If you’re reading this, there’s a good chance you’re on the edge of a new frontier.
Maybe it’s your first climb well above 5,000 meters. Maybe it’s Kilimanjaro, Aconcagua, or something you’ve been quietly thinking about for years.
Maybe you’ve climbed excessively already, but are looking for a new frontier in acclimatization optimization (especially how to PRE-acclimatize if you’re stuck at sea level and have limited time on the mountain).
So let’s dive into this highly-debated, battle-worn, oft-asked question:
“What should I do before my expedition to prepare my body for altitude?”
There it is, folks—the MILLION dollar question. And I’m not going to act like I have all the answers. But I do have one thing…TIME INVESTED in search of answers. So, I hope that my investment of time will save you your own.
Here’s my to-the-point attempt to lay out my findings clearly, using what the research actually supports — and being transparent about where the science is still incomplete in answering the “Pre-Acclimatization” question.
Hope you enjoy it.
What I mean by “pre-acclimatization.”
We need to start here. Pre-acclimatization is any structured exposure or training done before your trip that aims to reduce the physiological shock of altitude once you arrive. It does not replace staged ascent, conservative pacing, or good guiding. Nothing does.
What it can do is improve your starting position.
I often explain it this way:
You can’t show up to basecamp fully acclimatized — just make sure your body is less surprised.
Why does altitude stress the body in the first place
One more bit of groundwork to cover here: what altitude actually does to your body.
As you ascend, barometric pressure drops. The percentage of oxygen in the air stays the same (about 21%), but the molecules spread farther apart. That means each breath delivers less oxygen into your lungs → and from there, everywhere else. To compensate, your body has to:
- Work harder to move air in and out of the lungs (that lower-pressure air doesn’t move easily into your lungs)
- Breathe more
- Increase cardiovascular output
- Adapt how oxygen is transported and used
If that process outpaces your ability to adapt (hence why pacing your ascent is crucial), symptoms show up — headaches, fatigue, poor sleep, nausea, and in more serious cases, fluid shifts in your lungs or brain.
Pre-acclimatization is about nudging some of those systems in the right direction before you ever step on the mountain.
Great. Now we can talk about some of the methods, the research behind it, and what a pre-acclimatization plan might look like for you.
1. Hypoxic exposure: the backbone of pre-acclimatization
What I’m referring to
Hypoxic exposure means intentionally breathing air with less oxygen than normal — simulated altitude — while still living at sea level. This can look like:
- Short bouts – intermittent – of hypoxia exposure at rest / sleep
- Intermittent hypoxic training (hypoxia during exercise)
- Intermittent hypoxic–hypercapnic training (done primarily with strong breath-holds, where CO₂ is also elevated)
These methods are often grouped under terms like IHE, IHT, or IHHT.
What the research actually shows
The most balanced reviews of hypoxic conditioning agree on a few key points:
- Repeated hypoxic exposure can stimulate ventilatory adaptation, meaning your body becomes better at increasing breathing appropriately when oxygen drops
- With sufficient cumulative exposure, it can stimulate hematological changes (via erythropoietin EPO), though results vary widely
- The effects are dose-dependent — in reality, short, inconsistent exposure does very little
One comprehensive review by Burtscher et al. makes this clear: hypoxia conditioning can be useful, but only when it’s structured, progressive, and realistic about its limitations. The review says while short periods (7 hours) of pre-acclimatization by hypoxic conditioning may show improvements, longer periods (> 60 hours) are needed to elicit more significant effects. About 300 hour of HC (intermittently applied) may be the optimal preparation for very high altitude. Plus, it warns NOT to stop the exposure less than 1-2 weeks before heading out (or the reversal process begins).
About hemoglobin and “blood changes.”
This is the part people are most excited about — and most likely to misunderstand.
Yes, hypoxia can stimulate erythropoietin and red blood cell production. But the magnitude of change depends on how much hypoxia you actually accumulate.
There are studies showing meaningful increases. One often-cited example involved elite swimmers who performed intermittent hypoxic–hypercapnic training and saw roughly a 5.35% increase in hemoglobin concentration, resulting in a ~10.79% increase in VO₂max over the training period.
That’s real — but it’s also context-specific. Not everyone responds the same way, and short protocols won’t recreate what weeks at altitude do. (Quick plug: for you, oh serious trainer, yes, our breathwork training includes intermittent hypoxic-hypercapnic training. And it’s not joke).
My takeaway: hypoxic exposure – whether simulated through a tent/chamber or even breath-holds can shift the needle, but it’s not a guaranteed hematological shortcut. There are also studies that show passive breath-holds can increase your EPO, sure, but don’t necessarily result in hemoglobin mass increases. So, the research is murky.
In conclusion: think of this as support, not replacement, and don’t blindly expect your RBCs (red blood cells) to run wild from intermittent hypoxia.
2. Ventilation and respiratory muscle work — the underrated piece
One thing that often gets missed in altitude discussions (and I’m determined to change that) is this:
As air pressure drops, breathing itself becomes harder work.
The diaphragm and respiratory muscles have to generate greater pressure to move air into the lungs. If those muscles fatigue, ventilation drops — and oxygen saturation follows.
What the research says
Respiratory muscle training (especially inspiratory muscle training, or IMT) has shown some of the most consistent benefits relevant to altitude:
- Improved respiratory muscle strength and endurance
- Reduced sensation of breathlessness during exertion
- Higher oxygen saturation at altitude compared to untrained controls
One study by Dr. Mitch Lomax showed that individuals who completed IMT maintained higher SpO₂ at altitudes above ~4,800 meters compared to those who didn’t train their respiratory muscles.
That’s a meaningful finding, especially for climbers who already have well-conditioned legs and good general fitness but struggle with breathing fatigue. Note, this doesn’t “acclimate” you in the classical sense — but it makes the work of breathing more sustainable when the air thins.
Much like training your legs to hike uphill more, if you’re going to need to breathe more – which you will at altitude – why not, you know, TRAIN your breathing endurance?
3. Real altitude and simulated altitude environments
If you have the option to spend extended time at moderate altitude before your expedition, that remains one of the most reliable ways to stimulate acclimatization-related adaptations.
Living or sleeping at altitude for weeks can:
- Increase EPO and red blood cell mass
- Improve ventilatory response
- Reduce the severity of initial altitude exposure
Simulated altitude (tents, rooms, chambers) attempts to recreate this effect at sea level. These tools can work — but only if exposure time is substantial and consistent, as I mentioned earlier.
A few exposures here and there won’t do much. Like most physiological adaptation, it’s about cumulative dose.
Where the research gets fuzzy (and why that matters)
This is where I want to be very clear with you.
There is no single, standardized, evidence-based pre-acclimatization protocol that works for everyone.
The literature is messy because:
- Protocols vary widely
- Individuals respond differently
- Many studies measure physiology, not real-world outcomes like AMS incidence
That doesn’t mean pre-acclimatization is useless. It means it should be used carefully and conservatively, as part of a bigger preparation picture.
A word on cross-adaptation (heat and cold)
This is an “interesting but secondary” consideration that I would be remiss not to mention….
Heat exposure
Repeated heat exposure can expand plasma volume, improve cardiovascular stability, and upregulate protective cellular stress responses. Some interesting research suggests that these adaptations may partially improve tolerance to hypoxic stress during exercise.
- Note, for the science nerds, the reason this is intriguing is, in part, because research shows heat acclimation does two things:
- Increase plasma volume by ~5–15%. The increased plasma volume improves: stroke volume, oxygen delivery, and cerebral perfusion. And extended heat exposure has been shown to increase hemoglobin mass. Cheers to that.
- Increase heat shock proteins – which are interesting for altitude because they’re one of the few cellular adaptations triggered by heat exposure that overlap with hypoxic stress—potentially improving vascular stability and cellular resilience when oxygen drops.
True to my opening statement, heat acclimation does not replace your altitude acclimatization, but it can prep your body to be less SHOCKED (see what I did there?) by it.
Cold exposure
Cold exposure has far less direct evidence related to altitude readiness, but its benefits appear to be embedded in blunting the sympathetic (hyperventilation/over-breathing) response when arriving at altitude.
My take: heat may be a useful supportive tool; cold is more about nervous system resilience and recovery (oh, and exposure to the cold… because it’s cold on mountains)
What an effective, safety-focused pre-acclimatization plan actually looks like
If I were advising a climber preparing for a major altitude objective, I’d frame pre-acclimatization like this:
- Primary focus: Structured hypoxic exposure (if available), done progressively and conservatively
- Foundational support: Respiratory muscle training to reduce breathing fatigue and support oxygen saturation
- Optional layers: Heat exposure for cardiovascular robustness. Cold exposure and “breathwork” in a classic sense (not quite how we train you at Recal) as a way to improve ventilatory control and awareness.
- Non-negotiable: Proper staged ascent, rest days, hydration, nutrition, and listening to symptoms
The bottom-line: pre-acclimatization should reduce uncertainty, not encourage risk-taking.
Final thoughts
I’m a big believer in preparation — but not in shortcuts.
The research supports the idea that you can meaningfully prepare your body at sea level for altitude stress. It also makes it clear that no single method replaces time on the mountain. Used correctly, pre-acclimatization helps you arrive more resilient, more confident, and better able to adapt. Used poorly, it creates false confidence.
My goal with Recal has always been to help people stack the odds in their favor — thoughtfully, intelligently, and safely. You can find out your baseline here: The Recal Breath Assessment
Cheers and good luck in your own pre-acclimatization process,
Coach Anthony
References & Further Reading (Key Studies)
- Burtscher et al., Hypoxia Conditioning for High-Altitude Pre-acclimatization: A balanced review of simulated hypoxia protocols and effects.
- Xie et al., Intermittent hypoxia + exercise improves hypoxic tolerance.
- Dajana Zoretić et al., The effects of hypercapnic-hypoxic training program on hemoglobin concentration and maximum oxygen uptake. Hypercapnic-hypoxic training in elite swimmers linked to ~5.35% hemoglobin increase and ~10.79% VO₂max increase.
- Lomax (2010), IMT attenuates oxygen desaturation at ≥4,880 m altitude.
- Gibson et al., Cross-adaptation: heat adaptation can produce physiological benefits that carry into hypoxic environments.
- Meta-analyses on cross-adaptation suggest overlapping mechanisms, though evidence is emerging.