Most people who exercise or work in the heat are aware about heat stress and dehydration. Over-hydration - hyponatremia (also known a water poisoning or intoxication) - is also a risk. While dehydration is more common, over-hydration is more dangerous.
The first paper from Asia on exercise associated hyponatremia (EAH) was published in 2011 by a Singaporean team. It described cases of EAH from an ultra-marathon. Since then the body of studies from Asia and around the world has grown.
In this article we present:
Official sports safety guidance (2019) from Sports Singapore
Information from the International Scientific Consensus Statement 2015 on EAH
Links to these and other studies are at the bottom of page.
Please note, these resources are shared for information purposes only. Always exercise within your limits. If you are considering changing your diet, exercise or lifestyle practices, discuss plans with your primary medical practitioner before making any changes.
What is Hyponatremia?
Maintaining a balance between sodium and potassium in the bloodstream is crucial for overall health and is intricately linked to the body's total water content. Hyponatremia refers to the condition of having lower-than-normal blood sodium levels.
Excessive water consumption results in the dilution of sodium (i.e. an abnormal drop in blood sodium concentration) and subsequent swelling of cells, particularly those in the brain.
Symptoms
EAH, in its early stages, can be asymptomatic. Potentially confusing is that early symptoms of hyponatremia may overlap with those of dehydration or heat stress.
Mild
Lightheadedness, dizziness, nausea, bloating, weight gain
Moderate
Vomiting, headache, confusion
Severe
Hallucinations, seizures, coma, respiratory arrest
Risk and Prevention
Key risk factors involved in EAH are:
Over-drinking of water
Over-drinking of sports beverages and other hypotonic beverages i.e. those that contain a lower concentration of salt and sugar than the human body
Weight gain during exercise
Exercise duration >4 hours
Being female
Event inexperience or inadequate training
Slow running, or slow performance pace
The use of non-steroidal anti-inflammatory drugs (NSAIDs), for example, aspirin, ibuprofen
High or low body mass index
Readily available fluids
"The single most important risk factor is sustained, excessive fluid (water, sports drinks or other hypotonic fluids) intake in volumes greater than loss through sweat, respiratory and renal water excretion so that a positive fluid balance accrues over time."(1)
Drinking according to thirst (not drinking 'beyond thirst') is the most prudent and individualised strategy that can be used.
Be aware that "high fluid intakes will not necessarily prevent exercise-associated maladies such as muscle cramps or exertional heat stroke"(1) Remaining cool and hydration, the ability to sweat, are linked but having too much water does not keep you cooler.
In addition:
Know your personal level of hydration. The simplest method is to monitor your urine colour. If you are peeing 'clear' or 'straw yellow', you are hydrated.
Avoid going into any event dehydrated and feeling that you have to 'catch up' with hydration.
Understand your sweat rate (loss of sweat per hour) to create a personalised hydration strategy.
Regarding salt intake, eating salty snacks and adding extra salt to the diet during the event period. Note "while modest salt replacement is likely not harmful... it will be of modest to no benefit in situations where excess fluids are being consumed."(1)
A Note on Sports Beverages
A hypotonic drink has a lower concentration of solutes (such as salts and sugars) compared to the body's fluids, allowing for rapid hydration. An isotonic drink has a similar concentration of solutes to the body's fluids, facilitating efficient hydration and replenishment of electrolytes.
For hydration and to avoid EAH, an isotonic drink might be preferable. Isotonic drinks help mitigate this risk by replenishing electrolytes along with fluids, thereby supporting hydration without diluting sodium levels in the body. Notwithstanding the potential benefit of isotonic drinks, avoiding over-hydration is key to avoiding EAH.
Sport Singapore, The Sports Safety Committee Report 2019
Sport Singapore is the official government sports body in Singapore. The following is an excerpt from the The Sports Safety Committee Report 2019 information on hyponatremia. See full report here.
Page 78...
7.2.6 Event organisers should also provide athletes with advice on the risks of over-hydration leading to water intoxication.
Fact Box: Hydration. Hydration status can contribute to regulation of body temperature during intense exercise. When prolonged and without sufficient fluid replacement, the accompanying hypovolemia impairs shunting of blood to the skin thereby curtailing convective and radiative heat transfer to the external environment. Evaporative heat loss accounts for about 80% of heat dissipation during exercise. Therefore the more detrimental effect of severe dehydration is the decrease in sweat rate and volume, which directly limits evaporate heat loss.
a) It is important for athletes to understand how hydration status is determined by their own behaviour before, during and after participation in moderate to vigorous physical activity. High performance athletes consciously allow their bodies to encroach on dehydration in time and distance limited events in order to economise on time lost through drinking. Inexperienced athletes might face difficulties gauging and planning their own hydration needs.
b) Over-hydration occurs in athletes when the supply of fluid exceeds the body’s current needs and storage capacity. Excess fluid intake is closely linked with voluntary hyponatremia or water intoxication. There is consensus that it is normal for athletes to be dehydrated for up to 3% of body mass loss during the activity without health implications. The degree of dehydration that can be tolerated is highly individualized and athletes should monitor their body weight changes before and after exercise to track their state of fluid balance. Each kilogram of body weight loss after training is equivalent to one litre of sweat loss (or fluid deficit).
c) Athletes should drink enough water during exercise to prevent a deficit of 3% of body mass (e.g., 1.8 L of water for a 60kg person). Using this equation, athletes should measure their pre- and post-training body mass during routine training and develop an individual hydration plan for use during races and competition events. A fluid deficit of more than 3% of body mass may compromise physical performance (Cotter et al., 2014) (Glazer, 2005). Athletes should aim to maintain a level of hydration that is optimal for performance and safety.
Fact Box: Hyponatremia. Hyponatremia refers to low sodium concentration in the blood, due mainly to over drinking of water during exercise. Hyponatremia can lead to serious health consequences and fatality, including cerebral edema, pulmonary edema, coma and death.
The incidence of hyponatremia ranges from 0% (in New Zealand and South Africa) up to 13% (in USA) and is associated with excessive fluid intake habits. Gaining weight during exercise as a result of fluid intake is a risk factor for hyponatremia. Cases of hyponatremia in local athletes have been seen in endurance running events (Tan et al., 2016) (Tan et al., 2016) (Tan et al., 2017) (Lee et al., 2011). Generally, if participants drink according to a regime, the rate of hyponatraemia appears to be higher, whereas if they drink according to their thirst, the rate of hyponatraemia appears to be lower (HewButler et al., 2006). Signs and symptoms of acute hyponatremia overlap with those of exertional heat stroke.
EAH is less well known a condition amongst the public (and many sports people) than dehydration. In hot and humid conditions, keeping cool (as conditions may permit) is a valuable tool to mitigate the risks of over-hydration. For a comprehensive list of scientific studies that relate to hyponatremia, learn more here.
If heat stress and hydration is your interest, consider learning about (and following on socials) Singapore's own Heat Resilience & Performance Centre at the Yong Loo Yin School of Medicine, NUS.
For those exercising out in the heat, understanding and managing water intake requires knowledge and experience. Scroll down to access additional studies and resources on hyponatremia as well as exercise and performance in tropical climates.
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Related Studies
For in-depth collections of studies, see here...
(1) Consensus Statement
Hew-Butler T, Rosner MH, Fowkes-Godek S, Dugas JP, Hoffman MD, Lewis DP, Maughan RJ, Miller KC, Montain SJ, Rehrer NJ, Roberts WO, Rogers IR, Siegel AJ, Stuempfle KJ, Winger JM, Verbalis JG. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clin J Sport Med. 2015 Jul;25(4):303-20. doi: 10.1097/JSM.0000000000000221. PMID: 26102445.
Other Studies
(2) Hew-Butler T, Loi V, Pani A, Rosner MH. Exercise-Associated Hyponatremia: 2017 Update. Front Med (Lausanne). 2017 Mar 3;4:21. doi: 10.3389/fmed.2017.00021. PMID: 28316971; PMCID: PMC5334560.
Surapongchai J, Saengsirisuwan V, Rollo I, Randell RK, Nithitsuttibuta K, Sainiyom P, Leow CHW, Lee JKW. Hydration Status, Fluid Intake, Sweat Rate, and Sweat Sodium Concentration in Recreational Tropical Native Runners. Nutrients. 2021 Apr 20;13(4):1374. doi: 10.3390/nu13041374. PMID: 33923890; PMCID: PMC8072971.
Nolte HW, Nolte K, Hew-Butler T. Ad libitum water consumption prevents exercise-associated hyponatremia and protects against dehydration in soldiers performing a 40-km route-march. Mil Med Res. 2019 Jan 25;6(1):1. doi: 10.1186/s40779-019-0192-y. PMID: 30678725; PMCID: PMC6346586.
Lee JK, Nio AQ, Ang WH, Johnson C, Aziz AR, Lim CL, Hew-Butler T. First reported cases of exercise-associated hyponatremia in Asia. Int J Sports Med. 2011 Apr;32(4):297-302. doi: 10.1055/s-0030-1269929. Epub 2011 Jan 26. PMID: 21271498.
American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007 Feb;39(2):377-90. doi: 10.1249/mss.0b013e31802ca597. PMID: 17277604.
Other
Weather and Heat Stress. Singapore provides live heat stress information, stay up-to-date here.
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