Get hot to go high » BE THE EGG CYCLE COACHING

In 2024, I worked with coaching client Claire Stevens to a successful Silk Road Mountain Race finish (in 12D14H02M). The SRMR is an ~1900km with 30,000m elevation unsupported, ultracycling race through the stunningly beautiful, rugged mountain landscapes of Kyrgyzstan. The route is challenging with technical terrain, sparse resupply, and extreme environmental conditions.

In the words of organiser Nelson Trees: “The Silk Road Mountain Race is a serious undertaking that should not be underestimated. It takes place in an environment that can be as tough as it is beautiful. It requires not only the ability to ride big days back-to-back, but also knowledge and experience of self-reliance in a [remote] high-altitude, mountainous environment… Riders need to understand how to look after themselves in a harsh environment, when they’re exhausted and near their limits”.

The SRMR is tough – but so is Claire! Claire ticked many of the boxes for SRMR success. She is a seasoned ultra racer with oodles of endurance, strength, resilience and off-road riding skill and more experience of how to survive in remote places and challenging conditions than most. However, Claire’s big challenge was how to prepare for both the altitude and hot temperatures of SRMR. She lives at 21m above sea level and came into the race from Australian winter. Like most amateurs, she also lacked time and money for an altitude training camp or altitude tent.

So why did I suggest to Claire to ‘Get hot to go high’ ? Continue reading why understanding the physiological demands of ultra-endurance performance in extreme environments is key for challenges like the Silk Road Mountain Race.

How high is the Silk Road Mountain Race?

Below is the 2024 SRMR route Claire did as well as the 2025 SRMR route another of my coaching clients is lining up for (clink on the captions to explore the routes in more detail in RidewithGPS). As you can see from the elevation profiles, the SRMR reaches altitudes of up to 3,800m on several occasions during the race, with a peak altitude of 4,024m in 2025. What is more, the racers are above 3000m for extended periods of time often without a quick option to descend for sleep at lower altitude.

The physiological challenge of hypoxia during SRMR

Altitude poses a dual challenge of hypoxia and temperature (and humidity) changes. For this blog, the main focus is on hypoxia and the physiological challenge it poses.

In terms of categories of altitude – as defined by Bärtsch & Saltin (2008) based on the effects of relative altitude on performance and well-being) – much of the SRMR takes place in moderate altitude (2,000-3,000m) and high altitude (3,000-5,500m).

The higher up you go, the lower the barometric pressure. At sea level, barometric pressure is 760 mmHg, but at the highest peak during the SRMR (4,0240m) barometric pressure is only ~460 mmHg. Although the fraction of oxygen in the air remains constant (at ~20.9%), the partial pressure of inspired oxygen decreases in line with the drop in barometric pressure (from ~160 mmHg at sea level to ~96 mmHg at 4,024m). Consequentially, there are fewer and fewer oxygen molecules per breath as you go up in altitude, making the equivalent oxygen percentage at 4,024m as low as 12.7%. Because of the reduced availability of oxygen in the air you breath in, this reduces the amount of oxygen diffusing from the alveoli into the bloodstream, and your body will struggle to efficiently deliver oxygen to all body tissues. This does not just affect your breathing or power output but crucially also your brain.

Acute physiological responses to hypoxia

If you want the ‘skinny’, just glance at these bullet points. If you want to find out a bit more , click on the ! icons.

You can see from the acute responses why coaches advice the unacclimatised athlete to reduce training volume (and intensities) during the first few days after travel to altitude…

Hyperventilation

Both resting and exercise ventilation increase acutely at altitude. This hyperventilation response is triggered by the peripheral chemoreceptors sensing low arterial oxygen levels and signalling the respiratory control centres in the brainstem to (partly) compensate for this by increasing ventilation in an attempt to maintain oxygen delivery. The hypoxic ventilatory response is a key determinant of how much ventilation increases in low-oxygen conditions, although other factors such as CO₂ sensitivity, acclimatisation state, genetics and metabolic conditions also contribute.
When you breathe more to get more oxygen in, you also acutely exhale more CO₂ than your body is producing. This results in hypocapnia (i.e. low arterial partial pressure of CO₂) which in turn makes your blood more alkaline (respiratory alkalosis). Arterial blood pH can gradually return to normal by renal excretion of bicarbonate, but this metabolic compensation is less acute and both slower and less complete at very high altitudes. So take it easy at first !

Claire wearing a buff for SRMR - photo by Claire Stevens
As an aside, due to the lower temperatures and reduced humidity at altitude, hyperventilation of dry and cold air, especially during exercise, can cause irritated airways.
So wear a face covering/buff to protect yourself, not only from the cold, dry air but also the notorious dust on the dirt roads of the SRMR !

↑ HR

Any given exercise load will require more cardio-respiratory effort at altitude in order to deliver what little oxygen is available to all the tissues in your body. Although the responses differ largely between individuals, researchers have observed 20bpm higher resting heart rates at 3,700m altitude in male subjects compared with 500m (Rao, et al, 2015). Your resting HR, blood pressure and cardiac output (for a given oxygen uptake) will also increase acutely (within minutes), aided by the acute release of adrenaline. If you track your heart rate variability (HRV), you may notice an acute drop in HRV indicating increased sympathetic drive and a decrease in parasympathetic tone (and given the nature of an ultra race, this will likely remain that way for the rest of the race).

↑ Metabolic cost

The increase in HR and ventilation also result in a higher metabolic cost and greater relative anaerobic contribution (most notably in men), which has implications for fuelling your exercise at altitude. In one study acute altitude exposure to 4,300 m increased basal metabolic rate (BMR) by 27% by day 2 at altitude compared to sea level and remained elevated by 17% after 3 weeks of acclimatisation (Butterfield et al, 1992). Weight loss over the 3 weeks was limited however to ‘just’ 2kg as energy intake was increased to the increase in BMR. Women have less altitude-induced changes in bodyweight compared to men. At moderate altitude loss of body fat accounts for 70% of weight reduction; whereas, at high altitude there is more muscle catabolism (Boyer & Blume, 1984).
The double-whammy challenge is that when exercising at high-altitude leptin is acutely elevated and grehlin is blunted and this may be lasting with prolonged exposure to hypobaric hypoxia (Gatterer et al, 2023). Given that leptin is a hormone that suppresses appetite and grehlin stimulates appetite, the acute (and chronic) result is that you feel less hungry and may reduce your energy intake whereas, really, from day 1 you need to eat more at altitude. This – and limited refuelling opportunities – is why each year we see SRMR racers lose a scary amount of weight. In the ‘before’ and ‘after’ the 2024 SRMR pictures of Niel he lost 8kg and was not riding so well anymore at just 62kg. He was burning upwards of 8,000-10,000 calories per day but, once his freeze-dried 7×1,000 kcal bomb packs ran out, was unable to find enough resupply to fuel sufficiently for the demands of the physical effort and the conditions combined.

↑ Dehydration

At altitude, the temperatures are lower. Cold air holds less moisture than warm air (i.e. lower humidity at altitude). This, combined with an increased breathing rate and increased urge to pee in the cold and at altitude (diuresis due to early changes in water- and salt-regulating hormones that act on the kidneys), can result in dehydration. Dehydration can be worse when you are exercising hard and not paying sufficient attention to your hydration strategy (paradoxically altitude can cause a reduced sense of thirst …). In turn, dehydration can impair cognitive function, result in additional stress on the cardiovascular system as plasma volume decreases, and mask/mimic some of the symptoms of acute mountain sickness (more about that below).
So take care to compensate for these additional water losses with an additional ~1-1.5 L of electrolyte-packed fluid per day. Don’t over do it though as you don’t want to risk hyponatremia either! Checking your urine colour remains an easy and ‘rule of thumb’ way to assess your hydration status, even if it is a little less reliable during the first few days at altitude than at sea level.

↑ Oxidative stress & inflammation

Reduced oxygen availability at altitude triggers increases in oxidative stress which in turn triggers an increase in inflammation. Although your body will eventually mount an adaptive response (by upregulating antioxidants), when combined with ultra-endurance exercise, little sleep, and insufficient fuelling, the attack on your immune system and increased risk of infection and illness should come as no suprise. Each year many racers find that the ‘Silk’ Road can quickly become ‘Sick’ Road …

↓ Blood plasma volume

When urination and dehydration increase acutely at altitude, this results in a drop in blood plasma volume and increase in haematocrit and haemoglobin concentration (i.e. not yet the boost in red blood cell volume and hemoglobin mass that professional athletes seek from prolonged hypobaric hypoxic exposure). It is also this reduction in plasma volume that results in greater cardiac stress (due to less preload, reduced stroke volume and increased heart rate) and potentially also impairment to thermoregulation (Sawka et al, 2000).
This acute reduction in plasma volume at altitude is one of the reasons why a cross-adaptation strategy with prior heat acclimation – which drives an increase in plasma volume – may help you deal with altitude a little bit better …

↓ VO2max

With increasing altitude and due to reduced inspired oxygen partial pressure, there is a linear decrease in VO_2max and endurance performance (Wehrlin & Hallen, 2006), at ~6% decrease per 1000m altitude increase. This is why people coming straight from sea-level can feel pretty lousy for a few days when they arrive at moderate- to high-altitude destinations like Kyrgyzstan. So, don’t expect to be able to hold the same power output at altitude as you are used to at sea level. At 4,000m the drop in VO_2max (and corresponding power output) may be a large as 35-40% in unacclimatised and 25-30% in well-acclimatised athletes, with even slow hike-a-bike feeling like a Herculean effort ! That said, the drop in VO2max differs per person and can rebound a little bit with acclimatisation time.

↓ Sleep quality

Another acute effect of hypoxia is sleep impairment. Even at altitudes as low as 2,500m, unacclimatised athletes commonly complain of difficulty falling asleep, waking up frequently, crazy vivid dreams and feeling groggy when waking up.
Multiple sleep physiology studies back up these subjective reports with objective evidence of changes in sleep architecture at high-altitude. For example, as early as 1970, Joern et al noted that at an altitude of ∼3,350m at the (potentially rather light) South Pole deep sleep (stages 3 and 4) were near absent, coupled with a ∼50% reduction in rapid eye movement (REM) sleep. The combination of reduced oxygen availability and acute hypocapnia response to altitude can confuse the brain’s breathing control system causing increases in periodic breathing. This manifests as a cyclical breathing pattern that alternates between central sleep apnoea (i.e. the brain temporarily stops sending the signal to breathe) and brief episodes of hyperventilation.
So, when possible, you want to try and sleep below 2,500-3,000m, especially during the first night of the SRMR and even more so if you have not had any time or opportunity for adequate altitude acclimatisation.

↓ Neurological function

Your brain is very sensitive to reduced oxygen supply. Even mild hypoxia can affect neurological function within minutes and hours. Neurological symptoms will vary greatly from person to person and with rate of ascent, but can result in impaired coordination, cognitive slowing (have a go at a Stroop task at altitude …), visual and sensory changes (notably altered night vision – beware during the first night of the SRMR), headache and lightheadedness as well as mood changes and the aforemention sleep disturbances.
For a funny, but also important, illustration of some of the mental impairments caused by low oxygen saturation that can be experienced at high- and extreme altitude have a look at this ‘Four of Spades’ video.
Acute Mountain Sickness and other Altitude Illnesses

Chronic physiological responses to hypoxia

If the acute responses can be so challenging, why do many professional athletes do altitude training camps ?! Because they are seeking the performance benefits that come with chronic exposure to hypoxia … for performance both at altitude and at sea level.

So what are (some of) the chronic responses and how long does it take to adapt? Again, glance at the bullet points for the ‘skinny’ or click on them to find out a bit more …

Cardiac adaptations

Acutely, resting and (submax) exercise heart rate (HR) increase whilst stroke volumes reduces. Generally (unless continuing to ascend), after ~1 week exercise HR will come down a little again, albeit still elevated compared to a similar workload at sea level. Max HR and stroke volume will remain suppressed whilst blood pressure (due to continued sympathetic drive) may remain elevated. Resting HR may not be as crazy high as during the first few days at altitude but still remains elevated after a week, of course not aided by excessive exercise and little sleep during the SRMR …
If you fancy a very deep dive into cardiac adaptations to acute and chronic hypoxia, check out this open access article by Williams et al (2022).

→ Ventilatory adaptations

Ventilation progressively increases during the first few days at high-altitude, but levels off after 4–8 days. When spending time at 4,000m (high altitude), ventilatory acclimatisation is almost complete after 10 days, but it may take more than 2 months when ascending to extreme altitudes, (West, 1988)

→ Cerebral blood flow

Typically, cerebral blood flow acutely increases during the first 12h of high-altitude exposure (to compensate for the reduction in arterial oxygen content), but subsequently declines with acclimatisation (within 3–5 days) to near baseline values.

↑ Haemoglobin mass

The haematologital (blood) adaptations are what drives most athletes to altitude training for performance benefits. Acutely (within a day or two), there is an increase in haemoconcentration (i.e. thicker blood) due to the drop in blood plasma volume. This leads to a rise in haematocrit (the proportion of red blood cells in the blood) and an increase is haemoglobin concentration. But it is only with chronic hypoxic exposure that we see the increase in haemoglobin mass and red blood cell volume that is so beneficial to exercise performance and the increases in maximal oxygen uptake (VO_2max) observed when returning to sea level after altitude training.
This happens because of something you may have heard of: EPO. Erythropoietin is a hormone released by the kidneys (and to a lesser extent the liver) to boost and mature new red blood cells which in turn can augment your oxygen carrying capacity and oxygen delivery (Stockman & Fandry, 2006).
“Increased EPO production can be detected within a few hours of exposure to altitude and new erythroctyes are in circulation within 4-5 days. Plasma EPO levels tend to peak within 48-72 hour following initial exposure to altitude and slowly return to baseline in 2–4 weeks. Within a few weeks, RBC volume and Hb content can increase substantially … However, these hematological adaptations do not persist indefinitely” – Sinex, J.A. & Chapman, R.F. (2015)

↑ Capillary density

While most people will have heard of EPO, not everyone may have heard of HIFs. HIFs are hypoxia-inducible factors. HIFs promote the expression of various genes involved in maintaining oxygen supply. HIFs are not just responsible for upregulating EPO (see above), but also stimulate vascular endothelial growth factor (VEGF), a key mediator of vasodilation (making your blood flow more easily) and angiogenesis (i.e. the growth and formation of new capillaries). The increase in capillary density supports oxygen delivery to and aerobic metabolism of your muscles and other tissues (including the brain!).

↑ Glycolisis

There is a shift in metabolism away from oxidative phosphorylation to the glycolytic pathway as an adaptive way to create ATP (cellular energy) that requires less oxygen. But at the same time this leads to a greater dependency on glucose uptake in order to generate adequate amounts of ATP. This can in part be achieved by increased uptake of lactate by the liver where it can then serve as a substrate for gluconeogenesis (making of more glucose), but also needs to be supported by eating sufficient carbohydrate.

Although the time course for acclimatisation depends on the duration and severity of the hypoxia exposure, the below image gives a good general overview of the timeline of adaptations to high-altitude, with some changes taking longer (e.g. increase in haemoglobin mass) than others (e.g. ventilatory adaptation). Also see Mallet et al (2023).

Altitude illness awareness

Before deliving into the exciting stuff (!) , let’s first include an important health warning anyone who going to high-altitude needs to be aware of.

Taking part in the SRMR is a lot of fun (!) and I don’t want to scare anyone off. However, it is important to respect the challenges and be aware of potential altitude illness risks when doing an ultra-endurance race at atltitude. Probably the best-known challenge of altitude is Acute Mountain Sickness (AMS) which is uncomfortable and can lead to rare but more life-threatening conditions such as High-Altitude Cerebral Oedema (HACE) and High-Altitude Pulmonary Oedema (HAPE). For more information about each condition, see icons below.

Acute Mountain Sickness

Acute Mountain Sickness – the most benign form of altitude illness – typically does not happen below 2,000m, but incidence increases with altitude. Symptoms typically manifest within 6-12 hours after arrival at a new altitude above 2500m (if at all).
Some people are more susceptible to AMS than others and (annoyingly) it has little to do with how fit you are (although people with cardiovascular disease, respiratory disorders and other chronic diseases may be more susceptible). Genetic factors can play a role. But, above all, AMS risk is greater the higher and faster you ascend and the less acclimatised you are. Therefore, the main strategy to avoid AMS is to ascend slowly and descend to a lower altitude / wait before you ascend any further if you experience AMS symptoms. You should also calculate your Lake Louise Score to establish if you have ‘mild’, ‘moderate’ or ‘severe’ AMS (Roach, 2018).

Another way to monitor yourself for potential AMS is by monitoring oxygen saturation (SpO₂) i.e. the extent to which haemoglobin is bound/saturated to oxygen. Ideal SpO₂ at sea level is between 95% and 99%. At altitude, due to the reduction in the partial pressure of oxygen, you will notice a reduction in SpO₂ to moderate desaturation of ~85-89% at 3000m or as low as 65-75% at 5500m and it can drop further during exercise. Moderate desaturation can lead to fatigue, headache and breathlessness. But larger drops in SpO₂ (to below 80%) can cause impaired mental function and should be monitored. If SpO2 drops below 70% it can be dangerous – look out for some of the symptoms listed under HACE and HAPE below.
SpO₂ is typically measured through a pulse oximeter (a cheap and light device that you buy at many pharmacies) that clips onto your finger and can be used either at rest or during exercise. Smartwatches and other wearables can also measure SpO₂ but only passively during sleep and are generally perceived to be both less accurate (and of course also more expensive) than a traditional pulse oximeter.

High Altitude Cerebral Edema

AMS is uncomfortable but reversible. When ignored, it can progress to High Altitude Cerebral Edema (swelling of the brain). This is a serious conditions that can worsen quickly and can be life-threatening. If left untreated, HACE can progress to coma.
Symtoms of HACE:
Marked ataxia (loss of coordination) – displays as stumbling, unsteady gait and difficulty walking in a straight line
Severe, unrelenting headache
Confusion or altered mental state – including trouble speaking clearly, memory lapses and inappropriate behaviour
Severe lethargy – inability to get out of bed or perform simple tasks

High Altitude Pulmonary Edema

The other condition AMS can progress to (if ignored) is High Altitude Pulmonary Edema (fluid on the lungs. If left untreated, HAPE can be fatal.
Symptoms of HAPE:
Crackling or rattling sounds when breathing
Out-of-proportion breathlessness both on exertion and at rest
Cough producing frothy, white or pink saliva (and may progress to coughing up blood)
Blue/purple discoloration of lips and fingertips

Climb Slow, Sleep Low

Despite differences in individual susceptibility, altitude illnesses are largely preventable with appropriate acclimatisation and a slow ascent rate. Note that you may have a higher risk if:
You arrive unacclimatised
You have a history of acute altitude illness
You are sleeping above 3500m on day 1
You ascend by more than 500m/day above 3000m without extra days for acclimatisation every 1000m.
When planning your rate of ascent, note that the altitude at which you opt to sleep is considered more critical than the altitude reached during waking hours.

Luks et al (2024)
I am not a medical professional, so I cannot give you any medical advice. However, if you are interested in how to prevent and how to treat altitude illnesses (eg “Should I carry Acetazolamide (aka Diamox) for SRMR ?”, this 30-minute webinar by World Extreme Medicine is worth watching. World Extreme Medicine is a world-wide community or medics and adventurers which provides various CPD training courses that combine clinical practice with adventure in extreme environments.

For the time-rich: hypoxia conditioning at altitude

Pre-acclimatisation by hypoxia conditioning is important both to prevent high-altitude illnesses and maintain as much your aerobic exercise performance as possible. But how to go about it?

The money-rich may go for hypoxic conditioning in simulated altitude (e.g. sessions in an altitude chamber or sleeping in an altitude tent). But while these methods help you with the required chronic adaptations, sessions in the chamber are very costly and sleeping in an altitude tent for weeks on end is annoying and won’t do much good to any relationship !

Time-permitting, before the SRMR you can do your hypoxic conditoning at real altitude. This is not only cheaper and more fun, but also includes exposure to the specific climate conditions you will be facing (e.g. temperature changes, sunburn from increased exposure to ultra-violet light, wind and reduced humidity).

Essentially there are two strategies for hypoxia conditioning at real altitude:

“Staging” which refers to the concept of staying at moderate altitudes of about 2000–3000m for several days, which subsequently permits a more rapid ascent to higher altitudes during the race.
Slow, graded ascent – as described above under ‘climb slow’ sleep low’ common guidelines. This option may be more suitable if you are short on annual leave for the SRMR as it is…

Both strategies, or a combination of both, have been traditionally used in mountaineering to reduce altitude illness risk and limit the exercise performance drop at altitude.

How high to go for staging? Acclimatisation can be obtained using various combinations of altitudes and durations of exposure but is best accomplished at altitudes ≤3000m that do not induce significant AMS symptoms or sleep disturbances.

How many days of staging does it take? Older publications recommend extended staging time (e.g. a minimum of 4 days or ideally up to 2 weeks) at moderate altitude (2,100m) to reduce symptoms of AMS prior to ascent to 4000m or above. Beidleman et al (2018), however, suggest that staging time can be cut in half from four to two days at 3000m prior to ascent to 4300m and still achieve a 50% reduction in the prevalence of AMS (80% to 40%). In 2019 they followed this up with a study that focused on the development and use of an Accumulated Altitude Exposure (AAE) calculation method. This study showed that both 2 days at 3000m or 6 days at 2100 m would achieve the same level of AAE.

So, if you are short on time for your pre-SRMR altitude acclimatisation, and providing you can get up to 3000m gradually, a minimum of 2 days of acclimatization would appear to satisfy a minimal effective dose. In both scenarios, keep intensity low and avoid any exercise with a large glycolytic (lactate producing) contribution during the acclimatization period as your lactate buffering capacity will be reduced.

For the time-poor: heat training to prepare for altitude

For those who cannot afford more time in Kyrgyzstan, heat training is your friend! During the SRMR racers will encounter temperatures ranging from -10 to 40+C, so adding a heat training block into the training schedule of any rider preparing for SRMR makes a lot of sense (see my blog on heat training here).

In 2023, Claire and I had already successfully implemented a 2-week post-exercise hot water immersion protocol for The Rhino Run, a 2750 km unsupported solo ultracycling race from Plettenberg Bay, South Africa to Windhoek, Namibia. In fact, as a ‘warm-up’ Claire had casually added the 740km reverse Entrée to the race for a total of 3490 km in the heat! Having reaped the benefits of your heat training prep for The Rhino Run, Claire needed little convincing to repeat this protocol again for the SRMR. Perversely or expectedly for someone with an ultra-endurance racer mindset, she even ‘enjoyed’ the hot baths (40minutes at 40C really is not enjoyable!). Note to stop the hot baths latest 1 week before travel to lower overall stress and ensure an effective taper.

Whilst simultaneous heat and hypoxia training increases stress reduce their individual effectiveness for performance at altitude (Girard & Racinais, 2017; Esh et al, 2024), emerging science on (and application of) sequential approaches is promising.

Recently, you may have seen articles about how, upon their return to sea level, professional riders use heat after altitude to reduce the rate of decay / hold onto their altitude training camp gains.

“It [heat training] is becoming more normal for our riders now, more than before,” said Tim Heemskerk, the Visma-Lease a Bike trainer of Jonas Vingegaard and Matteo Jorgenson. “We mostly use heat training after altitude to maintain the blood adaptations. We also use it at the start of the year to prepare riders for altitude work. – extract from “The Wild World of Tour de France Training: How ‘Heat Suits,’ Wearables, and Recovery Hacks Accelerated the Race” by Jim Cotton for Velo (Outside Magazine)

Note – don’t try heat training after SRMR as the race will have totally wiped you out. However, for SRMR riders preparing to perform at altitude for up to 15 days, heat before altitude can make a lot of sense.

How can heat training help performance at altitude?

What were are looking for here is cross adaption in this case repetitive exposure to heat to boost subsequent tolerance to altitude, and reduce the actue drops in performance at altitude.

What mechanisms are behind this cross adaptation? Click on the icons for more information.

↑ Blood plasma volume

The heat-mediated increase in plasma volume aids performance in the heat, but also off-sets some of the acute drop in plasma volume at altitude. In turn this reduces cardiovascular strain both at rest and during (submax) exercise at altitude. What is not yet known however is how long how long the heat-acclimation induced plasma volume expansion can be retained for during subsequent hypoxic exposure (which is known to lower plasma volume).

↑ Haemoglobin mass

We have seen that the increase in haemoglobin mass (Hbmass) at altitude takes time.
With heat training it is also possible to bring about an increase in haemoglobin mass (and thus an increase in oxygen carrying capacity), but it is a ‘slow burn‘…
For example, a group of 27 male and 20 female elite cyclists were instructed to do 5 weeks of 5x 50min weekly supervised indoor sessions at a low intensity whilst wearing additional layers and drinking 500ml of water. At the end of the 5-week period they saw a 25g increase in Hbmass in female participants and 33g increase in males (Lundby et al, 2023). As highlighted in the image below, they also found increases in power at 3 mmol, V̇o2max – although not as large as the 4 ml/min increase per 1 gram of Hbmass increase as established by Montero & Lundby (2018) would suggest – and mean power output during 15-min maximal cycling in both sexes. They concluded all correlated with increases in Hbmass. In early studies, some of the same authors had already found an increase in Hbmass (by 43g) in elite cyclists when using a 5-week (5x60min per week) heat chamber training protocol (Rønnestad et al, 2021) and a 30g Hbmass increase when using a 5-week (5x50min per week) heat suit training protocol (Rønnestad et al, 2022).
As touched upon in the section above on ‘chronic adaptations to hypoxia, HIFs are known to be the responsible mechanism for increasing EPO and subsequently Hbmass. The authors of this 2023 study speculate – as previous authors did (e.g. Ely et al, 2014) – that heat and hypoxia share cellular stress response pathways with HIFs also responsible for upregulating Hbmass through heat training. A 2025 study by Li et al confirmed that heat acclimation enhances tolerance to both heat and hypoxia by stabilizing HIF-1α through increased interaction with HSP70 (heat shock proteins). However, the study was done in mice…

In a related study by Cubel et al (2024) a group of 20 elite male and female cyclists (heat, n=10; control, n=10) did 5 weeks of 6x 60min indoor (no fan) sessions per week at 70-75% of their max heart rate, similar to the heat rate reached in a heat tolerance test when their core temperature reached approximately 38.5°C.
After 5 weeks Cubel et al (2024) found an increase in haemoglobin mass by 40 ± 38 grams (4 ± 4%, p = 0.038) with 30 ± 30 grams (3 ± 4%, p = 0.048) increase in haemoglobin mass already seen after 3 weeks.
As can be seen in the image below, those doing the heat protocol also significantly increase their plasma volume (PV) and total blood volume (BV). However, haemoglobin mass had returned to pre-heat acclimation levels after 2 weeks of normal training without superimposed heat stress.
3 weeks of heat-to-altitude cross adaptation may be adequate, but timing your heat training carefully before travelling to altitude is key!
My heat-to-altitude cross adaptation experiment with Claire was done at the end of July / early August 2024 just before the study by Cubel et al on the time-course for onset and decay was published later in August 2024. ‘Only’ two weeks of post-exercise hot water immersion most likely was a little too short for full haematological adaptation benefits. Luckily, Claire also had time to do a couple of days of staged hypoxia pre-conditioning prior to the race. Of course, coaching from distance, I could not collect any blood samples from Claire during her heat training before the race or during the race at altitude itself. So, the ‘evidence’ had to be in the performance: feeling good and not getting ill.

Left panel – (a) total hemoglobin mass (tHbmass), (b) plasma volume (PV), and (c) total blood volume (BV) at baseline (pre) and post-testing for a control group (CON; n = 10) and a heat acclimation training group (HEAT; n = 10) after a 5-week intervention period (total n = 20). Data are presented as mean ± SD. Significantly different from pre and from CON group: ** = p < 0.01. Right panel – time-course for individual HEAT participants (n = 10). Open circle and thin lines represent individual values, and the closed circle and the thick line represent the group average (mean). The scratch (gray) area represents time-course for 5 weeks of heat acclimation training, followed by a subsequent 2-week evaluation of decay (2-wk decay). Significantly different from pre: * = p < 0.05.
The foregoing two study examples have demonstrated how heat acclimation, by exercising in a heat chamber or in a heat suit, can increase haemoglobin mass. My favourite mode of heat acclimation training however is post-exercise hot water immersion. It is cheap and allows for some high intensity exercise to be maintained during the heat block.
In May 2025, Rodrigues et al published the first study that validates this approach for cross adaptation. They had a group of 20 cyclists complete a 6-week post-exercise hot-water immersion intervention with 3 weekly interval training sessions (4×4 min at 90 ± 5% max HR) and 5 weekly 40-50min HWI sessions in 42°C . The study was not without limitations (lacking supervision and tight training load control and only having access to HB concentration rather than Hbmass measurement methods), but the authors concluded that:
“Six weeks of post-exercise HWI intervention resulted in greater improvements in time-to-exhaustion at 80% of Vo_2maxin acute hypoxia [at a simulated altitude of ~3500m with 13% fraction of inspired oxygen] compared to exercise training alone. This enhancement in normobaric–hypoxic exercise performance may be attributed to increased haemoglobin concentration, lower core temperature and improved respiratory efficiency”. – Rodrigues et al (2025)
Although cycling teams and coaches have experimented with this practical and accessible cross adaptation approach (using HWI) for longer, it is great to have the first scientific validation. It will no doubt spark future research and enable athletes and their coaches to fine-tune practical applications.

Other benefits ?

A systematic review and meta-analysis published in 2024 (Willmott et al) – which included 9 studies, totalling 79 all-male recreational athletes) found moderate, beneficial effect of heat acclimation increasing SpO₂ at rest and reducing HR, core temperature and skin temperature during submaximal exercise in hypoxic conditions. The majority of protocols used ‘medium-term’ heat acclimation (8-14 days). Although all 9 studies used exercise in the heat (at a fixed intensity), there were distinct differences in prescribed number of sessions, dose and type of activity as well hypoxic test protocols (which highlights some of the general limitation to interpreting results of meta-analysis). The observed increase in resting SpO₂ may be somewhat protective greater nighttime desaturation when sleeping at altitude and less potentiallybetter sleep during the for first few nights at altitude.

What else could be considered for altitude performance?

Iron

Sufficient iron stores are essential for the red blood cell adaptive response to altitude due to iron’s role as a component of hemoglobin. If you arrive at altitude with already insufficient iron stores your body won’t be able to produce the much needed red blood cells, which can result in anemia, but even nonanemic iron deficiency can decrease performance, reduce adaptations and impair thermoregulation.
Iron supplementation may also be of interest to athletes with normal serum ferritin levels because iron absorption is dimininished at altitude and as such, iron supplementation will benefit altitude training and performance for most.
Since female athletes have a (15-35%) higher prevalence of iron deficiency (Sim et al, 2019) they could be more at risk for reduced adaptations. Usually, serum ferritin cut-offs of <30 ng/ml for women and <40 ng/ml for men are used to diagnose iron deficiency. But when planning tavel to altitude higher cut-offs should be used of <50 ng/ml to assess if supplementation is needed. Although they can’t provide further evidence, (DeLoughery et al, 2025) recommend that women pursuing a high-altitude trip may benefit from having their ferritin level >80 ng/ml before departure.
When racing the SRMR in 2024 Claire had a medical check for her serum ferritin levels 6-8 weeks before travelling to Kyrgyzstan and she started iron supplementation as recommended by her consultant. Current evidence suggests that most people will maximize their altitude-induced Hbmass increase by consuming ~100-200 mg/day of oral elemental iron (Stellingwerf et al, 2019) which should ideally be taken as a single does after exercise so as not to elevate hepcidin (which reduces iron absorption) , alongside Vitamin C and away from coffee, tea and calcium, to optimise absorption. Hall et al, 2019 found that a single dose of 200mg each evening during a 3-week altitude training camp had a significantly greater increase (6.7 ± 6.3%; p = 0.048) compared to the split-dose (4.6 ± 3.9%).
Although iron availability via supplementation during altitude appears to be more important for optimal adaptations than pre-altitude iron stores, don’t start iron supplementation unguided as excessive iron can have negative health consequences.

Diamox

Claire had a medical consult for acetazolamide to prevent/treat acute mountain sickness and was very aware what the signs and symptoms AMS, HAPE and HACE are. She also did daily oxygen saturation checks.

Energy expenditure & intake

Eat, eat, eat. While at altitude during the SRMR your energy expenditure will increase significantly (more so than during any ultra at lower altitudes) and your energy intake make me insufficient due to a combination of lack of resupply, gut issues and lack of appetite. Low energy availability will not just effect your performance but also The suppression of sex hormones (estrogen and/or testosterone) levels due to low impair hematological adaptations to altitude. In women, altitude-induced weight changes tend to be smaller compared to men.

Caffeine

There are some interesting findings in the literature on caffeine and high altitude. On the one hand, unsurprisingly, caffeine – which works on blocking the adenosine receptors – can disrupt your sleep even more than it already is by the altitude itself. However, in the ultra-endurance context staying awake can be required. Some of the more interesting findings however are how a moderate dose of caffeine (4-6 mg/kg of bodyweight) can increase pulmonary ventilation, reduce perceived exertion and mitigate hypoxia-induced muscle oxygenation reductions during submax exercise at moderate and high intensity at moderate and high (simulated) altitude (Lei et al, 2024; Narang et al, 2025).

Creatine

Creatine is a chemical compound that resembles protein and is formed from the amino acids arginine, methionine and glycine. Creatine is produced naturally by the human body in the liver, pancreas, and kidneys, and it is stored mainly in skeletal muscle and in the brain.
Your body can make its own creatine (within the liver and pancreas), but to satisfy normal daily loss of creatine, your body either needs creatine from dietary sources or a supplement. Given that all dietary creatine comes from animal sources, vegetarian and vegan athletes will have a lower amount of creatine stores in their muscles and thus a greater need to supplement. Creatine is not only cheap but also safe and the most tested supplement. As per the Internationational Society for Sports Nutrition position statement on creatine (2017):
“Creatine monohydrate supplementation is not only safe, but has been reported to have a number of therapeutic benefits in healthy and diseased populations ranging from infants to the elderly. There is no compelling scientific evidence that the short- or long-term use of creatine monohydrate has any detrimental effects on otherwise healthy individuals”
Traditionally, people may associated creatine with strength and power sports because creatine can your muscels a greater capacity to generate energy quickly. However, there is an increasing body of evidence for the use of creatine to enhance endurance performance (see Forbes et al, 2023 including mechanisms behind its role in energy buffering and recovery).
For SRMR riders, there may be additional reasons to look at creatine:
Cognitive benefits – Overall, there is growing evidence that creatine supplementation can augment measures of cognitive function when brain bioenergetics are challenged, such as with sleep deprivation, mental fatigue, and hypoxia. To date, most studies on creatine supplementation for cognitive benefits (but only in metabolic stress situations such as sleep deprivation) involve a prolonged regime of 3-5 gram oral creatine monohydrate per day over a minimum period of 1 week. In 2024, a study was done that showed a high single dose (at 0.35 gram per kg of bodyweight) to be more effective. This is because the brain also has the ability to make its own creatine and therefore appears to be more resistant to the uptake of creatine. However, there are no formal recommendations yet for such a high dose, so stick to the tried and tested 3-5 gram/day in the meantime.
Creatine and hypoxia – The brain requires ∼20% of the body’s total energy (ATP). When deprived of energy, such as during hypoxia at high altitude, intracellular ATP levels in the brain drop. This is where creatine may play a role as a (temporary) energy buffer in the brain. For a deeper dive into creatine reseach, listen to this accessible podcast with Prof. Darren Candow.

Are there differences for female SRMR riders?

With my interest in female physiology and coaching many women, I can’t publish this blog without expanding on some of the differences between the sexes (as I did here in my blog on preparing for ultra races in the heat).

Frustratingly, the majority of studies examining physiological responses and adaptation to hypoxia have either solely or predominantly used male research participants. While the growth in the women taking part in high-altitude mountaineering and other high-altitude sports is undeniable, research on female physiological responses to hypoxia is only starting to come through slowly and there are not yet any practical recommendations if/how women should prepare for altitude differently to men.

It not often that we see an all-out female winner of an ultra cycling race (especially not off-road), but what are the odds that we will ever see a female SRMR winner? Female athletes are capable of extreme challenges, including the Guinness World Record for the Longest Solo Unsupported One-Way Polar Ski Journey Across Antarctica (female and overall). Yet, (at group level) female performance at altitude may be affected more starkly due to sex differences in responses to the concomittent stressors of heat/cold and altitude during the SRMR.

An examination of the scarce scientific evidence of distinct sex differences in (patho)physiological responses and adaptations to high altitude/hypoxia shows:

↓ Ventilatory response and ↑ breathing cost at altitude

Comparatively, females have smaller airway diameters and reduced lung volumes, even when matched for body size. This increases the airflow resistance and results greater mechanical work of breathing in females (Dominelli et al, 2015) which exposes females to greater respiratory stress, exacerbated diaphragm fatigue (Archiza et al, 2021) and greater pulmonary limitations to exercise in hypoxia compared to males.
Female endurance performance at altitude is affected by a reduced ventilatory response (i.e. a blunted signal to increase breathing compared to men with similar hypoxic exposure). Women also tend to increase their breathing less than men in response to high CO2 in the blood (hypercapnia), both at rest (Sayegh et al, 2022) and during exercise (Mann et al, 2022).
As concluded by Millet et al (2025):
“On balance, the evidence supports the view that the female pulmonary system during exercise, and consequently their aerobic performance, may be more negatively impacted by a hypoxic environment. Notably, the mechanisms involved are proportional to elevation, and … indeed, the potential sex-related differences due to pulmonary limitations will have a greater impact at higher elevations and be more limited at lower ones”.

↓ Vasoconstriction & ↑ vasodilation

Compared with men, premenopausal women’s blood vessels constrict less and open up more when the body is under certain stresses, such as hypoxia (Jacob, 2021). This (premenopausal) female advantage may be ascribed to the influence of oestrogen as well as greater levels of circulating endogenous nonadrenergic vasoconstrictors and lower release of vasoconstrictors (Hart et al, 2009).

Thermoregulatory differences

With reduced thermogenic heat production due to lower muscle mass as well as a higher body surface area-to-mass ratio, and a greater prevalence of Raynaud’s phenomenon (Alba et al, 2019), females riders have some thermoregulatory challenges at altitude during a mountainous ultra race like the SRMR.
This is exacerbated by repeated transitions during the race between cold and hot-dry conditions (Wait et al, 2023) which can result in elevated skin temperature yet greater heat dissipation due to competition between hypoxia-induced vasodilation (attempt to maintain oxygen delivery) and cold-induced vasoconstriction (attempt to conserve heat) (Mugele et al, 2021).
In the cold, women need to rely on shivering for heat production sooner than men (Kaikew et al, 2018) but other factors such as more fat stored under the skin may protect female riders somewhat – another reason to gain weight in advance of the SRMR! An additional challenge comes when racing during the luteal phase of the menstrual cycle when (due to the rise in progestrone and the related shift in core temperature) the vasoconstrictor response during cooling is reset to a higher body temperature. This adjustment may result in earlier exhaustion of maximal capacity for heat preservation by peripheral vasoconstriction (reducing blood flow to skin) and thus makes females rely on shivering for heat production sooner than males.
In dry heat (where the key heat dissipation mechanism is evaporation), females riders will be at a small thermoregulatory disadvantage as they tend to have lower sweat rates than men – another reason you want to jump on the band wagon of heat training before travelling to altitude! Whereas in humid heat (when the sweat just ‘sits’ on your skin) evaporation is constrained for all.

Less ↑ in carbohydrate reliance

Women are less sensitive to the substrate shift during exercise at altitude. They can still rely more on on lipids and have less increases dependence on carbohydrate oxidation during exercise. Women still require more carbohydrate at altitude than at sea level, but they may need a smaller proportional increase in carb intake at altitude than men. This is advantageous to women at altitude. However, higher intensity exercise may be comparatively harder at altitude for premenopausal women during the luteal phase when oestrogen triggers the body to spare carbohydrates (which are needed for hard efforts), and progesterone increases ventilation which is already increased at high altitude.

↑ AMS risk ?

A meta-analysis by Hou et al (2019) demonstrated a higher AMS prevalence in women compared with men (in 15 out of the 18 included studies). However, it remains debated whether women really have a great AMS risks and the potential mechanism underpinning such sex differences are still not well understood.

Menstrual cycle impact

Sex hormones may play a role in the responses to hypoxic exercise depending on the ovarian cycle phase. The image is a schematic representation of the potential impact of sex hormone fluctuations during the menstrual cycle influence (with estrogen in red and progesterone in blue) on physiological responses to altitude as theorised by Raberin et al (2023).

Progesterone is known to increase the rate and depth of breathing at rest, stimulating a slightly higher breathing rate in women than in men at the same altitude.
The proposed augmented hypoxic ventilatory responses during exercise (HVRe) was confirmed in a study with 1060 women by Richalet et al (2020) who found that premenopausal women with a regular/normal menstrual cycle (i.e. not using hormonal contraception) had a larger HVRe response (resulting in less oxygen desaturation) in the luteal phase (when both oestrogen and progesterone are high) than in the early follicular phase (when both sex hormones are low). However, after finding only small or insignificant differences in HVRe (and other markers of cardiorespiratory function) within a group of 16 untrained premenopausal women tested after 18 hours of hypoxia exposure at high altitude (3375m), Tagiliapietra et al (2024) concluded that no blanket recommendations should be made for women based on their menstrual cycle phase. Instead, individual variability in each woman’s specific responses to hypoxia across her menstrual cycle should be considered for optimising altitude travel.

Menopause impact

The effect of age on the hypoxic ventilatory response appears to differ based on sex, with Citherlet et al, 2025 concluding that age – and the related significant reduction of estrogen and progesterone levels – has minimal impact on HVR in women. Although postmenopausal women appear to have similar HVR during exercise as premenopausal women, older men see less-pronounced desaturation both at rest and during exercise at altitude (Richalet, 2015). However, cardiac responses to hypoxia are lower post- than pre-menopause with post-menopausal women loosing the greater compensatory vasodilation in hypoxia that we see in premenopausal women compared with age-matched men (Casey et al, 2014). This is ascribed to postmenopausal women losing oestrogen which is considered to aid vasodilation and adjust blood distribution during dynamic exercise and in response to hypoxia.

In mountain races at altitude with a lot of uphill, Millet et al (2025) identify three main factors that widen the performance gap between the sexes more so (18–22% gap) than in flatter races at sea level (9–12% gap), namely:

Sex differences in physiological responses to altitude
Sex differences in physiological responses to ambient temperature
Sex differences in uphill-downhill locomotion, with steeper gradients

Beyond the general and well-described factors explaining the usual 9–12% performance gap between the sexes (see for example ), they pointed to specific factors (i.e., particularly the differences in body composition and in skeletal muscle characteristics) that widening sex-differences during uphill locomotion.

Although cycling reduces this differential in uphill locomotion somewhat (as the bike reduces the mechanical load), the SRMR still includes long and steep (!) hike-a-bike sections that are weightbearing and much akin to ultra-trail running in this sense! When we see female riders finish in the Top 10 overall of big ultracycling races we are looking at outliers and extremeley strong riders, but the day a woman wins the SRMR we are looking at something quite extraordinary indeed ! Never say never…

Do women need to follow different advice for altitude?

I hope you enjoyed reading this blog and may reap some benefit of it when you plan to line up for your own Silk Road Mountain Race one day. For now, it is time to enjoy dotwatching the 2025 SRMR racers !