Karoline Bjerkeli Grøvdal

Hyponatraemia can affect endurance athletes, and is caused when excessive body water dilutes your sodium levels in the blood. This can occur when you’re taking part in prolonged (ie more than 2 hours) exercise and sweating excessively, but fail to replace the sodium lost through sweat or drink a very large volume of very low-sodium beverages such as water.1-4 Some of the symptoms you may experience are:

  • Bloating
  • Nausea
  • Headaches
  • Loss of co-ordination
  • Fatigue

Hyponatraemia can be a problem for anyone doing endurance sports but it’s more likely to affect you if you’re running a marathon for the first time. This is because it’s likely that you’re less lean, running slowly, sweating more (lose more sodium) and/or are tempted to drink a lot of very low- sodium beverages (such as water) before, during, or after an event.

There are two main reasons why excessive fluid intake can cause a problem. Firstly, because urine production decreases during exercise which limits your body’s ability to excrete excess fluids; and secondly, because sodium is also lost in sweat, it makes it easier for your body’s sodium levels to become diluted.

To combat hyponatraemia when you’re exercising, reduce the amount of low- or sodium-free fluids you’re drinking during exercise and drink sports drinks or drinks that have a higher-sodium content than plain water. As with all things, balance is key, so ensure that you do not over consume fluids, whatever they are.

  • 1Sawka 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; 39(2): 377-390.
  • 2Noakes T: Hyponatremia in distance runners: fluid and sodium balance during exercise. Curr Sports Med Rep1 :197 –207,2002.
  • 3Twerenbold R, Knechtle B, Kakebeeke TH, et al: Effects of different sodium concentrations in replacement fluids during prolonged exercise in women. Br J Sports Med37 :300 –303, 2003.
  • 4Vrijens DMJ, Rehrer NJ: Sodium-free fluid ingestion decreases plasma sodium during exercise in the heat. J Appl Physiol86 :1847 –1851,1999.

This information is not medical advice and should not replace consultation with your health care provider or nutritionist before starting a new exercise program or eating plan.