The Science of Altitude Training

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If you're travelling to higher altitude or using Altitude Training to improve performance, it's worth understanding the science of how altitude effects athletes. The key takeaways are that acclimation takes about two weeks and most people will benefit from iron supplements, ideally starting weeks or months before altitude exposure. (Iron supplements should be taken under medical supervision and iron levels checked regularly. I use Walk In Labs to check my Serum Ferritin levels.)

1 The Effects of Altitude

  • At altitude there is lower air pressure. This lower pressure means that each lung full of air has less oxygen (lower partial pressure of O2). This results in lower oxygen saturation in the blood (Hypoxia).
  • Altitude is generally considered High altitude as 1500 to 3500m (5,000' to 11,500'), Very high altitude as 3500 to 5500m (11,500' to 18,000'), and Extreme altitude as above 5500m (18,000')[1].
  • Rapid ascent from near sea level to above 2500m/8,000' can result in problems ranging from mild sickness to life-threatening Acute Mountain Sickness (AMS), but with gradual acclimation extreme altitudes can be tolerated[1].
  • One "rule of thumb" is that above 3000m/10,000', you shouldn't sleep more than 300m/1,000' higher than the previous night[1].
  • A key feature of acclimation to altitudes up to 5,000m/16,000' is an increase in breathing[2]. Other changes include an increase in heart rate, increase in blood pressure, increase in red blood cells, reduction in blood volume (increased urine output), increase in capillary density, and an increase in mitochondria and oxidative enzymes[3]. However, the increase in capillary density might be at least partly due to a reduction in muscle fiber size[4].
  • The human body adjusts to lower blood oxygen saturation in many ways, and one key adaptation that is of interest to athletes is an increase in red blood cells, but the performance improvements from Altitude Training may come from additional sources[5][6].
  • The effects of altitude are non-linear. From sea level to Leadville (10,170 ft), your blood oxygen levels may drop 6% from 96% to 90%. Going up another 4,000 ft to Pike's Peak (14,110), blood oxygen levels may drop a further 8% to 82%. Running is harder at altitude as seen by the VO2max drop. At Leadville your VO2max may drop by ~15% (range 4-30%)
  • SpO2 at altitude may be slightly misleading as the oxygen deliver to the muscles may be modified by O2 dissociation curve shifts caused by changes in pH, PCO2, and blood temperature[7]. However, SpO2 is cheap and easy to monitor and should not be ignored.
  • There is great individual variability in the response to altitude[8]. Some studies have classified subjects as 'responders' and 'non-responders' due to the significance of this variability. This variability can change over time within an individual. I met someone in Tanzania who had been a porter on Kilimanjaro (19,334 ft) until he lost his ability to cope with the altitude.
  • Some variability may be due to differences in iron intake/availability. Low blood iron (serum ferritin) may limit the body's ability to generate new red blood cells, which is part of the altitude adaptation. See below for more details on iron supplementation.
  • Generally, 'live high, train low' seems to work better than 'live high, train high'. Intermittent Hypoxic Exposure may have additional benefits over other Altitude Training Approaches.
  • Altitude acclimatization takes time, with 2 weeks being a point of diminishing returns. This is based on a study of athletes traveling to 2340m/7,766' that showed a performance decrease of 26% on arrival, they recovered by 6.0% after 7 days, another 5.7% after 14 days, but only another 1.4% after 21 days[9]. These findings seem broadly similar for those sleeping in an altitude tent (normobaric hypoxia) [10].
  • Training needs to be reduced at altitude, and this reduction can lead to detraining. 'Live high, train low' and Intermittent Hypoxic Exposure help mitigate this problem.
  • It is a myth that if you can't arrive at altitude with time to acclimate, it's best to arrive near within a day of your event. This is based on the idea that performance at altitude declines for a period before improving. However, research shows that the reduction in performance occurs immediately and improves gradually over time[11]. At moderate altitudes (1700m/5,600') performance was better after just 18 hours compared with 6 hours[12].

2 Nutrition and Altitude

There's good evidence that nutrition is important for altitude acclimation, at least as far as iron goes. For other nutrients, the evidence is a little less clear.

2.1 Iron

For adaptation to altitude, probably the most critical nutrient is Iron. Low iron stores can result in reduced adaptation to altitude[13] and altitude training will reduce the body's stores of iron[14]. It's been suggested that iron supplementation may need to be started some months prior to the needed altitude acclimation due to the time taken for iron store to accumulate[15], and six weeks may be insufficient time[8]. One study found that in a group of 178 athletes, iron stores (serum ferritin) reduced by 33% without supplementation, reduced by 14% with 105mg/day of iron and increased by 37% with 210 mg/day of iron[16]. Further, the non-supplemented athletes only increased hemoglobin mass by 1.1%, those on 105mg/day by 3.3% and those on 210 mg/day by 4.0%. The supplements were started one week before, and during, altitude exposure. The supplements were not given randomly but based on serum ferritin status. Those with levels above 100 ug/L were not supplemented, those with 30-100 ug/L were given 105 mg/day and those below 30 ug/L were given 210 mg/day. This suggests that even those with good iron status may need supplementation. However, that's a lot of iron, considering the RDA for men is only 8 mg/day and women 18 mg/day, and the tolerable upper intake is only 45 mg/day. Taking more than four times the tolerable upper intake is a little worrying, and the study made no mention of reported side effects. The study used a prolonged release supplement that included 105 mg iron with 1,000 mg Vitamin C (which can increase absorption of Iron) in a product called "Ferro Grad C."

2.2 Antioxidants

There's some indications that "live high, train low" may increase the need for antioxidants[17]. However, while some studies suggest that antioxidant supplementation might be beneficial[18][19], it seems the preponderance of evidence is that antioxidant supplements may hinder recovery and adaptation to training stress[20][21][22].

2.3 Carbohydrate

It's unclear if the macronutrient mix of carbohydrate/protein/fat should be different at altitude, as most claims of the need for a high carbohydrate diet at altitude are based on sea level studies rather than any change due to altitude[23].

2.4 Vitamin D

There's no evidence for Vitamin D supplementation at altitude specifically, though there is the suggestion[23] that Vitamin D benefits might be particularly valuable as it may act as a vasodilator[24][25]. There's also the possibility that longer term (months) exposure to extreme altitude could result in bone loss[26].

3 Hydration and Altitude

Within 90 minutes of exposure to higher altitudes, urine output increases[27][28], resulting in the loss of water and sodium[29]. The diuretic effects of low pressure at altitude may be exacerbated by the temperature, as cold conditions can result in "cold diuresis"[30][31]. One study suggests that hydration could exacerbate performance problems at altitude, but the study was short term (1 hour) and used dehydration that was not related to the altitude[32]. While there is a net loss of water as a response to altitude, it's unclear if increased fluid intake would help or hinder performance. There is an argument that this loss of fluid is an important adaptation to altitude that concentrates the blood and reduces the demand on the heart[33]. There is some evidence that increased hydration does not increase the blood volume and may exacerbate fluid retention[34]. (Fluid retention is the build up of fluid between or inside the cells rather than in the blood.) There is further risk of Hyponatremia with excessive drinking, so caution is needed, and the advice to "drink to thirst" would seem to remain valid. (Note that while people with kidney problems may be able to tolerate short durations at modest altitudes, the risks are unclear[35].)

4 Assessing Altitude Impact

The Lake Louise Scoring System (LLSS) is used to assess the severity of AMS (Acute Mountain Sickness, or altitude sickness). The 2018 version of the LLSS scores headache, Gastrointestinal symptoms, fatigue, dizziness, and functional disruption to provide an overall assessment of AMS severity[36]. (Sleep disruption was removed as it does not appear to be well corelated with AMS[37].) However, analysis has shown that a single question is just as effective as the LLSS[38]. This is the Clinical Functional Score, which asks "overall if you had any symptoms, how did they affect your activity?", with possible answers of

  1. "Not at all."
  2. "Symptoms present but did not force any change in activity or itinerary."
  3. "My symptoms forced me to stop the ascent or to go down on my own power."
  4. "I had to be evacuated to a lower altitude."

5 Effects of hypoxia

Low levels of SpO2 effect brain functioning as shown in the following table[39].

SpO2 Description Effect Notes
100-80% Mild Hypoxia Normal brain functioning This mild level of hypoxia does not affect the functioning of the brain, but some people can be sensitive enough to detect changes.
80-60% Moderate Hypoxia Decreasing brain function Vision can be altered, including tunnel vision. Coordination is impaired in things like handwriting will deteriorate. Below 80% the skin may become blue (cyanosis). Mental functioning is impaired, sometimes creating euphoria or tranquility, including indifference to everything including pain. At this level some people become fixated on whatever they were doing when the hypoxia began, which can be dangerous. Memory and speech can also be impaired. There may be older treat visual hallucinations, feelings of depersonalization and even out of body experiences.
60-40% Severe hypoxia Muscle paralysis Apparent unconsciousness.
<40% Extreme hypoxia Unconsciousness and eventually death

6 Hypoxia and Altitude

The following table[40] gives an idea of different SpO2 levels at different altitudes. Intermittent Hypoxic Exposure can increase SpO2 levels at a given altitude[41], which are specified in the table below for some altitudes. However, the actual SpO2 value at a given altitude will vary on many factors, so use this as a rough guide only.

Altitude(feet) Altitude(meters) Air Pressure(mmHg) Oxygen Pressure(mmHg)  % of sea level Oxygen Equivalent O2 partial

pressure at sea level





0 0 760 159 100 20.9 98%
5,000 1,524 639 134 84 17.6 95%
7,500 2,286 584 122 77 16.1 93%
9,000 2,740 554 116 73 15.3 90.3% (+/-3.4%) 93.8% (+/-2%)
10,000 3,048 534 112 70 14.6 89%
11,000 3,360 514 107 68 14.2 86.4 % (+/- 4.8%) 90.2% (+/-2.7%)
12,500 3,810 487 102 64 13.4 87%
14,000 4,267 460 96 61 12.7 83%
15,000 4,570 443 93 58 12.1 81.7% (+/-6%) 89.1% (+/-3%)
16,500 5,029 418 87 55 11.5 77%
18,000 5,490 395 83 52 10.9 84.9% (+/-4%)
20,000 6,096 365 76 48 10.0 65%
21,000 6,400 351 73 46 9.6 79.2% (+/-6%)
25,000 7,620 299 62 39 8.2 <60%

7 See Also

8 References

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