Do you know what happens to the human body at altitude?

My business partner, John Bolton, is currently taking on the mammoth task of cycling the Great Divide to raise funds for Laois Hospice. This challenge will take him 35 days and involves 4,500km of crazy mountain biking (self-supported) across the Rocky Mountains starting in Canada and finishing in Mexico. John is currently on day 12 and doing absolutely fantastic. He posted a video shortly after his first day to say he was ‘off form’ and ‘struggling a bit’ which is very unusual for John and I think people were surprised to hear this. This is what prompted me to write this post.

Day 1

John is currently cycling ~150km per day (~15hours of cycling) on his heavy mountain bike that is also weighed down with gear, in extreme weather conditions, and across very rough terrain. A tough task on flat ground! However, in addition to this he is also doing numerous climbs of 5,000-8,000ft every day. This is where he encounters the additional stress of altitude. John has prepared expertly for this event in terms of training, nutrition, equipment, logistics, planning etc., but the one thing he could not prepare for while training in Ireland is altitude!! There are altitude tents etc. available in Ireland that simulate these conditions, and we discussed John training in them, but we couldn’t manage to integrate this into his schedule. John’s working day in True Fitness generally starts at 7am and finishes at 10pm Mondays to Fridays, and he also works a half day on Saturdays doing personal training. Over the last 12 months John trained for 1-2 hours most week days in between clients and classes, and this training consisted of a variety of TRX sessions and cycling in different training zones. At the weekend he cycled up to 20 hours on average. With this schedule there was literally no time to travel around Ireland visiting simulated altitude facilities, so we decided to focus on all of the other elements that needed to be improved to help him cope with the altitude when he got there.

What components of fitness does John need for this event?

The first thing we do when preparing for any event is we think about the components of fitness that are required specifically to complete the event. The components of fitness required for an 800m race are very different to those required for a marathon or a triathlon or an ultra endurance event. Some of the components of fitness that John’s needs for example are a high VO2max (cardiovascular fitness), a high lactate threshold (the cycle pace he can sustain before building up too much lactate which causes all sorts of problems in the body), muscular endurance (ability of the muscles in his body to repeat an action over a long period of time / cycle for 15 hours), and strength endurance (particularly useful when cycling up massive climbs or dragging the bike up mountains!). Once we determine these we put a plan in place to train them.

Martyn 5

(i)                  VO2max

When we exercise we take in oxygen from the air we breathe, this transfers from our lungs to our blood, our blood carries it to our muscles, and then our muscles extract it from our blood and use it to produce energy to support the exercise that is being done. The maximum amount of oxygen that we can take in and use to produce energy is called our VO2max, and the greater this is the fitter we are, so the more exercise / work we can do. A persons VO2max depends on two things (i) blood flow (and thus oxygen delivery) to the muscles and (ii) the ability of the muscle to extract the oxygen from the blood and use it in very special cells called mitochondria to produce energy. In relation to John, we designed his training programme to increase the functionality of all of his organs, tissues, blood and cells involved in this process (lungs, muscles, heart, blood volume, red blood cells, mitochondria, aerobic enzymes etc.) to improve his VO2max so that it would be sufficient to cope with the task of cycling the Great Divide. This was achieved with TRX training and cycling in different training zones.

(ii)                Lactate threshold

When you exercise your body needs fuel for energy. You have two sources of fuel (i) carbohydrates and (ii) fats. We use fat for energy at rest and in low intensity exercise e.g. walking. As the intensity increases e.g. jogging, the body starts to use a mixture of both fuels for energy, and at high intensity exercise we use predominantly carbohydrates. The more aerobically trained you are the more you can use fat for fuel at higher intensities of exercise compared to untrained individuals. The thing is we have plenty of fat stores in our body but only small carbohydrates stores. Even a very lean individual with only 15% body fat could potentially run 40 marathons back to back if they could utilize all of their fat stores for energy production. However, we only have enough carbohydrate stores to work at race pace for 2-3 hours. Once your carbohydrates stores become depleted this coincides with fatigue. For this reason, if you are involved in endurance running or cycling you ideally want to train your body to be able to use your fat stores for fuel, and spare your limited carbohydrate stores, and this is exactly what we did with John. When you break down carbohydrates one of the bi-products is lactic acid. If you have a very good level of aerobic conditioning, the lactate you produce can be cleared away and metabolised so that it doesn’t build up. The point at which lactate starts to build up is called your lactate threshold, and once this happens it causes all sorts of problems in your body e.g. impaired muscle contraction. If you are involved in endurance events you need a very high lactate threshold so you can work comfortably at a high percentage of your VO2max / good pace for a long period of time without your body being under too much stress or eating up your limited carbohydrate stores. This was a major focus for us in John’s preparation. For more info on lactate threshold testing and training see . We tested John’s lactate threshold periodically over 12 months, and used the data collected from this test to calculate training zones for his cycle sessions. 


(iii)               Muscular strength and endurance

This is very important in endurance events as it helps you to sustain repeated contractions by the same muscles over a long period of time. If you need to cycle for 15 hours per day for 35 days up and down mountains like John does, then you need to have lots of this! This was achieved in his TRX sessions, and the different types of cycle sessions that he did e.g. long cycles, hill sessions, tempo sessions, aerobic interval sessions.

TRX pic

 Back to altitude! So what is it?

Moderate altitude is considered ~1,500-3,000m above sea level, high altitude is ~3,000-4,500m above sea level, and extreme altitude is >5,400m above sea level.

Day 2

Exercise at altitude is very difficult because the human body experiences arterial hypoxemia, which basically means low oxygen content in blood. The amount of oxygen present in the air at sea level (20.93%) is the same as the amount of oxygen present in air at altitude (20.93%), but the pressure in the atmosphere is different, it is lower at altitude, and the higher you go the lower it is. The consequence of this is that oxygen transfer from the air to the tissues of the body is lower, and oxygen saturation in blood is lower. This is called ‘hypoxia’. In this situation, the body cannot produce energy the way it did. A persons VO2max and lactate threshold is lower at altitude than it is at sea level. This causes a whole host of responses in the body to compensate. Other issues (among many!) that occur are things like increased water loss from the body due to elevated respiration (breathing). This is called respiratory water loss. It could reach up to 2L per day (males) in moderate altitude (2,500-4,300m)! This is combined with extra water loss in the form of urine, which increases the chances of dehydration. Exercise at altitude is extremely challenging!

What negative (challenging) adaptations occur in the human body at altitude?

* We have to breath quicker and harder

* The body dehydrates quicker

* Exercise feels a lot tougher

* At the start of the event you could also be suffering from jet lag and    travel fatigue

* Your energy stores in your muscles (glycogen) deplete quicker

* Your blood volume decreases

* Your blood flow decreases

* Increased risk of sunburn

* Acute mountain sickness

* Sleep disturbances

* Suppression of your immune system

So, John would have experienced a mixture of these issues in the first 5-7 days of his Rocky Mountain Cycle which is why he was ‘off form’. It is very difficult for us to comprehend what this feels like when we are sitting at sea level, but I can absolutely guarantee you that cycling up the climbs that John has done so far is some feat of human strength, endurance, and determination. The great news is that our body adapts very quickly to whatever we do to it. So, after the first week John’s body has started to adapt.

What positive (helpful) physiological adaptations occur in the human body at altitude that will help John?

* ↑ haemoglobin which carries oxygen around the body

* ↑ movement of fat from fat tissue which is used as fuel to produce energy for exercise

* ↑ activity of enzymes that help the body produce energy aerobically↑ size and number of mitochondria (the structures that produce energy)

* ↑ amount of capillaries which are the little blood vessels that carry oxygen and nutrients to tissues in the body and remove waste products from same 

John is currently in this phase of adaptation and by the time he finishes his Rocky Mountain Cycle he will be absolutely phenomenal in physiological terms. I think I will give it 6 months before I do a training session with him when he comes home!

I hope this sheds some light on all of the amazing things that happen in your body with training and exercise in altitude. This whole topic is of course far more complex but this should give you an idea of some of the things John is going through at the moment.

Thanks for taking the time to read this post 😊.

Yours in health,

Dr Diane Cooper.

We use cookies to improve our services and to enhance your browsing experience. Your continued browsing will mean that you accept the use of cookies. Click here to view our Cookies Policy

The cookie settings on this website are set to "allow cookies" to give you the best browsing experience possible. If you continue to use this website without changing your cookie settings or you click "Accept" below then you are consenting to this.