Mountain Air: How a Stay at Altitude Can Affect Blood Formation, Metabolism and Sleep

Maybe you know this moment. You're not ill, you're not out of shape. And still, your body feels strange to you during the first days in the mountains.

May I offer you a different perspective? Being out of breath on the stairs is not a failure. It is a response. Your body has noticed that its surroundings have changed, and it is adjusting.

Around 400 million people live permanently above 1,500 meters. Millions of others head to the mountains every year, to hike, to ski, to breathe freely. Mountain air is considered healthy. But what happens in the body, and how much of it is backed by evidence?

High-altitude medicine has gathered a great deal of knowledge on this, especially about risks and altitude sickness. That is important and remains the foundation. What an integrative view can add is a look at the stimulus itself: What does a dosed lack of oxygen set in motion in your cells?

Thin air: What changes up high and how your breathing responds

Remember the stairs from the scene? At home, you take them without thinking. At 1,800 meters, they suddenly turn into a small mountain hike.

The reason isn't in your legs. It's in the air, though not in the way many people think.

Key point: Up high, the air does not lack a share of oxygen, it lacks pressure. According to a statement by European and international professional societies, the partial pressure of oxygen in inhaled air at 2,500 meters is about 25 percent lower than at sea level.

Mountain air contains the same proportion of oxygen as the air by the sea. What drops is the air pressure. Imagine a suitcase packed with the same mix of things, only more loosely. Each breath holds less oxygen, and it pushes into your blood with less force.

Your body notices this right away. In your neck, where the carotid artery divides, sit tiny oxygen sensors: the carotid bodies. In a review, Teppema and Dahan of Leiden University describe them as the place where the breathing response to low oxygen begins. From there, a signal goes to the respiratory centers in the brainstem: more air.

At the same time, your heart beats faster. In their review, Bärtsch and Saltin describe how breathing and heart rate rise on acute exposure to altitude. Many of these adaptations begin at around 1,000 meters and become clear above 2,000 meters.

In the first 24 to 48 hours, plasma volume also falls, meaning the liquid part of the blood. As a result, each milliliter of blood can carry a little more oxygen. In an Austrian field study at about 1,700 meters, body water had measurably decreased after the first night. Some people notice this because they need to use the bathroom more often during the first days.

This shift is not a flaw in the system. The statement by the professional societies led by Parati summarizes how the body secures its supply: more breathing, a higher cardiac output via heart rate, altered vascular tone and more hemoglobin in the blood.

Give yourself time during the first two days. Take stairs and climbs more slowly than at home, drink regularly and don't plan a summit hike for the day you arrive. Bärtsch and Saltin consider it possible, even for altitudes of 2,500 to 3,000 meters, to arrive, wait two days and then slowly start training. If you're interested in how air pressure and well-being interact, also read my article on weather sensitivity and barometric pressure.

And now you know why the same stairs leave you more out of breath at 1,800 meters.

The oxygen sensor in your cells: How HIF translates the stimulus

The sensors in your neck are only the beginning. How do your kidneys, your muscles, your blood vessels know that less oxygen is arriving up there?

The answer is one of the most elegant systems in biology. Kaelin, Ratcliffe and Semenza received the 2019 Nobel Prize in Physiology or Medicine for its discovery.

Key point: To your cells, a lack of oxygen is information. Via the HIF signaling pathway, genes for blood formation, blood vessels and energy metabolism can be switched on. Whether this leads to benefit or harm depends on the dose.

HIF stands for hypoxia-inducible factor. In a review, Semenza of Johns Hopkins University describes these factors as master switches of oxygen balance in all animal life.

The principle works like a messenger who keeps getting intercepted. As long as there is enough oxygen, enzymes called prolyl hydroxylases tag the HIF messenger, and the cell breaks it down. These enzymes themselves need oxygen. That is why Kaelin and Ratcliffe describe them as ideal oxygen sensors.

When oxygen runs short, the enzymes lack their tool. The messenger gets through, reaches the cell nucleus and rings the doorbells of certain genes. According to the review by Kaelin and Ratcliffe, HIF influences signaling pathways for development, metabolism, inflammation and the interplay of entire organs.

The mitochondria themselves adapt as well. Jacobs, Lundby and colleagues at the Zurich Center for Integrative Human Physiology took muscle samples from eight lowlanders before and after 28 days at 3,454 meters. The maximum respiratory capacity of the mitochondria decreased, while their efficiency increased. That is a high altitude and a small group. But it points in a direction: less capacity held in reserve, more economy in everyday operation.

The dose makes the stimulus

Now comes what may be the most important idea in this article. A lack of oxygen is not automatically good or bad. It depends on how strong it is, how long it lasts and how often it occurs.

Navarrete-Opazo and Mitchell of the University of Wisconsin-Madison reviewed the research on repeated, brief oxygen deprivation, known as intermittent hypoxia. Mild hypoxia with 9 to 16 percent oxygen in the inhaled air and 3 to 15 episodes per day mostly led to beneficial effects without harm. Severe hypoxia with 2 to 8 percent oxygen and 48 to 2,400 episodes per day was associated with increasingly pathological changes.

The second pattern is reminiscent of the many nightly breathing pauses in untreated sleep apnea. The first is more like well-dosed training. This distinction is at the heart of the concept of hormesis: A small stress stimulus can strengthen, a large one can harm. You can find more on this in my articles on cold and hormesis and on heat hormesis.

So treat altitude like any training stimulus and increase it gradually. If you're traveling above 2,000 meters, Bärtsch and Saltin recommend a staged ascent averaging about 300 meters of altitude per day. The most important reason: so that you sleep well and feel good.

And now you know why a lack of oxygen can be a signal in a small dose and a burden in a large one.

Blood formation: What EPO sets in motion within hours and what takes weeks

Maybe you've heard that a mountain vacation brings “new blood.” Athletes go to altitude training camps to build more red blood cells. Can you expect the same from a hiking vacation?

The honest answer has two parts. The first part comes quickly. The second takes patience.

Key point: The blood-forming hormone EPO can rise after just one to two hours at altitude. But more red blood cells take time: Below 3,000 meters, a meta-analysis showed no increase in their volume within four weeks.

Does the body make more red blood cells during a mountain vacation?

EPO is short for erythropoietin. It is a hormone made mainly in the kidney that tells the bone marrow: Make more red blood cells. Its gene is one of the best-known targets of HIF.

The Berlin physiologists led by Gunga found a similar picture in the same study series with 22 men. EPO was elevated from day 1 to day 19, and reticulocytes from day 4 onward.

Iron: the silent bottleneck

Your bone marrow needs iron for every new red blood cell. Your body seems to know this. At altitude, it opens up its iron stores.

In the HIGHCARE project, Piperno, Parati and colleagues from Monza and Milan studied 47 healthy volunteers at sea level and at 3,400 to 5,400 meters. The hormone hepcidin, which slows the release of iron, fell within 40 hours. At 5,400 meters, it was 80 percent lower. EPO rose within just 16 hours, and the iron storage protein ferritin fell very quickly.

What happens when the stores are small? Govus and colleagues from Australia analyzed data from 178 athletes who spent two to four weeks at altitudes of 1,350 to 3,000 meters. Without an iron supplement, hemoglobin mass rose by only 1.1 percent on average, and ferritin fell by 33.2 percent. With iron, hemoglobin mass rose more, and the stores were more likely to be preserved.

These are competitive athletes, not vacationers. Still, the direction is revealing: Empty iron stores could slow the response to altitude. But don't take iron on your own initiative. A blood test with ferritin shows whether you have a deficiency, and the decision belongs in a conversation with your doctor. You can find more on this topic in my article on fasting and iron deficiency.

If you tend toward iron deficiency, do a lot of sport or are planning a longer stay at altitude, it can make sense to check your ferritin beforehand. And don't expect a week of mountain vacation to give you a new volume of blood. Instead, enjoy what comes faster: the adjustment of your breathing and circulation.

And now you know why the blood signal arrives within hours, but new blood takes weeks.

Metabolism: Appetite, weight and blood sugar at altitude

You come back from your mountain vacation, and your belt sits one notch tighter. And that's even though you ate well, Kaiserschmarrn included. Is it the air, the hiking or both?

This question is not so easy to answer. Because when you go to the mountains, you usually change more than just the altitude.

Key point: In a randomized trial, men with metabolic syndrome on the same hiking program lost the same amount of weight at 1,700 and at 200 meters, an average of 3.13 kilograms. The benefit could be attributed more to the exercise than to the altitude.

This result matters to me because it is honest. It takes nothing away from altitude. But it puts it in its place: as a possible amplifier, not a shortcut.

The Berlin group led by Gunga also saw lower body weight after three weeks at about 1,700 meters. This was mainly due to less body fat, while fat-free mass stayed the same.

And blood sugar? Woolcott, Bergman and colleagues at Cedars-Sinai Medical Center in Los Angeles summarize that people living at higher elevations have lower fasting blood sugar and better glucose tolerance than people near sea level. Why this is so remains unclear. The lack of oxygen itself is an obvious candidate, but this has not been proven in humans.

Use the mountains for what they do well: daily movement in a landscape that draws you outside. If your appetite is lower during the first days, still eat regularly and drink enough. You can read how to use everyday movement for your blood sugar in my article on moving after meals. And I explain why being outdoors can increase your enjoyment of exercise in the article Green Exercise: moving outdoors.

And now you know why your pants can fit more loosely after a mountain vacation, and why that may be mostly down to your legs.

Mountain residents: What observational studies show, and what they don't

Have you ever watched older people in a mountain village walk steep paths as if it were nothing? Many people then wonder: Is it the air that keeps people here so fit?

Researchers have studied this question in millions of people. The results are fascinating. And they call for careful reading.

Key point: In Switzerland, mortality from coronary heart disease fell by 22 percent per 1,000 meters of residential altitude. This is an association from observational data, not proof that altitude is the cause.

Seven years later, the same research group, together with colleagues from Bern and Basel, examined whether sun, temperature, precipitation, slope or proximity to major roads explain the effect. Among 4.2 million people, the risk of dying from ischemic heart disease was one third lower above 1,500 meters than below 600 meters. The hazard ratio was 0.67. After accounting for all these environmental factors, a value of 0.74 remained.

Do people in the mountains live longer?

In the US, a team led by Ezzati of Imperial College London took a closer look. In counties above 1,500 meters, men lived 1.2 to 3.6 years longer before adjustment than in counties near sea level, and women 0.5 to 2.5 years longer. After accounting for socioeconomic factors, migration, solar radiation and smoking, this advantage largely disappeared. Mortality from ischemic heart disease remained lower at altitude, while mortality from COPD was higher.

The authors conclude that the two effects might at least partly cancel each other out. On balance, altitude showed no net effect on life expectancy.

Similar patterns appear in metabolism. In a US cross-sectional study of 285,196 adults, the odds of diabetes between 1,500 and 3,500 meters were 12 percent lower than below 500 meters. The association was clear in men, but not in women.

In the Spanish SUN cohort of the University of Navarra, with 9,365 university graduates, people in cities above 456 meters had a 14 percent lower risk of developing overweight over ten years than people below 124 meters. Notably, these were rather low altitudes.

In a review, Burtscher of the University of Innsbruck summarizes the data like this: Living at higher elevations is associated with lower mortality from cardiovascular disease, stroke and certain types of cancer. Moderate altitudes might be more favorable than very high ones. And while altitude could protect against the development of some diseases, it could be unfavorable once a disease is already advanced.

What you can take home with you to the lowlands: inclines, stairs and getting around on foot. You can read how strongly endurance can be linked to your life expectancy in my article on VO2max and life expectancy.

And now you know why the statistics on mountain residents are fascinating, but no reason to move.

The first nights: Why sleep at altitude can be restless

Three in the morning. You're lying awake even though you're tired from hiking. Sometimes you notice your breathing stall for a moment, followed by a few deep breaths. Maybe you wonder whether something is wrong with you.

You're not alone in this. Many lowlanders sleep worse during their first nights at altitude, and this has been well studied.

Key point: Even at 1,630 meters, the number of breathing pauses and episodes of shallow breathing in healthy young men rose from 4.6 to 7.0 per hour on the first night. On the second night, it was 5.4.

Why do I sleep badly on my first night in the mountains?

Where does this swinging come from? A common physiological model goes like this: At altitude, you breathe harder and exhale more carbon dioxide in the process. But carbon dioxide is itself an important trigger for breathing. If it drops below a threshold during sleep, the drive to breathe can pause briefly. Then oxygen falls, the sensors in your neck sound the alarm, and a few deep breaths follow. Experts call this pattern periodic breathing.

Imagine a thermostat that is set too sensitively. It heats, it gets too warm, it switches off, it gets too cold. In the same way, your breathing regulator can swing back and forth during the first nights until it finds its new center.

In a review, Bloch and colleagues from Zurich summarize: Lowlanders frequently report poor sleep during their first nights at altitude. The most consistent finding is a decrease in deep sleep. Whether restless sleep triggers periodic breathing or the other way around is still unclear.

In Sestriere at 2,035 meters, too, breathing disturbances increased in healthy people. Central breathing pauses and episodes of shallow breathing in particular became more frequent, most markedly in men over 40.

If you have the choice, don't plan your first nights too high up. Let the day of arrival wind down calmly and don't expect perfect sleep. A calm breathing exercise before falling asleep can help you unwind, for example 4-7-8 breathing. I explain how breathing and the nervous system are connected in my article on breathing techniques and the vagus nerve.

And now you know why your first night in the mountains is often the most restless.

Mountain vacation or altitude chamber: What the trends promise

An altitude tent in the bedroom, hypoxic training at the gym, classes in an altitude chamber: Oxygen deprivation has become a product. The promise sounds tempting. The effect of the mountains, without the journey.

Most of these offerings work with normobaric hypoxia. The air pressure stays normal, but the proportion of oxygen in the room air is lowered. In the mountains, it's the other way around: The proportion stays the same, the pressure falls. And in the mountains, a lot else comes into play as well.

Key point: In a double-blind, placebo-controlled study, four weeks of 16 hours a day in low-oxygen room air changed neither the hemoglobin mass nor the endurance performance of cyclists.

And for weight loss? Ramos-Campo and colleagues from Murcia analyzed 13 randomized trials involving 336 people with overweight or obesity. Training in low-oxygen conditions reduced body weight, fat mass, waist circumference, LDL cholesterol and blood pressure. But these effects were no greater than with the same training in normal air. For triglycerides and muscle gain, there were only trends in favor of hypoxia, without statistical certainty.

If a hypoxia offering tempts you, ask about the dose, the supervision and the studies. And ask yourself whether you could reach the same goal with exercise outdoors. For the foundations of your endurance, take a look at my article on Zone 2 training.

And now you know why an altitude tent is no substitute for the mountains.

The four KPNI lenses on a day in the mountains

Imagine a day of hiking at 1,800 meters. Cool air in the morning, sun on the ridge at midday, tired legs in the evening. What happens inside you, beyond pulse and elevation gain?

In Clinical Psychoneuroimmunology, KPNI for short, we look at how the nervous system, hormonal system, immune system and metabolism work together. Altitude speaks to all four.

And now you know why altitude speaks to more in you than just your lungs.

How solid the evidence is

Not everything in this article is equally well supported. To help you put the statements in context, here is an overview of the levels.

But first, three weaknesses that many studies share. One: They are small. Two: Often only men were studied. Three: Much of the data comes from higher altitudes than a typical vacation destination.

Who should plan carefully

For many, a mountain vacation is a gift. For some, it is a challenge that needs good preparation. This applies above all to people whose oxygen supply is already tighter in the lowlands.

Key point: In people with moderate to severe COPD, nighttime oxygen saturation fell from 92 to 89 percent at just 1,650 meters, and to 85 percent at 2,590 meters.

It is similar with obstructive sleep apnea. In a study by the same Zurich group in the journal JAMA, 51 people with sleep apnea who used a breathing mask at home traveled to Swiss mountain villages. With automatically adjusted mask therapy plus placebo, the number of breathing disturbances was 10.7 per hour at 1,630 meters and 19.3 per hour at 2,590 meters. At 2,590 meters, they spent a median of 57 percent of the night with a saturation below 90 percent. In the study, an additional drug treatment improved both values. Whether this is an option for you is something to clarify with your doctor beforehand.

Altitude sickness can affect anyone. In the Colorado study, however, some groups were affected more often: people who live near sea level, younger people under 60, women, people with little training, people with overweight or lung disease, and anyone who had had altitude sickness before. More than half of those affected cut back on their activities. Severe forms are rare at moderate altitude: According to Bärtsch and Saltin, pulmonary edema rarely occurs below 3,000 meters, and cerebral edema does not occur below 4,000 meters.

And now you know why good preparation doesn't make the mountains smaller, but safer.

Three levers for your next stay in the mountains

Maybe you're now wondering: What do I do with all this on my next vacation? Three directions are enough to start with.

And now you know why the best mountain vacation is not the highest one, but the one with the right dose.

Frequently asked questions about mountain air and altitude stays

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