Racing in the heat

 In Nutrition & Hydration, Running training

Its been just over a month since I collapsed at the Great North Walk 100 miler. Recovery was relatively quick thankfully and I have spent the last few weeks researching all I can on what happened to me and how to prevent it happening again. I have also heard from many other runners who have experienced the same type of symptoms – echoing in the ears, dizziness, breathlessness, cramping, tingling in my face and hands and extreme fatigue.

So if you have experienced any of those symptoms in training or racing read on and I’ll go through why you experience them and how to prevent them.

The echoing in the ears I experienced started off very mild and gradually increased to the point where I couldn’t hear anyone talking to me. This was due primarily to low blood pressure. The brain simply wasnt getting enough oxygen which is why my breathing rate was high. It is also why as soon as I placed me head lower than my heart I felt so much better as it allowed blood to get to my brain. If the brain isnt getting enough oxygen its very reluctant to allow the legs to have oxygen to run. The brains demands come first. So thats why anytime my legs demanded more oxygen such us running uphill the brain made it just about impossible to do so. Running downhill was possible due to far less oxygen being needed by the muscles.

So why was my blood pressure so low? From my research there are a number of factors that can cause this

1. Resting blood pressure

My resting blood pressure is around 95-105/ 60-65, which is on the low end of the spectrum. If it drops below this for some reason then there is not far to go before trouble occurs. Exercise normally raises blood pressure so having a low blood pressure isnt usually a problem but in certain conditions it can be.

2. Dehydration

By checkpoint 2 (51km) of the race despite drinking around 1 litre per hour I had lost 2.8kg or 4.3% of my body weight . This isn’t extreme and marathon runners have been observed to have lost almost 10% of their body weight by the finish without any detrimental affects. But dehydration will lower blood pressure since it lowers blood volume and when your blood pressure is low to start with it doesnt leave much of a margin for error.

The other concern for me was that unlike the marathon runner I still had another 123km to run. My rate of fluid loss clearly wasn’t sustainable for another 123km .

The doctor who assessed me after I had withdrawn from the race suggested I was dehydrated but not extremely so. I didn’t go to the toilet between just before race start and around 6pm that afternoon so dehydration was certainly an issue but it wasn’t the main one.

3. Heat

One of the bodies response to heat is to dilate blood vessels. This helps bring more blood to the surface of the skin to aid heat loss. The negative side to this is that if blood vessels are dilated then the heart has to work harder to pump blood around. Since the blood vessels are larger blood pressure must drop or heart rate must increase to maintain the same pressure. In my case my heart rate wasn’t able to increase enough to offset the increase in blood vessel size and hence my blood pressure dropped.

4. Electrolytes and cramping

I was cramping in most of the muscles of my legs by the time I collapsed which many would point to low electrolyte levels as the cause. However research has shown people suffering cramp in an endurance event rarely have low sodium levels , often their sodium levels are higher than non crampers. (read Tim Noakes book Waterlogged for a comprehensive review on the topic)

Does this mean we shouldn’t bother taking electrolytes ? There is overwhelming anecdotal reports that taking electrolytes reduces incidence of cramp – we just don’t know why. I was taking them on a regular basis throughout the run and upped the intake as my body started to shut down in an effort to resurrect it.

Neuromuscular fatigue is the latest theory on why we cramp.For more on cramp see here. Running faster than we are accustomed to is a main cause of cramp so maybe I was running too fast for the conditions. The pace was very comfortable by training standards and I deliberately focussed on not trying to run with anyone or keep up with anyone for that very reason. However in those conditions maybe it was too fast.

5. Heatstroke

I was diagnosed with heat stroke by the doctor which upon investigation appears to be wrong. Heat stroke is usually caused by an underlying pathology that influences either the bodies ability to dissipate heat or causes excess heat production. My symptoms weren’t that severe and after a cold drink and some ice water poured over me I gradually recovered. So I was probably suffering heat illness rather than heat stroke. Heat stroke seems very unlikely at the speeds I was running.

To give you an example in 30 degree weather a 100kg man running at 4 hour marathon pace could run almost 8 hours before his body temp got to heat stroke levels. If he weighed only 85kg then running at 4 hour pace in 30 degrees he would never be able to generate enough heat to succumb to heat stroke (figures from

Conditions were a lot hotter than 30 degrees on race day but I weigh 20kg less and the running speed would have varied between 3.30 marathon pace to 7 hour pace depending on the terrain so its unlikely that running too fast could have caused heat stroke.

Was my speed too fast for the conditions and that caused my body temperature to rise high enough to cause heat illness?

I don’t believe its that possible for the speed I was running at to be the main contributor to my collapse.

It is true we run slower in the heat but the intensity we run at is the same. So it feels like we are running the same pace but due to more blood being diverted to the skin less can be used by the muscles and therefore we run slower. If we try to run the same pace our heart rates will be higher. I had no set pace I was trying to run, I was running by feel and was making an effort to keep the perceived effort level low. It may however caused me to sweat more and contributed more to dehydration.

6. Heat Acclimatisation

This is a crucial aspect in performing well in the heat and one that is hard to plan for. Heat acclimatisation takes around 7-10 days training an hour day which doesn’t sound like much but the problem is it only lasts 1-3 weeks so if you train in the heat in September and then October is cooler then you lose all that effect come race day in November.

I trained in the middle of the day many times throughout Sydney’s hot September and managed a 8 hour training run 3 weeks before the race in 30 degree weather so thought that would have been sufficient in terms of acclimatisation. But 40 degrees is a lot hotter than 30 degrees!

7. Genetics

I have had a tendency to pass out when standing in hot conditions since early school days. At school assemblies on hot days I’d often have to sneak off, sit down and put my head between my knees to stop myself passing out – much the same as I had to do during the GNW. It looks as if I am genetically unsuited for the heat.

However when training for the Ironman I have run and biked in 40+ degree heat and always fared better than almost everyone else. Having an low body fat helps with heat dissipation and I always took the opportunity to train in the middle of the day in the hottest conditions.

So why did I suffer at GNW?

The main difference between and Ironman and Ultramarathon is that in an Ironman there are aid stations every 2km. So every 2km you have access to ice cold water. At GNW the gap between checkpoint 1 and 2 was over 2 hours and between 2 and 3 is almost 5 hours.

To last 5 hours with no access to cold water in 40 degree heat is a lot more difficult.

Mistakes I made

1. Slightly dehydrated
2. Pushed to fast when my body was giving me warning signs I should slow down
3. Didn’t spend enough time at checkpoints reducing body temperature
4. No access to cold water other than at checkpoints
5. Heat acclimatisation could have been better.

How to avoid heat illness

If you are competing or training in extreme heat learn from my mistakes and do the following

1. Listen to your body and at the first sign of problems take action. In between checkpoints the only action you may be able to take is to slow down but thats better than nothing. Don’t push on sticking to race plan if you are getting signals that something is wrong.

2. At checkpoints spend more time bringing your body temperature down. Ice bath, ice cold water etc. Don’t be in a rush to leave the checkpoint until body temp is down. If you haven’t properly cooled yourself it is likely body temp will very quickly rise not long after you leave the checkpoint.

3. Take frozen water bottles as well as cold drinking water at each checkpoint. This will allow you access to cold drinking water several hours later.

4. Stop and rest at the top of any big climbs to help bring body temperature back down.

5. Don’t force yourself to run uphills when the body clearly doesn’t want to. It’s most likely not you being mentally soft, its the body trying to protect itself.

6. Make a conscious effort on the last two weeks to get out in the middle of the day to aid heat acclimatisation

Recent Posts
Showing 10 comments
  • charles hew

    hi Andy ,
    Being a doctor , i fully support what you have written and would say that you had heat exhaustion , with your relatively quick recovery once you stopped and instated cooling procedures yourself rather than heat stroke which requires hospitalisation etc with invasive medical treatments.
    Your core body temperature would certainly gives us the diagnosis as in heat stroke it is significantly elevated.


  • kirsty

    Well said Andy. Kirsty (the doctor)

    • Andy DuBois

      Thanks for your help post race Kirsty – much appreciated

  • Nick

    I remember reading an interview with Rob Krar after his second place finish at Western States this year. At the time it was twice as far as he had ever run and the weather was very hot. At every checkpoint his crew gave him an ice-filled bandanna which he put around his neck, and he also filled his cap with ice. At every stream they crossed he would fully submerge himself. All these things played a big part in keeping his core body temperature down.
    Women’s winner Pam Smith raided her mum’s goodwill bin for a plain cotton t-shirt, which stayed damp, rather than wicking away the moisture as tech-style shirts do. This helped with a constant evaporative cooling effect.

    • Andy DuBois

      Thanks for the comments Nick

      It is something I’ll be taking into consideration into any other hot races I do – my top was bone dry within seconds of pouring water over me. Oh how i wished for a wet top – moisture wicking clothes not that good in hot conditions!

  • Wolfgang

    Hi Andy,

    Glad to hear you didn’t do any harm to your body and are feeling better. Thanks for the – as always – great article.

    Merry Christmas and all the best for 2014 !

    Wolfgang (from Austria)

    • Andy DuBois

      Thanks Wolfgang

  • Scott Enfield

    I agree with your analysis after conking out at 134km. Following my own research I would like to include that I feel I took too many calories early on in the race. This mean blood was diverted from the extremities including the brain as the blood was used by the gut to process the food. Thank you for the insight Andy. Scott

  • Jeff

    A question, Andy: In retrospect, do you think it would have been beneficial to use a waist pack hydration system and/or handhelds, instead of your backpack? I find my Salomon pack to be extremely hot because it covers both my chest and my back, and as a result, I do everything I can to avoid it on extremely hot days. I know it makes me less efficient to use handhelds, but I think I’m more comfortable and faster as a result. Any thoughts?



    • Andy DuBois

      Good question Jeff. I use a ultraspire pack which is a little smaller than the Salomon ones and covers less of the body but yes it would certainly reduce the effectiveness of heat dissipation.

      A waist pack probably wouldnt have been big enough for the essential kit

      If it is hot and you dont have to carry that much then what you lose in efficiency with hand helds may be offset by the extra cooling benefit of not wearing vest

      Thanks for your comment

Leave a Comment