What is exercise-associated hyponatremia (EAH)?
A rare but preventable condition of low serum sodium during exercise. Normal levels of sodium are 135 – 145mmol/L, mild hyponatremia <135mmol/L, moderate hyponatraemia 125 – 129mmol/L, severe hyponatremia <120mmol/L.
What are the symptoms?
Mild EAH: dizziness, lightheaded, fatigue, bloating, puffiness, mild nausea.
Moderate EAH: headache, vomiting, confusion, mood changes, dyspnoea, phantom running.
Severe EAH: convulsions, coma, seizures, frothy sputum, and worst-case scenario - death.
What causes it?
It can occur due to a range of overlapping factors, the primary cause is (1) drinking water or other hypotonic fluids in excess (this is a repeat offender). Hypotonic fluids are those that have a lower concentration of electrolytes compared to those within the body.
Other causes of EAH can also include (2) suppression of the hormone arginine vasopressin (AVP) which is a key regulator in renal water excretion. If AVP is not working appropriately, then water retention occurs. Water retention + excessive water intake = diluted serum sodium.
(3) Higher sweat sodium losses. This is highly variable between individuals. Some people have very salty sweat, the kind where you are left with gritty skin, white streaks on the side of your face/neck and can taste it when you lick your lips. Other people less so. Higher losses of sodium through sweat may contribute to EAH.
(4) Glycogen metabolism (the stored form of glucose in your liver and muscles). When we metabolise this during exercise, the by-product of this reaction is water. This water is then released within the body and may contribute to dilution of serum sodium.
(5) Prolonged sweating, i.e. the longer the event, the greater the risk.
(6) Consuming a low sodium diet.
Who is at greatest risk of EAH?
Anyone participating in endurance events e.g. triathlons, marathons, ultra-marathons, etc., people drinking copious amounts of fluid exceeding renal water excretion, people with very low body mass/very high body mass, first-timers who are competing at a slower pace/longer race times (>4 hours), and females.
More water does not equal better, in can be dangerous if consumed excessively. Mix up water consumption with electrolytes/sports drinks in endurance events.
Fluid needs are individual. Sports dietitians can work with you to help calculate your specific sweat rate and race-specific fluid needs.
It’s important to keep in mind that you also want to stay adequately hydrated, aiming to keep body weight change <2% (i.e. fluid loss). More than this will be detrimental to sports performance. Hydration is a constant balancing act.
Athletes need to have their own fluid plan matching their specific race. It’s key to consider aid stations, what the athlete can carry, individual sweat rate, race day conditions etc.
Sodium helps drink palatability and sodium replacement is useful for endurance athletes.
I typically recommend athletes consume electrolytes/sports drink for activities exceeding 60-90 minutes. For ultra-endurance events it is important to consider specific sodium replacement for some athletes (e.g. ironman).
Hew-Butler T, Loi V, Pani A and Rosner MH (2017) Exercise Associated Hyponatremia: 2017 Update. Front. Med. 4:21. doi: 10.3389/fmed.2017.00021
Burke L, Deakin V (2015). Clinical Sports Nutrition (5th edition). Australia: McGraw-Hill Education.