The 1968 Olympics in Mexico City was the first and last time the games have been held at altitude, but many endurance events still take place where the air’s a little thinner.
And it’s increasingly common practice for elite endurance athletes to attend several altitude training camps each year as part of a systematic strategy to maximise their performance at major events taking place at sea level.
Given the prevalence of time spent at altitude, a common question we get from athletes preparing to train and race at altitude is, “do I need to change my fueling and hydration strategy?” We’ll get to that, but first we need to understand what happens to your body at altitude…
What happens to your body at altitude?
To start, it’s worth tackling a common misconception that there’s less oxygen at altitude. This isn’t true – there’s the same amount of oxygen (~21%) in the air as at sea level.
It’s actually the pressure of the air that’s lower at altitude. This lower air pressure reduces the pressure gradient to drive oxygen through your lungs and into your bloodstream. And the lower level of oxygen in the blood is a fantastic stimulus for adaptation…
When your body senses that there’s less oxygen coming in, it stimulates the production of a protein (HIF-1α) which is the signal your kidneys need to naturally produce more of the hormone erythropoietin (aka EPO, of doping notoriety). More EPO stimulates the production of more haemoglobin, the oxygen-carrying protein in your red blood cells.
When the amount of haemoglobin in your blood is increased, there’s a greater capacity to carry oxygen around your body and to your muscles. More oxygen means a greater proportion of energy can be generated aerobically.
The benefits of altitude training also extend beyond an increase in oxygen-carrying haemoglobin. Activation of that HIF-1α protein in your muscles may also bring about other beneficial adaptations, including an increase in mitochondria – the powerhouse of all cells in your body – and an improved capacity to buffer lactate.
How does altitude affect fueling requirements?
Most studies investigating the effects of altitude on nutrition requirements have been conducted at 3,000m (9,842ft) or above – which is defined as high or extreme altitude.
At this elevation, rates of muscle protein synthesis – the process of building new proteins in the body – are lower, and carbohydrate utilisation is often greater. So, the broad consensus from the research surrounding this topic is that, when heading to altitude, athletes have an increased need for energy, carbohydrate and protein.
But most altitude training camps usually take place at a lower elevation, somewhere between ~1,600 and ~2,400m (5,249-7874ft). From a research perspective, much less work has been conducted at these altitudes, but plenty to come I’m sure as altitude is continually being used (and talked about) more and more. But we can still make some educated inferences on how your fueling requirements might change based on a combination of known physiological challenges and athlete training behaviours.
Because of the lower oxygen availability, your VO₂max – the maximum amount of oxygen that can be taken up by your body – is lower at altitude. There’s a fairly linear relationship between altitude and VO₂max. For every 1,000m (3,280ft) of elevation gain, VO₂max is reduced by about 6%.
So, to cycle at the same power, or run at the same pace as you do at sea level, you’ll be working at a higher percentage of your VO₂max. This results in the training session having a greater carbohydrate cost. In other words, more of the carbohydrate stored in your body will be used to generate the energy you need to perform.
Of course, any coach worth their salt (pun definitely intended) should be instructing athletes travelling to altitude to reign in the intensity of their training, at least for the first 5-7 days. Just being at altitude and being exposed to the hypoxic environment, places more physiological stress on the body, even before training is layered on top. So, taking it easy for the first week at altitude is a must.
What often happens is that athletes on an altitude camp see it as the perfect opportunity to ramp up their training. This can be tough to avoid, especially when training as part of a group – and can lead to something often referred to as the “training camp effect”.
The result? Athletes are exposed to an even higher total internal training load at altitude, and overall energy requirements go up as the body tries to deal with this additional stress.
In fact, one study showed that resting metabolic rate (RMR) – the energy expended by the body at rest to maintain its basic function – was around 300 kcal higher per day for athletes who spent four weeks training at Flagstaff in Arizona (~2,200m / 7,217ft) compared to those who trained in Canberra, Australia (~600m / 1,968ft).
While ~300 kcal per day might not seem like much, this equates to ~8,400 kcals over a four-week period. Not adjusting your food intake – and being in this level of energy deficit – would have a significant impact on training quality and impair adaptation.
Should you eat more when training at altitude?
We’ve established that training at altitude typically has a higher carbohydrate cost than the equivalent session at sea level.
As such, consuming carbohydrate during the session becomes even more important to help spare your liver glycogen stores and sustain your performance.
The bad news – when looking at the research – is that the ability to burn carbohydrates consumed during exercise at altitude is actually blunted by as much as 20 to 50%. This means that the higher carbohydrate costs for a session at altitude would have to be paid for by your muscle and liver glycogen stores, which could accelerate the development of fatigue.
The obvious way to address this issue is to consume more carbohydrate during the session. Pro cyclist Victor Campenaerts told us that he adds ~15 more grams of carbohydrate per hour on training rides when attending an altitude camp. This strategy should theoretically spare his liver glycogen stores to a similar degree as a lower level of fueling would during the equivalent session at sea level.
I say ‘theoretically’ here, because this additional carbohydrate will only be effective if it can also be oxidised (i.e. utilised by your body) at a higher rate, and there’s not a lot of published research into whether that’s the case yet.
As part of his preparation for the Leadville Trail 100 mountain bike event, three-time Gravel Worlds Champion, John Borstelmann, told us that he spends six weeks leading up to the race sleeping and training at altitude. Becoming acclimatised to the thin air is critical for anyone racing at altitude – mainly to benefit from the increase in haemoglobin mass that comes with altitude exposure.
From a fueling perspective, this period of acclimatisation is important too. Like we talked about, altitude can reduce your ability to burn carbohydrate ingested during exercise, but consuming more carbohydrate may help overcome this.
When you're racing, you're already pushing the envelope in terms of carbohydrate consumption. As an example, John Borstelmann has consistently been consuming carbohydrate at a rate of more than 100g/h in all of his 100-mile plus events – right at the top of the range we would typically advise - since 2023. This is also toward the top end of the range that the body can effectively use – at least at sea level.
So, is John wasting his time fueling at such a high rate at altitude? Thankfully no. Research tells us that three-to-four weeks of acclimatisation will ‘rescue’ our carbohydrate burning ability, meaning athletes can employ their well-practised ‘sea level’ fueling strategy when competing at altitude – provided they have spent time acclimatising first.
How to fuel at altitude
If you are lucky enough to be heading to altitude for training or racing, here are our key tips to help you perform at your best in thinner air:
- Consume more carbs in your day-to-day diet than you would at sea level, particularly before longer and/or harder sessions. The aim is to make sure your muscle and liver glycogen stores are better 'stocked' before starting the day's training.
- Increase your carb intake during sessions themselves. Particularly for racing, aiming towards the top of the carb intake range - provided you have trained your gut to tolerate more carbs - is likely to be a good strategy.
- Opt for more carbohydrates post-session to replenish your muscle glycogen stores.
If you’re heading to altitude, book a free one-to-one video consultation with our Athlete Support Team and we can help you plan a strategy for fueling your next altitude training camp or competition.
