An updated position statement on the dangers of hyponatremia (also known as water intoxication) was just released (1). It once again highlights how drinking too much water or any fluids during physical activity in the hopes of preventing dehydration can be potentially fatal.
Taking in too much fluid dilutes the sodium levels in your blood, and severely low sodium levels can lead to brain swelling, seizures, coma, and death. Less severe, symptoms of hyponatremia include nausea and vomiting, headache, confusion, loss of energy and fatigue, restlessness and irritability, and muscle weakness, spasms or cramps.
Hyponatremia has become a problem in recent years following the push to stay hydrated during all exercise, but until recently has primarily been associated with marathon races and other prolonged endurance events, especially among slow participants. Their main problem is that they don’t sweat that much, but they drink at every opportunity, often to excess over many hours due to fear of getting dehydrated. More recently, though it has been reported as being a problem during half-marathons, sprint triathlons, long hikes, yoga classes, and team sport practices and games, particularly football at all levels.
Drinking sports drinks or other fluids with electrolytes in them instead of straight water can help a little bit, but hyponatremia appears to be more related to your total fluid intake, not whether or not the drinks contain some sodium in them (2). Sports drinks containing added sodium are still more dilute than what’s in your blood, and drinking any fluids in excess during exercise can lower your blood sodium levels.
This condition is almost completely preventable. The key is simply to drink only when you feel thirsty during exercise. It really isn’t necessary to stay ahead of your thirst. The small impact that slight dehydration is going to have on your performance is nothing compared to the medical emergency created by drinking too much. Listening to your “innate thirst mechanism” provides a safe and reliable guide to hydration (1).
Using thirst as your guide should not increase your risk for cramping either since dehydration may only contribute minimally to cramps, which are likely more associated with fatigue than dehydration/electrolyte losses (3). Even becoming severely dehydrated during exercise in the heat is not likely to cause muscle cramps (4).
Surprisingly, becoming dehydrated during exercise will not necessarily increase the risk of developing a heat-related illness like heat stroke either. A body mass loss of up to 3% (mostly fluid losses through sweating) was found to be tolerated by well-trained triathletes during an Ironman competition in warm conditions without any evidence of heat illnesses (5). In fact, athletes who collapse from heat illness often are quite well-hydrated, and it’s more likely that cramping and heat illnesses come from exercising too intensely. Muscles are more prone to spasms when fatigued, and heat illnesses generally occur in people who aren’t used to exercising in the heat and who continue to work out even when feeling unwell.
People with diabetes are also more likely to have electrolyte imbalances (e.g., sodium and potassium) to start with, including hyponatremia associated with extended hyperglycemia (6). As discussed in a recent article (7), an increased risk of electrolyte disturbances with diabetes can result from poorer blood glucose management, diabetes medications (some of which alter electrolyte balance), and organ damage associated with diabetes (such as nephropathy).
When it comes down to it, preventing overhydration during exercise is likely more important that worrying about the effects of dehydration. Use thirst as your guide when you exercise, and avoid consuming excess fluids, especially during prolonged workouts or sporting events. Also, keep your blood glucose levels in better control overall prior to engaging in any activities to ensure that hyponatremia is not an issue.
Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Br J Sports Med. 2015 Jul 30. pii: bjsports-2015-095004. doi: 10.1136/bjsports-2015-095004. [Epub ahead of print] (http://www.ncbi.nlm.nih.gov/pubmed/26227507)
Dugas J. Sodium ingestion and hyponatraemia: sports drinks do not prevent a fall in serum sodium concentration during exercise. Br J Sports Med. 2006 Apr; 40(4): 372. doi: 10.1136/bjsm.2005.022400 PMCID: PMC2577547 (http://www.ncbi.nlm.nih.gov/pubmed/16556798)
Miller KC, Mack GW, Knight KL, et al. Three percent hypohydration does not affect threshold frequency of electrically induced cramps. Med Sci Sports Exerc. 2010 Nov;42(11):2056-63. doi: 10.1249/MSS.0b013e3181dd5e3a. (http://www.ncbi.nlm.nih.gov/pubmed/20351595)
Braulick KW, Miller KC, Albrecht JM, Tucker JM, Deal JE. Significant and serious dehydration does not affect skeletal muscle cramp threshold frequency. Br J Sports Med. 2013 Jul;47(11):710-4. doi: 10.1136/bjsports-2012-091501. (http://www.ncbi.nlm.nih.gov/pubmed/23222192)
Laursen PB, Suriano R, Quod MJ, et al. Core temperature and hydration status during an Ironman triathlon. Br J Sports Med. 2006 Apr;40(4):320-5; discussion 325. (http://www.ncbi.nlm.nih.gov/pubmed/16556786)
Palmer BF, Clegg DJ. "Electrolyte and Acid-Base Disturbances in Patients with Diabetes Mellitus." N Engl J Med. 2015;373(6):548-59. (http://www.ncbi.nlm.nih.gov/pubmed/26244308)
Beware: Diabetes Results in Significant Electrolyte Disturbances, Diabetes In Control, Issue 798, September 11, 2015 (http://www.diabetesincontrol.com/articles/53-diabetes-news/18464-beware-diabetes-results-in-significant-electrolyte-disturbances#unused)