the combustion of substrates:

Fats = low CO₂ production
Carbohydrates = high CO₂ production

👉 When your breathing explodes… so does your carbohydrate consumption.

2. Why long sessions should remain under VT1

Studies show that:

Maximum fat oxidation (FatMax) is almost always below VT1 .
Exceeding VT1 immediately reduces the proportion of fats used.
Staying below VT1 conserves glycogen reserves (essential in competition).

This is demonstrated in particular by:

Achten & Jeukendrup (2004),
San-Millán & Brooks (2018),
Meyer et al. (2005).

👉 During a long run, if your breathing remains smooth → goal achieved. If it becomes labored → you are no longer in the “fat burn” zone.

3. Breathing and drift: the signal that shows you're tipping over

Over the course of several hours, a respiratory drift is often observed:

same power,
but faster breathing
need to breathe more often,
shorter expirations.

This is a sign that your body:

✔ begins to run out of usable lipids,

✔ increases the use of carbohydrates,

✔ becomes more acidic → more CO₂ → increased respiration.

👉 Respiratory drift is one of the best indicators of metabolic fatigue.

4. Performance: your breathing shows your "energy efficiency"

Experienced endurance athletes are distinguished by one thing:

➡️ They maintain stable breathing at intensities where others become breathless.

This is a sign:

better utilization of lipids,
a higher ventilatory threshold,
improved respiratory efficiency,
increased tolerance to CO₂.

This efficiency is a direct predictor of long-distance performance (articles by Poole, Jones, Wasserman).

5. How ZoneX measures all of this in real time

Traditionally, to analyze your respiratory and metabolic profile, it was necessary to:

a mask,
a laboratory,
a complete stress test.

ZoneX makes this analysis portable and continuous .

By measuring ventilation flow and respiratory rate, ZoneX can detect:

the actual fat-burning zone,
your respiratory drift,
your VT1 and VT2 transitions,
the switch from carbohydrates → fats → carbohydrates depending on the effort.

👉 You know precisely when you are in the fat burn zone ,

👉 and when you start overconsuming your carbohydrates ,

👉 even during an outdoor outing.

6. In summary

During a long session, your breathing reveals:

What you feel	What this means	Area
Very fluid breathing	FatMax → predominant fats	Under VT1
Breathing a little rapid but controlled	Fat + Carbohydrate Mixture	Between VT1 and VT2
Gasping breath	Carbohydrate-based metabolism	Above VT2
Breathing that shifts with the effort	Metabolic fatigue + cardio drift	Over time