If your definition of ‘fitness’ is your ability to do work – not just your VO₂ max score – and you want to improve that ability, then there’s a strong argument that one of the best things you can do each day is also one of the simplest.
And it’s something you can do anywhere, any time.
Hold your breath.
Because when you’re reaching the edges of your fitness – in the gym, on a run, in life – it’s not a lack of oxygen that stops you, and sometimes it’s not even your muscles – it’s carbon dioxide. The rising CO₂ in your blood is what drives that urge to breathe, that sense of panic, that voice telling you to slow down.
Build tolerance to CO2, train that response, and you can majorly change the game – and not just in the gym.
Why it Works
The sensation of breathlessness is driven primarily by CO₂, not a lack of oxygen . If your tolerance to CO₂ is low, you’ll feel out of breath sooner, even if your muscles could keep going.
What to read next
That’s where breath-hold training comes in.
By exposing yourself to rising CO₂ in a controlled way, you:
- Reduce your sensitivity to that panicked ‘stop’ signal
- Build the ability to stay calmer under stress
- Improve your ability to maintain output despite fatigue
In other words: you can get more work done.
But there’s also a mental fitness crossover. Breath holding has been used in research as a proxy for distress tolerance, linking it to how well people handle discomfort and anxiety .
If you can stay calm when CO2 is rising, you’re building downstream tolerance for a tough set of lunges and a stressful meeting. In fact individuals with higher CO₂ sensitivity (low tolerance) show greater anxiety and future anxiety risk. Training your response to CO₂ (via breath holds) may reduce your fear/anxiety response.
The BOLT Test
The easiest way to assess your current CO2 tolerance is the BOLT test (Body Oxygen Level Test):
- Sit upright and breathe normally
- Exhale gently through your nose
- Pinch your nose and start a timer
- Stop at the first clear urge to breathe
That number (not a max hold until you nearly pass out) is your score.
Under 20 seconds? You’ve got work to do. 30+ seconds? You’re in a good place.
A higher score reflects better CO₂ tolerance, more efficient breathing and a more regulated nervous system.
A Simple Daily Protocol
You don’t need ice baths or apps. Just a few minutes at your desk:
1. Measure your BOLT score (once per day)
Track progress over time.
2. Submaximal breath holds (5 minutes total)
- Inhale through the nose, exhale gently
- Hold your breath until the first strong urge to breathe
- Recover with calm nasal breathing for 60–90 seconds
- Repeat for 3-5 rounds
3. Progress gradually
Over weeks, your hold times – and BOLT score – should climb.
The Takeaway
This isn’t necessarily about building a bigger engine, but removing a limiter – if you can tolerate more CO₂, you can stay composed for longer. If you stay composed for longer, you can do more work. And if you do more work, you get fitter.
All from something you can do sitting at your desk.
With almost 18 years in the health and fitness space as a personal trainer, nutritionist, breath coach and writer, Andrew has spent nearly half of his life exploring how to help people improve their bodies and minds.
As our fitness editor he prides himself on keeping Men’s Health at the forefront of reliable, relatable and credible fitness information, whether that’s through writing and testing thousands of workouts each year, taking deep dives into the science behind muscle building and fat loss or exploring the psychology of performance and recovery.
Whilst constantly updating his knowledge base with seminars and courses, Andrew is a lover of the practical as much as the theory and regularly puts his training to the test tackling everything from Crossfit and strongman competitions, to ultra marathons, to multiple 24 hour workout stints and (extremely unofficial) world record attempts.
You can find Andrew on Instagram at @theandrew.tracey, or simply hold up a sign for ‘free pizza’ and wait for him to appear.
