QUICK TAKES

1. Altitude Effects Start Lower Than You Think 

⛰️ Altitude sickness symptoms can begin at just 8,200 feet (2,500 meters) above sea level. In Swiss Alps studies, 9% of individuals experienced symptoms at 9,350 feet, rising dramatically to 53% at 15,000 feet.

2. Speed Kills (Your Mountain Experience) 

🏎️ The rate of ascent is the most critical factor determining altitude sickness risk. Studies show that when ascending to 15,000 feet, illness rates jump from 54% at slow simulated ascent (300 ft/hour) to 89% at rapid simulated ascent (5400 ft/hour). Your likelihood of getting sick increases 40% for every day you ascend 3,280 feet.

3. Gender and Body Composition Matter 

👨 👩 Women have a physiological advantage at high altitude; higher BMI predicts both immediate and long-term altitude problems. Interestingly, age and most medical conditions don't significantly impact altitude sickness risk in most studies.

4. Early Fluid Retention Predicts Severe Symptoms 

💧 People who develop severe acute mountain sickness show water retention within the first 3 hours of altitude exposure, contrasting with the expected diuresis (increased urination) seen in those who adapt well.

5. Recovery Happens Naturally (Almost always) 

🤞 Most altitude sickness symptoms resolve within 24-36 hours without descending. Only 1% progress to dangerous complications like pulmonary or cerebral edema.

33% of people who experience it once will get it again despite slow ascent.

FAVORITE FINDS

I don’t have any that relate to altitude sickness but have plenty of backpacking favorites! As usual, no affiliate links because I just haven’t set any up. However, if you are enjoying these newsletters and my overall approach to health, check out the Simple Science workbooks, handouts, mini-courses, and membership.

• My tried and true Hyperlite pack, tent, and sleeping bag.

• I use my Garmin watch to tell me how far we’ve walked, how long, how fast, and how much we’ve ascended

• And the GaiaGPS app to match this to our origin and destination

• The South Sierra and Yosemite are my favorite place to go backpacking. You need permits and preparation but when you’re addicted…

DEEP DIVE

The Physiology of Altitude Adaptation

The Challenge: How We Adapt to Altitude

At high altitude, reduced barometric pressure creates hypobaric hypoxia—less oxygen enters the body with each breath, due to lower pressure in the mountain air (Murdoch, 2010). Differences in respiratory and heart rate occur very quickly upon exposure, suggesting immediate mechanisms like chemoreflex responses are at play.

The increased respiratory rate creates “respiratory alkalosis” which the kidneys then have to correct by getting rid of excess bicarbonate, thus maintaining pH balance.

We also depend on the kidneys for a diuretic (water-losing) effect of high-altitude exposure that soon results from decreased levels of antidiuretic hormone (ADH) (Wang et al., 2022). By getting rid of some of the water between the cells, the oxygen concentration in the blood increases (Palubiski et al., 2020).

However, this adaptation process varies dramatically between individuals—up to 10-fold differences in respiratory compensation and natriuresis responses during the initial 24-48 hour acclimatization period.

When Adaptation Goes Wrong

A critical finding reveals that people who develop acute mountain sickness show water retention within the first 3 hours of altitude exposure, contrasting sharply with the expected diuresis seen in those who adapt successfully (Loeppky et al., 2005). This early fluid retention appears to be due to:

• too much antidiuretic hormone (ADH) release

• too much epinephrine and norepinephrine

The sympathetic nervous system activation prevents proper adaptation by promoting sodium and water retention—part of the body's misguided attempt to maintain blood flow and pressure. This may explain why exercise, which further activates the sympathetic system, can make altitude sickness much more likely.

As a result of not being able to concentrate the blood, people with altitude sickness have measurably lower oxygen saturation (85% vs. 91%) and higher heart rates (96 vs. 87 beats per minute) compared to those who adapt well (Ouyang et al., 2024). Organs such as the brain, heart, and kidneys then receive less oxygen than would be ideal.

Risk Factors

Studies consistently show that ascent rate is the most important risk factor, followed by previous history of altitude sickness (Poudel et al., 2025). Sufficient doses and optimal timing of acetazolamide (Diamox) provides significant protection, though it cannot prevent symptoms if ascent is too rapid.

Interestingly, demographic factors show unexpected patterns. While age and most comorbidities don't significantly impact altitude sickness risk in most studies, older people, men, and those with higher BMI do have lower initial oxygen levels (Vignati et al., 2021). Post-menopausal women may be more susceptible, though training can reduce or eliminate this effect (Richalet & Lhuissier, 2015).

Higher BMI predicts adverse effects of altitude exposure in long-term high-altitude workers, suggesting this factor has both immediate and chronic implications in this setting (Vinnikov & Krasotski, 2022).

Special Considerations for Kidney Disease

Given the pivotal role of kidneys, people with chronic renal disease face unique challenges at altitude. There is increased risk of excess fluid retention due to reduced urinary sodium excretion capability (Furuto et al., 2020).

Hydration becomes particularly challenging. Drinks high in sugar can negatively impact kidney function, especially with chronic consumption. Meanwhile, electrolyte-enhanced beverages present their own risks—excess sodium often cannot be properly processed by compromised kidneys, while excess magnesium and phosphorus intake can create imbalances.

However, plain water alone poses problems too, particularly for postmenopausal women whose hormonal changes make water absorption less efficient during physical exertion. In this complex scenario, somewhat diluted electrolyte beverages may offer the best compromise, providing necessary minerals for hydration without overwhelming already-stressed kidneys.

Treatment and Prevention Strategies

For people without chronic renal insufficiency, staying well-hydrated will help the kidneys excrete bicarbonate. Avoiding exertion when first arriving at altitude and instead focusing on relaxing activities may also help. Sleeping at lower altitude and having excursions at higher altitude during the day is recommended.

A good ascent strategy involves spending the first night no higher than 9,800 feet, then ascending no more than 1,600 feet per day thereafter. Anyone who has had altitude sickness may want to try an even more cautious strategy.

Pharmacological interventions include acetazolamide (Diamox) for prevention (works by promoting bicarbonate excretion and improving oxygenation) or dexamethasone (reduces blood vessel impairment). For symptomatic relief, acetaminophen and NSAIDs effectively manage headache symptoms.

🔑 The key insight for travelers is that one should try to prevent altitude sickness through proper ascent planning and, when indicated, prophylactic medication. However, individual physiological variation means that even with precautions, some people will experience symptoms and need to be prepared for appropriate management or even descent if severe complications develop.

References

Furuto Y, Kawamura M, Namikawa A, Takahashi H, Shibuya Y. Health risk of travel for chronic kidney disease patients. J Res Med Sci. 2020.

Loeppky JA, Icenogle MV, Maes D, Riboni K, Hinghofer-Szalkay H, Roach RC. Early fluid retention and severe acute mountain sickness. J Appl Physiol (1985). 2005.

Murdoch D. Altitude sickness. BMJ Clin Evid. 2010

Ouyang Q, Yang Y, Zou D, Peng Y, Zhang W, Yang Y, Ma Y. Incidence and risk factors of acute mountain sickness during ascent to Hoh Xil and the physiological responses before and after acclimatization. Turk J Emerg Med. 2024.

Palubiski LM, O'Halloran KD, O'Neill J. Renal Physiological Adaptation to High Altitude: A Systematic Review. Front Physiol. 2020.

Poudel S, Wagle L, Ghale M, Aryal TP, Pokharel S, Adhikari B. Risk factors associated with high altitude sickness among travelers: A case control study in Himalaya district of Nepal. PLOS Glob Public Health. 2025.

Richalet JP, Lhuissier FJ. Aging, Tolerance to High Altitude, and Cardiorespiratory Response to Hypoxia. High Alt Med Biol. 2015.

Savioli G, Ceresa IF, Gori G, Fumoso F, Gri N, Floris V, Varesi A, Martuscelli E, Marchisio S, Longhitano Y, Ricevuti G, Esposito C, Caironi G, Giardini G, Zanza C. Pathophysiology and Therapy of High-Altitude Sickness: Practical Approach in Emergency and Critical Care. J Clin Med. 2022.

Vignati C, Mapelli M, Nusca B, Bonomi A, Salvioni E, Mattavelli I, Sciomer S, Faini A, Parati G, Agostoni P. A Breathtaking Lift: Sex and Body Mass Index Differences in Cardiopulmonary Response in a Large Cohort of Unselected Subjects with Acute Exposure to High Altitude. High Alt Med Biol. 2021 Dec;22(4):379-385.

Vinnikov D, Krasotski V. Healthy worker survival effect at a high-altitude mine: prospective cohort observation. Sci Rep. 2022 Aug 16;12(1):13903.

Vinnikov D, Saktapov A, Romanova Z, Ualiyeva A, Krasotski V. Work at high altitude and non-fatal cardiovascular disease associated with unfitness to work: Prospective cohort observation. PLoS One. 2024.

Wang SY, Gao J, Zhao JH. Effects of high altitude on renal physiology and kidney diseases. Front Physiol. 2022.