You arrive in a mountain town expecting crisp air, spectacular views and epic
adventures.

Instead, you develop a headache.

Walking upstairs leaves you breathless. Sleep becomes restless and fragmented. Your
appetite disappears. One beer feels like three. Normal activities suddenly feel
exhausting.

The reason lies in how the body responds to an environment it was never fully designed
to inhabit.

It’s Not Less Oxygen. It’s Less Pressure.

One of the most common misconceptions about altitude is that there is less oxygen in
the air.

In reality, the atmosphere contains approximately 21% oxygen whether you are at sea
level or standing on a mountain summit.

The difference is atmospheric pressure.

As altitude increases, atmospheric pressure falls. Oxygen molecules become more
dispersed, reducing the pressure gradient that drives oxygen from the lungs into the
bloodstream.

The oxygen is still there. It is simply harder to access.

Every symptom of altitude can ultimately be traced back to this problem.

Why You Start Breathing Faster

Within minutes of arriving at altitude, specialised oxygen sensors in the carotid arteries
detect falling oxygen levels.

The brain responds by increasing breathing rate and depth in an attempt to bring more
oxygen into the lungs.

This response is known as the hypoxic ventilatory response and represents the body’s
first line of defence against altitude.

It helps, but not enough to completely overcome the reduced oxygen availability.

Why Your Heart Starts Racing

The next response is an increase in heart rate.

If less oxygen is available with each breath, the body partially compensates by beating
faster, delivering that oxygen more rapidly to tissues.

As the heart pumps faster, it increases blood flow to muscles and vital organs.
This is why climbing a short flight of stairs can suddenly feel disproportionately difficult.
It is not that you have become less fit. Your cardiovascular system is simply working
harder to achieve the same result.

Why Headaches Occur

The altitude headache is almost a rite of passage.

Reduced oxygen availability triggers changes in blood flow within the brain. Blood
vessels dilate in an attempt to increase oxygen delivery, while fluid shifts may
contribute to mild swelling of brain tissues.

The result is the familiar dull, persistent headache many people experience after
ascending.

For many people, this is the earliest warning sign that the body is struggling to adapt.

Caution: Most altitude-related headaches improve as acclimatisation occurs. However,
a persistent or worsening headache, particularly when accompanied by symptoms
such as nausea, dizziness or difficulty walking, may indicate altitude sickness and
should never be ignored. These are not headaches that can simply be ‘pushed through’.
The golden rule of mountain medicine is simple: never ascend with worsening
symptoms. If altitude sickness is developing, descending to a lower altitude remains
the definitive treatment.

Why Sleep Falls Apart

Altitude and sleep are notoriously poor companions.

Many people find themselves waking repeatedly during the night or feeling as though
they never reach a deep, restorative sleep.

Part of the problem lies in how breathing is controlled.

At sea level, breathing is driven primarily by carbon dioxide levels. At altitude, falling
oxygen levels begin exerting a much stronger influence. This can create an unstable
breathing pattern known as periodic breathing, where breathing alternates between
rapid breaths and brief pauses.

Your body is effectively trying to learn a new breathing strategy while you sleep, and in
doing so, makes you feel less rested.

Why Your Appetite Disappears

Many people notice they feel less hungry at altitude.

Low oxygen levels appear to alter hormones involved in appetite regulation, including
ghrelin and leptin. Blood flow may also be redistributed away from the digestive system
as the body prioritises oxygen delivery elsewhere.

The result is appetite suppression at precisely the time the body requires more fuel.
This helps explain why prolonged stays at altitude are often associated with weight loss.

Why Alcohol Hits Harder

Travellers will often discover this lesson for themselves.

Altitude does not necessarily increase blood alcohol concentration, but alcohol’s
effects often feel amplified.

Fatigue, dehydration, disrupted sleep and reduced oxygen availability combine to make
alcohol feel more potent than it would at sea level.

A hangover at altitude can feel similar to altitude sickness itself.

Why Exercise Suddenly Feels Impossible

Exercise performance falls at altitude because oxygen delivery becomes the limiting
factor.

Muscles rely on oxygen to generate energy efficiently. When oxygen availability falls,
aerobic performance declines.

VO₂ max, the body’s maximum capacity to utilise oxygen during exercise, decreases
progressively with increasing altitude. Even highly trained athletes experience
substantial reductions in performance.

The mountain simply doesn’t care how fit you are.

Fitness provides a larger physiological reserve, but it does not eliminate the effects of
reduced oxygen availability. A highly trained athlete may perform better at altitude may
the average person, but they are no less susceptible to altitude sickness.

Why Do Some People Seem Fine While Others Suffer?

Altitude affects people very differently, and fitness is not a reliable predictor of who will
develop altitude sickness. Elite athletes can become unwell, while less athletic
individuals may naturally have better adaptation.

The reason lies largely in genetics and individual physiology. Some people mount a
stronger breathing response when oxygen levels fall, helping maintain higher blood
oxygen levels. Others don’t respond so well and are more susceptible to symptoms.
Previous altitude illness is one of the strongest predictors of future altitude illness.

Altitude Sickness

Some breathlessness and fatigue are normal at altitude.

Altitude sickness, also known as Acute Mountain Sickness (AMS), is different.
Symptoms typically develop between six and twenty-four hours after ascent and may
include:

• Headache
• Nausea
• Loss of appetite
• Fatigue
• Dizziness
• Disturbed sleep

Most cases improve with rest and acclimatisation.

In some cases, untreated altitude sickness can progress to more serious forms of
altitude illness. Fluid can accumulate in the lungs, causing severe breathlessness,
coughing, exhaustion and, ultimately, respiratory failure. Swelling of the brain can lead
to confusion, loss of coordination, unusual behaviour, reduced consciousness and
coma. Both conditions are life-threatening and can be fatal. Immediate descent and
urgent medical attention are essential.

How the Body Adapts

Everything so far sounds rather discouraging.

Yet perhaps the most remarkable lesson is not how vulnerable we are, but how
adaptable we can be.

Your Breathing Control System Resets

One of the most important altitude adaptations happens in the kidneys.

When oxygen levels fall, the body responds by breathing faster. This helps increase
oxygen uptake, but it also causes more carbon dioxide (CO₂) to be exhaled.
This matters because CO₂ is not simply a waste gas. When dissolved in blood, it forms
carbonic acid and contributes to the body’s acid-base balance.

As breathing increases and CO₂ levels fall, the blood becomes more alkaline. This is
known as respiratory alkalosis.

The problem is that the brain does not like large changes in blood pH.

So while low oxygen levels are telling the body to breathe faster, the developing
alkalosis is simultaneously encouraging it to slow down.

One system is pressing the accelerator.

The other is pressing the brake.

The body finds itself caught in a physiological tug-of-war.

Fortunately, the kidneys step in to help.

Over several days they begin excreting bicarbonate into the urine. Bicarbonate is
alkaline, so losing it gradually lowers blood pH back towards normal.

As blood pH normalises, the physiological brake on breathing is removed.

The body can now maintain a higher breathing rate without causing excessive alkalinity.
More air moves through the lungs, oxygen levels improve and symptoms often begin to
ease.

This adaptation is one of the key reasons people generally feel better after several days
at altitude.

Your Red Blood Cell Factory Switches On

Within hours of altitude exposure, the kidneys begin releasing erythropoietin (EPO).
EPO signals the bone marrow to produce additional red blood cells.

Over days and weeks, haemoglobin concentration increases, allowing the blood to
carry more oxygen with every heartbeat.

This adaptation takes time, which is why a weekend trip to altitude is unlikely to result in
meaningful increases in red blood cell mass.

Can You Prepare for Altitude?

Many people assume there must be a way to train for altitude before they go.
The answer is partly.

Good cardiovascular fitness improves exercise capacity and provides a larger
physiological reserve. Inspiratory muscle training may strengthen the muscles involved
in breathing and reduce perceived breathlessness in some individuals.
However, neither reproduces true acclimatisation.

Similarly, commercial “altitude masks” do not simulate altitude. They restrict airflow,
making breathing harder, but they do not reduce oxygen concentration in the way
genuine altitude does.

The adaptations that matter most still require exposure to a low-oxygen environment.
Specialised altitude tents and hypoxic chambers can stimulate some of these changes,
but for most people there is no complete substitute for spending time at altitude itself.

Can Medication Help?

Acetazolamide (Diamox) is the most commonly used medication for preventing altitude
sickness.

Rather than masking symptoms, it works by accelerating the body’s natural
acclimatisation processes. The drug increases bicarbonate excretion by the kidneys,
helping counteract the respiratory alkalosis caused by hyperventilation.

In effect, it helps remove the physiological brake on breathing more quickly.

Acetazolamide is generally well tolerated. Common side effects include tingling of the
fingers, toes or face, altered taste sensation and increased urination – this last one is
often the side effect people remember most.

It should not be taken without medical advice. Caution may be required in people with
significant kidney disease, liver disease, electrolyte abnormalities or certain medication
interactions.

Acetazolamide is most effective when started before ascent rather than after symptoms
have already developed.

Adapting to Thin Air

Altitude has a remarkable ability to make healthy people feel incredibly unwell.

Breathlessness, headaches, poor sleep, reduced appetite and fatigue are all signs that
the body is struggling with reduced oxygen availability.

Yet these symptoms also reveal something extraordinary.

The body responds through changes in breathing, blood chemistry, red blood cell
production and cellular oxygen utilisation. Given sufficient time, these adaptations
allow humans to function in environments that initially seem hostile and exhausting.

The body can adapt amazingly well, but it cannot be rushed. At altitude, patience is
often the most powerful piece of equipment you can carry. The mountains reward those
who give their physiology time to catch up.

Information and other content provided in these blogs should not be construed as medical advice and should not be considered a substitute for professional medical expertise. If you have any medical concerns, you should consult with your health care provider.