Skip to main content
The British Journal of General Practice logoLink to The British Journal of General Practice
. 2006 Jul 1;56(528):542–545.

Too much of a good thing?

The danger of water intoxication in endurance sports

Angus H N Whitfield
PMCID: PMC1872071  PMID: 16834886

INTRODUCTION

The deaths of four runners in the 2005 Great North Run alerted the public to the potential dangers of distance running. In the immediate aftermath, the Sports Minister recommended that runners be advised to ‘drink more water’.1 These comments would appear to be extremely sensible, and it is likely that most people seeking advice prior to participating in such an event would be encouraged to ‘drink plenty of fluid’. It is therefore a seeming paradox that the medical director for the 2005 New York City marathon went to extraordinary lengths to advise participants to limit their fluid intake to no more than 750 ml per hour.2 The dangers of water intoxication are well known among sports physicians, and are starting to become recognised by ultra-endurance athletes (competing in ultra-marathons or Ironman-distance triathlons), even if there is doubt in some lay minds about the cause. However, the message does not appear to be strong enough to re-educate a public that is given out-dated advice in magazines, and is bombarded with advertising from manufacturers of sports drinks, all stressing the importance of avoiding dehydration.

GPs are ideally placed to help educate participants in endurance events about the dangers of hyponatraemia of exercise. The London Marathon is the largest annual fundraising event in the UK. One-third of the available entries are allocated to charities and, as a result, each year a significant number of first-time runners seek medical clearance before they begin training. The slower novice or ‘amateur–enthusiast’ is at greater risk of developing hyponatraemia than the competitive athlete, and this is because they are prone to consuming too much fluid. Prevention of hyponatraemia rests on education about the dangers of excessive fluid intake. However, this is unlikely to be effective on its own without correcting the widely held belief that dehydration is the greatest danger faced by endurance athletes.

Endurance athletes are generally advised to consume enough fluids to prevent any degree of dehydration. This stems from the beliefs that dehydration is the greatest risk to health and performance during exercise, that the sensation of thirst is a late indicator of dehydration, and that all of the fluid lost during exercise must be replaced and can be done so without consequence. The scientific evidence does not support this position.

DISPELLING THE FEAR OF DEHYDRATION

Until the late 1960s, distance runners were actively discouraged from consuming fluids, partly for fear of gastrointestinal disturbance. Even so, it was possible to complete a full marathon (26.2 miles) in times that are still impressive.3

However, the focus of dehydration as the predominant problem faced by distance runners can be traced back to a 1969 paper entitled ‘Danger of an inadequate water intake during marathon running’,4 even though marathon runners were not actually studied. The report showed that the fastest runners were the most dehydrated, but concentrated on the correlation between dehydration and core temperature, and not on performance. The erroneous conclusion that has persisted from this study is that fluid restriction leads to progressive dehydration, which in turn leads to heatstroke and potential collapse. However, this conclusion is completely at odds with the repeated observation that winners of marathons are among the most dehydrated and the most hyperthermic.5 Over the course of a marathon and despite available fluids, elite runners will become voluntarily dehydrated, in part, because it is not possible to consume fluid in large volumes at competitive running intensity.6 Dehydration by 5% is common in these runners, and voluntary dehydration during exercise has been observed elsewhere in those with free access to fluids.7 Exercise is only terminated at much higher degrees of dehydration, and fluid resuscitation provides rapid recovery.

FLUID INTAKE DURING EXERCISE

Taken together, the above observations indicate that it is entirely possible to run a marathon at competitive intensity without consuming any fluids (or by consuming a limited volume), and that this will obviously result in a degree of dehydration. However, fluid consumption during prolonged exercise has been repeatedly shown to improve performance8 beyond what can be achieved without fluids. Conversely, progressive dehydration leads to a multitude of physiological disturbances, including increased heart rate and core temperature, reduced cardiac output and stroke volume, and reduced blood flow to the skin and exercising muscles—all in proportion to the degree of dehydration.

The misinterpretation of the 1969 paper (as above) has led to the recommendation that athletes achieve 100% fluid replacement, the current position stand of the American College of Sports Medicine (ACSM).9 Athletes are advised to drink the maximum amount that can be tolerated, or 600–1200 ml per hour.

In terms of performance, this recommendation does not stand up to experimental testing. Repeated studies have shown that, over prolonged (>2 hours) exercise, 100% fluid replacement does not improve performance compared to consumption ad liberatum.8 At competitive racing intensity, 100% fluid replacement often leads to gastrointestinal distress.6

The second scientific criticism of this recommendation is based on the incorrect assumption that fluid losses account for all weight loss during prolonged exercise. The predominant fuels for endurance sports are glycogen (from muscle and liver) and fat. Oxidation of these in the tricarboxylic acid (Krebbs') cycle leads to the production of ‘metabolic’ water. This is lost as sweat, but does not require replacing. Additionally, glycogen is stored in association with water, which is released when the glycogen is broken down. Again, this water is lost as sweat but does not require replacement. The total of this non-replacement water has been calculated as being up to 2 kg over a standard marathon,10 and higher in longer events. This means that a runner who finishes a marathon at the same weight as at the start is overhydrated by as much as 2 litres.

The final criticism of this advice is that it assumes no danger from large volume fluid intake. This, too, is erroneous.

WATER INTOXICATION AND HYPONATRAEMIA OF EXERCISE

Hyponatraemia of exercise has generally been considered a rare event in endurance sports. However, the morbidity and mortality from this condition has been well reported in the medical literature.3 While the mild form may be asymptomatic, severe hyponatraemia causes confusion, seizures and death. In contrast, there does not appear to be a single report of the death of an athlete in which dehydration was the clear cause.

A recent prospective study of runners in the Boston Marathon11 revealed a 13% prevalence of hyponatraemia (serum sodium <135 mmol/L) and a 0.6% incidence of critical hyponatraemia (serum sodium <120 mmol/L) in race finishers. Equating this to the London Marathon: of a finishing field of 33 000, 4290 would have mild hyponatraemia and 198 would suffer from critical hyponatraemia. Obviously, this figure does not include those unable to complete the event. The Boston study also shed interesting light on the risk factors for this condition: slow (over 4 hours) race time, weight gain and body mass extremes, all of which confirmed the prevailing theories arising from previous case studies. Of interest was the lack of any increased risk from drinking water compared to sports drinks.

Hyponatraemia of exercise is caused primarily by excessive fluid intake.12 In ultra-endurance events such as ultra-marathons and Ironman triathlons, the situation becomes somewhat more complicated. Since the body's sodium levels determine water content, the continued loss of sodium in sweat can lead to a progressive loss of fluid reserves. In such ultra-endurance events, an adequate sodium intake is therefore necessary to correct dehydration without causing a dilutional hyponatraemia. Despite this, excessive fluid intake remains the predominant cause of hyponatraemia in all endurance events.

This aetiology explains the significance of weight gain as a risk factor in the Boston study. As explained above, competitive runners are unable to consume excessive volumes of fluid. However, high volume fluid consumption is relatively easy in slower runners, especially if they adopt a run-walk strategy and walk through the drinks stations. The problem is then made even more likely by the presence of drinks stations at almost every mile in most marathons. But because of their lower running intensity and lower sweat rates, these are the very runners who do not need to consume fluid in large volumes.

Sports drinks certainly lead to increased fluid absorption in the intestine (due to co-absorption of carbohydrate), and provide a source of carbohydrate that may help to prevent the phenomenon of ‘hitting the wall’. Formulas containing sodium (almost all at lower concentration than found in sweat) do help to limit the fall in serum sodium, in part by allowing maintenance of urine output.13 In theory, sports drinks should help to prevent hyponatraemia, although this is unproven.

CONCLUSION AND RECOMMENDATIONS

Hyponatraemia of exercise is a well-recognised condition and a recent report has indicated the incidence during a large city marathon race, although further studies are needed to confirm this. The condition results from excessive fluid intake during prolonged exercise, which stems from erroneous advice given to athletes that their goal should be 100% fluid replacement. This recommendation is based around the belief that any degree of dehydration is highly dangerous, despite fluids having been shunned by marathon runners for many years, and today's elite runners finishing with a variable degree of dehydration. A 100% fluid replacement strategy is neither necessary for performance nor physiological, since it actually results in over-hydration. Similarly, over-hydration prior to an event should be discouraged, since this also promotes hyponatraemia. However, the reasoned scientific debate on this topic over the last 10 years has had little effect on the public, predominantly due to the influence of advertising and non-medical coaching advice in popular running and triathlon magazines.

Re-education of endurance athletes about the dangers (and ease) of over-hydration is the most important means of preventing this condition. Slower athletes are particularly at risk. In a marathon, those runners who are likely to complete the race in over 4 hours, or those likely to utilise a run-walk strategy, are at a higher risk of hyponatraemia.

A high degree of responsibility also lies with the event organisers to ensure that runners are made aware of the dangers of hyponatraemia, and how to guard against it. The efforts of the New York Marathon medical director demonstrate that it is possible to alert all participants in a marathon to this problem. Entrants in Ironman-distance triathlons are already provided with warnings about the dangers of hyponatraemia, but advice to guard against excessive intake is not listed as the predominant cause. There are even warnings about hyponatraemia at the Grand Canyon National Park in Arizona, US. However, the website for the 2006 London Marathon contains no specific information about hyponatraemia or warnings concerning excessive fluid intake.14

Athletes need to be reassured that a mild degree of weight loss over an endurance event is acceptable, whereas no weight loss represents over-hydration. Drinking according to the perception of thirst appears safe. Specific advice on hourly volume intake is ill advised, but would typically be between 400 and 800 ml per hour. The faster, larger athlete racing in hot weather will have higher requirements, whereas those for the smaller slower runner taking part in cold conditions will be lower.

From personal experience of developing hyponatraemia during ultra-endurance sports, the first objective symptom is confusion (consistent with published reports15). However, the assumption must not be that an athlete in this condition is simply dehydrated. Confused marathon/Ironman triathlon finishers should always have their weight checked, and this compared to pre-race weight (which can and should be recorded on the race number at the prerace registration formalities). While athletes who are dehydrated will respond rapidly to intravenous fluid administration, this same course of treatment would prove disastrous to an already fluidoverloaded individual.

Experts in this area of exercise medicine firmly believe that the case for 100% replacement has been satisfactorily thrown out.16,17 Unfortunately, the world's largest organisation of sports physicians has yet to issue a formal change of its recommendations and so, in the face of advertising and erroneous magazine advice, the message is not reaching many athletes, novice or experienced. It is the unfortunate amateur–enthusiasts and novices who are most likely to suffer.

Acknowledgments

The author wishes to express his gratitude to Mr Alex Billioux and Mr Hugh Whitfield for their assistance.

Competing Interests

The author has stated that there are none.

REFERENCES

  • 1.Anonymous. Four competitors die as heat hits Great North Run. The Times. 2005;19 Sept:9. [Google Scholar]
  • 2.Kolata G. Marathon; runners are warned about too much water. The New York Times. 2005;20 Oct:7. [Google Scholar]
  • 3.Noakes TD. Lore of Running. 4th edn. Champaign, IL: Human Kinetics; 2003. Temperature regulation during exercise. [Google Scholar]
  • 4.Wyndham CH, Strydom NB. The danger of an inadequate water intake during marathon running. South Afr Med J. 1969;43:893–896. [PubMed] [Google Scholar]
  • 5.Pugh LGCE, Corbett JL, Johnson RH. Rectal temperatures, weight losses and sweat rates in marathon running. J Appl Physiol. 1967;23:347–352. doi: 10.1152/jappl.1967.23.3.347. [DOI] [PubMed] [Google Scholar]
  • 6.McConnell GK, Stephens TJ, Canny BJ. Fluid ingestion does not influence intense 1-h exercise performance in a mild environment. Med Sci Sport Exerc. 1999;31:386–392. doi: 10.1097/00005768-199903000-00006. [DOI] [PubMed] [Google Scholar]
  • 7.Adolph EE. Physiology of man in the desert. New York, NY: Interscience; 1947. [Google Scholar]
  • 8.Noakes TD. Fluid replacement during exercise. Exerc Sport Sci Rev. 1993;21:297–330. [PubMed] [Google Scholar]
  • 9.American College of Sports Medicine. Position stand: Exercise and fluid replacement. Med Sci Sport Med. 1996;28:i–vii. doi: 10.1097/00005768-199610000-00045. [DOI] [PubMed] [Google Scholar]
  • 10.Pastene J, Germain M, Allevard AM, et al. Water balance during and after marathon running. Eur J Appl Physiol. 1996;73:49–55. doi: 10.1007/BF00262808. [DOI] [PubMed] [Google Scholar]
  • 11.Almond CSD, Shin AY, Fortescue EB, et al. Hyponatremia among Runners in the Boston Marathon. New Eng J Med. 2005;352:1550–1556. doi: 10.1056/NEJMoa043901. [DOI] [PubMed] [Google Scholar]
  • 12.Irving RA, Noakes TD, Buck RH, et al. Evaluation of renal function and fluid homeostasis during recovery from exercise induced hyponatraemia. J Appl Physiol. 1991;70:342–348. doi: 10.1152/jappl.1991.70.1.342. [DOI] [PubMed] [Google Scholar]
  • 13.Vrijens DM, Rehrer NJ. Sodium free fluid ingestion decreases plasma sodium during exercise in the heat. J Appl Physiol. 1999;86:1847–1851. doi: 10.1152/jappl.1999.86.6.1847. [DOI] [PubMed] [Google Scholar]
  • 14. London Marathon. www.londonmarathon.org (accessed 12 Jun 2006)
  • 15.Noakes TD, Sharwood K, Collins M, et al. The dipsomania of great distance: water intoxication in an Ironman triathlete. Br J Sport Med. 2004;38:E16. doi: 10.1136/bjsm.2002.004614. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Hew-Bulter T, Almond C, Ayus JC, et al. Consensus statement of the First International Exercise-Associated Hyponatremia Consensus Development Conference, Cape Town, South Africa 2005. Clin J Sport Med. 2005;15:208–213. doi: 10.1097/01.jsm.0000174702.23983.41. [DOI] [PubMed] [Google Scholar]
  • 17.Noakes TD. Overconsumption of fluids by athletes. BMJ. 2003;327:113–114. doi: 10.1136/bmj.327.7407.113. [DOI] [PMC free article] [PubMed] [Google Scholar]

Articles from The British Journal of General Practice are provided here courtesy of Royal College of General Practitioners

RESOURCES