Adventures, Core Skills — 3 May 2021 at 8:33 pm

Myth-busting in Endurance Physiology

Dr Stuart Allan / GP / Cumbria, England

If you are interested in this article, you may be interested in the following article related to endurance sports:

Doping in Endurance Sports

Comrades Ultra-Marathon

Duraphat 5000 – The Secret Dental Elixir For Endurance Athletes

Dr Stuart Allan combines his day-job as a GP in Cumbria with teaching on the UK Diploma of Mountain Medicine, and runs…a lot! Having experienced first-hand the challenges in optimal hydration whilst completing in epic races such as the Frog Graham Round, Bob Graham Round, and the Ultra-Trail du Mont Blanc (UTMB), Stuart has learned through practice and subsequent research the truths behind hydration for optimum performance in endurance events.

Recently, I worked collaboratively with Jeremy Windsor, my faculty colleague on the UK Diploma of Mountain Medicine, as well as Edi Albert and colleagues from the Healthcare in Remote and Extreme Environments Program at the Tasmanian School of Medicine on a practical management paper discussing hydration during high altitude endurance events. It has recently been accepted for publication in the Clinical Journal of Sport Medicine. This project was hugely insightful and was excellent to carry out during lockdown.

The paper discusses the physiological changes in water homeostasis with changing altitude. Building on this it also examines the effects on poor hydration at altitude compared to sea level. We also investigated the controversial debate between “ad libitum hydration vs programmed hydration”. Both sides have their passionate supporters: read the paper to see which side our team agreed with…or maybe both have validity! We made recommendations on hydration strategies in high altitude environments and my colleague Larissa Trease will be discussing these in a follow-up article.

I am briefly going to discuss the myths around hydration at altitude in endurance events. This has been an interest of mine for many years after reading a plethora of inaccurate information on the internet, and it appears that I am not alone in my concern 1. The controversy is over a condition called exercise-induced hyponatraemia (EAH), caused by over-hydration which can and does kill athletes 2. Fluid consumption must be carefully balanced to maintain adequate hydration for athletic performance and health. For those interested in providing medical support in endurance races of any distance, practice guidelines have been published on this subject and should be essential reading before being involved in endurance events of this sort 3.

Myth 1: Maintaining body weight through drinking (euhydration) is essential to maintain performance

One study showed that half of the top 10 finishers in one 161km ultramarathon lost more than 2% body weight by 90km 4. Marty Hoffman and his colleagues also showed that athletes should ‘allow for weight loss up to 4–5 % of body weight during extended periods of exercise’ 5 ; largely due to weight loss from the energy gained from glycogen and other substrates. It is also worth mentioning however, that weight loss of up to 8% has not been shown to lead to adverse clinical or performance outcomes 5. Killian Jornet, the elite ultarunner, reportedly drank three litres of Coke and ate two Nutella and jam sandwiches in the 2008, UTMB (168 km), and won the race! This is obviously an extreme example relating to an athlete who by his own admission, has made numerous dietary-related mistakes in his sport, but it does call into question the perceived wisdom as well as the scientific basis behind hydration and performance.

The key message here is that weight loss should be expected during endurance events and should not be a yardstick for measuring performance. In conclusion, 2-4% weight loss is a reasonable measure of ‘euhydration’ given energy expenditure. With regards to altitude, it is worth reflecting that hydration status in mountaineering appears not to be related to summit success 6, and the risk of developing high altitude illness (HAI) is cut down by reducing total body water 7.

Myth 2: By the time the athlete develops thirst, it is too late and performance is affected

This is a statement I have often heard during endurance events and on sports internet sites. However, thirst is and has always been a safe and effective physiological mechanism to maintain adequate hydration 3. In our paper, our clinical recommendation is to use thirst as the driver behind an athlete’s approach to hydration – i.e the ‘drink to thirst’ hypothesis. But we are also aware that HAI may impact on an individual’s judgment and their ability to drink (e.g. confusion, nausea). In addition, access to adequate clean water supplies at altitude might also reduce the risk of over-hydration.

Myth 3: Athletes must take electrolyte supplements during exercise

There is good evidence now to support the avoidance of excessive sodium supplements in endurance races as well as their use at times of ‘high thermal stress’ 4,8. Sodium supplementation has been proven ineffective in correcting the hyponatraemia caused by overhydration 9, however that being said, sodium supplementation may be required if total body weight loss exceeds 5% 3. It has also been shown that sodium-rich electrolyte supplementation is ineffective in alleviating exercise-related muscle cramp 10. The bottom line is that excessive or indeed regimented sodium supplementation is not required.

Clinicians must also be aware the potential adverse effects on renal and salt metabolism caused by consumption of acetazolamide and anti-inflammatory medications (e.g ibuprofen) 11,12 which could subsequently have an impact on acclimatization and development of HAI.

Myth 4: Drinking regularly and without the driver of thirst (i.e. overhydration) is safe and inconsequential

This myth brings together some of the points made above. It’s sobering to realise that hyponatraemia is associated in endurance events with a 23% incidence (e.g Ironman Triathlon distances), and with a growing incidence at shorter distances 13. So, drinking beyond thirst is unnecessary and may in fact be detrimental.

Clinicians working in endurance events at high altitude should be aware that EAH has similar presenting symptoms and signs to AMS (nausea), HAPE (pleural effusion and breathlessness) and HACE (seizures, cerebral oedema). This further challenges medical assessment and decision making surrounding medical management. As always, question what you read, consider challenging perceived dogma, but above all enjoy the process of learning and adventuring!


  1. Martin D. Hoffman, Theodore L. Bross III & R. Tyler Hamilton (2016): Are we being drowned by overhydration advice on the Internet?, The Physician and Sportsmedicine, DOI: 10.1080/00913847.2016.1222853
  2. Lebus DK, Casazza GA, Hoffman MD, et al. Can changes in body mass and total body water accurately predict hyponatremia after a 161-km running race? Clinical journal of sport medicine : official journal of the Canadian Academy of Sport Medicine 2010;20(3):193-9. doi: 10.1097/JSM.0b013e3181da53ea [published Online First: 2010/05/07]
  3. Bennett BL, Hew-Butler T, Hoffman MD, et al. Wilderness Medical Society practice guidelines for treatment of exercise-associated hyponatremia. Wilderness & environmental medicine 2013;24(3):228-40. doi: 10.1016/j.wem.2013.01.011 [published Online First: 2013/04/18]
  4. Hoffman MD, Stuempfle KJ. Hydration strategies, weight change and performance in a 161 km ultramarathon. Research in sports medicine (Print) 2014;22(3):213-25. doi: 10.1080/15438627.2014.915838 [published Online First: 2014/06/21]
  5. Hoffman MD, Pasternak A, Rogers IR, et al. Medical services at ultra-endurance foot races in remote environments: medical issues and consensus guidelines. Sports Med 2014;44(8):1055-69. doi: 10.1007/s40279-014-0189-3 [published Online First: 2014/04/22]
  6. Ladd E, Shea KM, Bagley P, et al. Hydration Status as a Predictor of High-altitude Mountaineering Performance. Cureus 2016;8(12):e918. doi: 10.7759/cureus.918 [published Online First: 2017/01/14]
  7. Bärtsch P, Pfluger N, Audétat M, et al. Effects of slow ascent to 4559 M on fluid homeostasis. Aviation, space, and environmental medicine 1991;62(2):105-10. [published Online First: 1991/02/01]
  8. Hoffman MD, Hew-Butler T, Stuempfle KJ. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Med Sci Sports Exerc 2013;45(4):784-91. doi: 10.1249/MSS.0b013e31827985a8 [published Online First: 2012/11/09]
  9. Twerenbold R, Knechtle B, Kakebeeke TH, et al. Effects of different sodium concentrations in replacement fluids during prolonged exercise in women. Br J Sports Med 2003;37(4):300-3; discussion 03. doi: 10.1136/bjsm.37.4.300 [published Online First: 2003/08/02]
  10. Hamilton R, Bross T, Hoffman M. Hydration Guidelines During Exercise: What Message Is the Public Receiving? Wilderness & environmental medicine 2015;26:e3. doi: 10.1016/j.wem.2015.03.009
  11. WADA. Prohibited list [accessed 13 September 2020.
  12. Lipman GS, Krabak BJ, Waite BL, et al. A Prospective Cohort Study of Acute Kidney Injury in Multi-stage Ultramarathon Runners: The Biochemistry in Endurance Runner Study (BIERS). Research in Sports Medicine 2014;22(2):185-92.
  13. Rosner MH. EXERCISE-ASSOCIATED HYPONATREMIA. Transactions of the American Clinical and Climatological Association 2019;130:76-87. [published Online First: 2019/09/14]

Images provided by Stuart Allan and Shona Main