Nutrition - Is it important for an endurance athlete to use an electrolyte supplement or replacement?

Hydration status of an athlete is a critical component to performance and a commonly discussed factor among physiologists. Equally important, electrolyte balance can greatly affect health and performance if not considered. Electrolytes in the body regulate fluid balance, blood acidity, and allow muscle and nerve activity through maintenance of a precise concentration gradient. An endurance exercise event can profoundly affect this balance. Major factors include exertion level, ambient temperature and humidity, fluid intake, and sweat rate.

How though should an athlete address their electrolyte balance? There are lots of products for electrolyte supplement and replacement but is it important? When should an athlete be supplementing electrolytes? After every workout or only after a marathon? Advertisements for products like the one below just confuse matters. It can’t be that every athlete needs salt tablets during every workout can it?

Daily water needs can range from 1-2L in sedentary individuals to greater than 10L in those extremely active [1]. The physiological need for sodium (Na+) is 184-230 mg/day but most nations have mean sodium intakes greater than 230 mg/day [2], suggesting that generally, most people get more than enough salt in their normal diets. Athletes, especially those exercising in extreme weather conditions, may have sweat losses that exceed their normal electrolyte intake levels. 

Many studies have examined the relationship between race distance, temperature, sweat rate, and electrolyte losses but the results are varied [3]. One study describes normal sweat losses around 1L/Hr (range: 500ml/hr to 2L/hr) and Na+ loss at 1000mg/l (range: 560mg/L to 1840mg/L) [1,4]. Replacing this during the event would be unpalatable and therefore could only be replaced by consumption in food over 24 hours [3].

One study compared electrolyte levels in runners who cramped during races with another group that did not experience cramps and found no differences [5]. A similar study in triathletes found similar results [6]. According to a 2011 study by the same group, just running speed itself is a better predictor of cramps than electrolyte levels [7]. 

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According to an American College of Sports Medicine analysis of several studies on electrolyte balance, normal food and fluid intake at mealtime is sufficient for most athletes to regain electrolyte losses when sessions are spaced more than 24 hours apart [1]. Potassium and Magnesium are also lost in the sweat and urine during exercise but only in very small amounts (<1g even during very prolonged exercise) and for this reason, additional supplementation is unnecessary [4]. Large fluid volume replacement increases urine losses while smaller volume at longer intervals encourages optimal hydration. When rehydration and electrolyte balance must be attained within six hours, fluid and electrolyte supplementation are necessary: a fluid volume at 125-150% of the water loss is necessary to ensure rehydration while accounting for urine losses. These replacement fluids can include up to 2-7g/l electrolytes [1].

I think the take away message is that for normal exercise, electrolyte supplementation is unnecessary. For activities greater than four hours (or shorter and in the heat) a small amount of electrolyte supplementation may be beneficial and allow maintenance of proper hydration levels.

1. Casa, D. J., Clarkson, P. M. & Roberts, W. O. American College of Sports Medicine Roundtable on Hydration and Physical Activity: Consensus Statements. Current Sports Medicine Reports 4, 115 (2005).
2. Elliott, P. Sodium intakes around the world. … meeting on Reducing Salt Intake in Populations (Paris … (2006).
3. C Dennis, S., Noakes, T. D. & Hawley, J. A. Nutritional strategies to minimize fatigue during prolonged exercise: Fluid, electrolyte and energy replacement. Journal of Sports Sciences 15, 305–313 (1997).
4. Noakes, T. D. Sports Nutrition: Fluid, Electrolytes, and MineralsSports Nutrition: Fluid, Electrolytes, and Minerals. World Forum on Physical Activity and Sport (1995).
5. Schwellnus, M. & Nicol, J. Serum electrolyte concentrations and hydration status are not associated with exercise associated muscle cramping (EAMC) in distance runners. British journal of … (2004).
6. Sulzer, N. U., Schwellnus, M. P. & Noakes, T. D. Serum electrolytes in Ironman triathletes with exercise-associated muscle cramping. Med Sci Sports Exerc 37, 1081–1085 (2005).
7. Schwellnus, M. & Drew, N. Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort …. British Journal of Sports … (2011).