For a runner, breath training is a lever on CO₂ tolerance, breathing economy and comfort — not on your VO₂max ceiling. It can make a given pace feel calmer, quiet the early air hunger that wrecks your form, and lower how hard you have to breathe at submaximal effort. What it will not do is raise the size of your engine. Once you separate those two claims, breath work becomes a genuinely useful tool instead of a magic pill.
Key takeaways
- Breath work builds CO₂ tolerance, not aerobic capacity. In the 2025 runner trial, resting breath-hold time rose 11.7 s in the breathwork group against 1.9 s in controls (p = 0.04, d = 1.13), while VO₂max showed no group difference.
- Nasal breathing at easy pace lowers the ventilatory equivalent for CO₂, slows breathing frequency and raises end-tidal CO₂ — calmer, cheaper breathing for the same submaximal workload.
- Nasal-only breathing has a hard ceiling. At peak effort the nose cannot shift enough air, so an open mouth at threshold is physiology, not weakness.
- Three to four dry CO₂ table sessions a week is the core protocol, and two weeks is usually enough to feel early air hunger arrive later and quieter.
Why breathing can limit a run — and what is actually trainable
Two things happen to your breathing when you run that have nothing to do with your legs.
The first is air hunger. Air hunger is the urge to breathe harder that arrives well before your muscles are actually starved of oxygen. Just like in a breath hold, that urge is driven mostly by rising carbon dioxide, not by falling oxygen. A runner who panics at the first wave of air hunger tenses up, over-strides and burns energy fighting their own diaphragm.
The second is over-breathing. Over-breathing is dumping far more air than the effort requires, in short, shallow, upper-chest gulps. It feels like you are working hard for your oxygen. Mostly you are just moving stale air in and out of the top of your lungs. In the lab it shows up as a high ventilatory equivalent for CO₂ and a low end-tidal CO₂ — the exact pattern Eser and colleagues reversed by switching people to nasal breathing (Frontiers in Physiology, 2024, PMID 39165283).
Neither of those is your VO₂max. VO₂max is the maximum rate at which your body can take in, transport and use oxygen, and it is set by heart, blood and muscle rather than by breathing habits. Air hunger and over-breathing, by contrast, are habits and tolerances, and both are trainable. That is the honest promise of breath work for runners: not a bigger engine, but a calmer, cheaper, more controlled way of feeding the one you have.
CO₂ tolerance vs VO₂max — the myth-check
This is where most breath-training marketing quietly overreaches, so let’s be precise.
A 2025 randomised controlled trial put recreational runners through a short functional-breathing program and measured what changed. The result is the cleanest myth-check you could ask for. Resting breath-hold time improved significantly in the breathwork group and barely moved in the control group (+11.7 s vs +1.9 s; p = 0.04, with a large effect size, d = 1.13). But VO₂max did not differ between the groups (Sports, 2025, PMID 39997962).
The methodology is what makes that split usable. Runners were randomly assigned to the breathing program or to a control group, and both were re-tested on the same measures afterwards. So “breath-hold up, VO₂max flat” is a comparison inside one trial, not two findings stitched together from different papers.
Read that twice. CO₂ tolerance is how much carbon dioxide you can let accumulate before the urge to breathe takes over, and a resting breath-hold time is the cheapest way to measure it. The breathing program clearly built it. It did not enlarge the aerobic engine. So if a program promises breath drills will raise your VO₂max, the evidence says otherwise. What breath training reliably moves is your tolerance for the CO₂ signal and how comfortable a given pace feels — which is worth having, as long as you buy it for what it is.
| Claim | What the evidence supports | Verdict |
|---|---|---|
| “Breath drills raise VO₂max” | No group difference in the runner RCT | Myth |
| “Breath training improves CO₂ tolerance” | Breath-hold +11.7 s vs +1.9 s (p = 0.04, d = 1.13) | Supported |
| “Nasal breathing is more efficient at easy pace” | Lower VE/VCO₂, slower breathing, higher end-tidal CO₂ | Supported |
| “You can breathe only through your nose at race pace” | Nasal-only impairs peak capacity via ventilatory limit | Myth |
Nasal breathing: what the evidence really shows
Nasal breathing is the other claim that gets stretched. The truth is genuinely good — just bounded.
At rest and at easy-to-moderate effort, nasal breathing is more efficient. A 2024 study found that breathing through the nose lowered the ventilatory equivalent for CO₂ (VE/VCO₂), lowered breathing frequency and raised end-tidal CO₂ across every group tested, including healthy controls (Frontiers in Physiology, 2024, PMID 39165283). In plain terms: calmer, slower, more economical breathing for the same submaximal workload. That is exactly the state you want on an easy or base run.
But there is a ceiling. Researchers pushed people to peak effort using exclusively nasal breathing, and at rest and submaximal intensities it was fine. At maximum it impaired peak exercise capacity because the nose cannot move enough air — a ventilatory limitation, not a lack of willpower (PLOS One, 2025, PMID 40668801).
So the rule writes itself: nose for easy and base runs; open the mouth as intensity climbs. Forcing nasal-only breathing at race pace does not make you tougher. It caps your ability to shift air exactly when you need the most, so use nasal breathing where it pays and stop treating an open mouth at threshold as a failure. For instance, say you can hold 6:00/km with your mouth closed but have to open it at 4:30/km. That gap is not a weakness you can breathe your way through; it is the ventilatory ceiling the PLOS One group measured.
A dry protocol for runners
You train CO₂ tolerance off the road, then let it show up on it. None of this needs water, and none of it belongs at max effort.
CO₂ tolerance at the same pace
Drag the slider from low to high CO₂ tolerance and watch what happens at one unchanged running pace: the same effort, fewer and calmer breaths.
Slide the widget from low to high tolerance and watch the same pace turn from ragged breathing into something controlled. That shift is the whole point — same engine, calmer breathing. Here is how to build it:
- CO₂ tables, dry, 3–4× a week. Fixed comfortable holds with shrinking rests train exactly the CO₂ signal that shows up as early air hunger on a run. Build yours from your current max with the CO₂ tables generator. For example, a relaxed maximum of 2:00 gives you a table of eight holds at about 1:00 each, with the rest shrinking from 2:00 down to roughly 0:45 by the last round.
- Apnea walks as a movement bridge. Holds while walking sit between a static table and a run — a low-intensity way to meet air hunger while your body is actually moving. Start with the apnea walk protocol.
- Nasal easy runs. Keep your recovery and base runs at a pace you can hold through the nose. If you are forced to open your mouth, you are running too hard for an easy day — which is useful pacing feedback in itself.
- Mouth open at intensity. On tempo, threshold and intervals, breathe however moves the most air. The efficiency you built at easy pace carries over; the ceiling at max effort is real and physiological.
Two weeks of consistent dry CO₂ work is enough to feel the early air hunger arrive later and quieter. That is the same direction the runner trial measured — a longer resting breath hold, +11.7 s against +1.9 s in controls, from a short structured program (Sports, 2025, PMID 39997962). That is the win — not a new VO₂max, but a pace that stops feeling like a fight.
Safety for runners
Never do breath holds while running, and never in or near water — a blackout on the move is far more dangerous than one seated. Keep all hold work dry and still: sitting or lying down. Stop any drill at the first sign of dizziness, tingling or tunnel vision. And never hyperventilate before a hold or a run to “feel fresher” — it silences the CO₂ alarm without adding oxygen and can drop you without warning.
Sources
- Recreational-runner breathwork trial (2025). Sports 13(2):31. PMID 39997962
- Mapelli, M. et al. (2025). PLOS One 20(7):e0326661. PMID 40668801
- Eser, P. et al. (2024). Frontiers in Physiology 15:1380562. PMID 39165283
Every source above links to its abstract, and each link was opened and re-read against this text during the last editorial pass. Where a study measured trained athletes rather than beginners, for example, the article says so instead of generalising.
Written by Jan Luther — keen amateur freediver, and the developer behind Apnea Trainer since 2010. About this blog explains how sources are chosen; the imprint has the contact details for corrections. All articles
Frequently asked
Does breath training increase VO₂max?
No. In a 2025 randomised trial with recreational runners, the breathwork group and the control group showed no difference in VO₂max. What did change was resting breath-hold time, which rose by 11.7 seconds in the breathwork group against 1.9 seconds in controls. Breath work moves CO₂ tolerance and breathing comfort, not the size of your aerobic engine.
Should runners breathe only through the nose?
Only on easy and base runs. At submaximal effort, nasal breathing lowers the ventilatory equivalent for CO₂, slows breathing frequency and raises end-tidal CO₂, which is exactly the calm, economical state you want on a recovery day. At peak effort the nose cannot move enough air, so forcing nasal-only breathing there caps your performance for a purely mechanical reason.
How do I train CO₂ tolerance for running?
Dry CO₂ tables, three to four times a week — fixed comfortable holds with rests that shrink each round. Add apnea walks as a bridge between a seated table and a run, and keep your easy runs at a pace you can hold through the nose. Two weeks of consistent dry work is usually enough to feel early air hunger arrive later and quieter.
Is it safe to hold your breath while running?
No. Never combine breath holds with running, and never do them in or near water. A blackout while moving is far more dangerous than one taken seated, so keep all hold work dry and still, and stop at the first sign of dizziness, tingling or tunnel vision. Never hyperventilate before a hold or a run.
Apnea Trainer is a breath-hold training app for iPhone, Apple Watch and Android. It guides you through timed breathing cycles and builds progressive tables around your personal bests. On Apple Watch the whole session runs on your wrist — live heart rate and haptic cues for every phase.
Keep reading
Apnea Walks: The Dry CO₂ Training You Can Do Anywhere
Movement makes CO₂ faster. Apnea walks turn that into the most time-efficient dry tolerance session there is — with a configurable interval timer.
CO₂ Tables Explained: The Science of Training Your Urge to Breathe
The urge to breathe is a CO₂ alarm, not an oxygen alarm. Here is how CO₂ tables train that alarm — and how to build one from your current max.