The most important electrolytes lost during sweating are sodium, potassium, magnesium and even calcium.
Sodium is the most abundant in sweat as well as blood plasma and would be a natural electrolyte for replacement. Sweat always contains some sodium, reaching a reported maximum upto 70 mmol/L. A number of articles explain why sodium needs replacing, how and at which point (Shirreffs et al., 2004; Jeukendrup and Gleeson, 2004). We all sweat at different rates depending on who we are, how much and intense the exercise that is being performed or the temperature. I was surprised to read that upto 3 litres per hour can be generated when the conditions are warm and humid.
An athlete involved in endurance exercise should regularly imbibe drinks containing between 30 and 50 mmol/L sodium for best or optimal absorption to reduce hyponatraemia (Jeukendrup et al., 2005). This last condition causes very unpleasant effects such as confusion and vomiting and occur when almost unnaturally large amounts of water upto 10 litres are ingested. EFSA states claim levels for sodium in a rehydrating sports drink should be in the range of 20 to 50 mmol/L. One research article recommended that in extreme cases of heat stress, and upper limit of 690 mg/L (300 mmol/L) was reasonable (Brouns et al., 1992). It also has to be borne in mind that the osmolality of a sports drink is critical in promoting intestinal water absorption. That is another post !
When I last looked, a number of health claims had been submitted to EFSA regarding sodium content:-
• For drinks containing at least a 15%RDA sodium:
Water and electrolyte balance – Sodium is necessary for water and electrolyte balance throughout the body.
• For drinks containing 20-50mmol/L (460-1150mg/L) with 80-350kcal/L energy:
Rehydration – For the replenishment of lost salts due to sweating and dehydration.
• For drinks containing 15-50mmol sodium (345-1150mg/L):
Hydration – Replaces electrolytes lost through sweat.
Maintains fluid and electrolyte balance.
Potassium is found predominantly in intracellular fluid than in plasma and is thus not lost as heavily during sweating. There appears no real advantage in supplementing sports drink with this electrolyte and most of what is obtained comes in the diet anyway (Cunningham, 1997; ACSM, 2007). It is commonly found in many drinks as the counter-ion in preservatives using potassium sorbate. It might help in rehydration of the intracellular fluid (Gisolfi et al, 1992). The average losses during sweating are 5 mmol/L (195 mg/L) with a range of 3.2 to 5.5 mmol/L. For any EFSA claims, a 15% RDA of 525 mg is required. Gisolfi et al., (1992) suggested 117 to 195 mg/L might be the content in sports drinks although at this time there is no optimal level recommended.
In terms of EFSA claims:-
• When the source is at least 15%RDA:
Water and electrolyte balance – Potassium supports normal water and electrolyte balance throughout the body
• When used in conjunction with carbohydrate and providing 15%RDA for potassium:
Rehydration – Re-hydrates during and post exercise bouts.
Magnesium is also at its highest concentration in intracellular fluid compared to plasma and again small amounts are lost in sweat. Supplementation in a sports drink might reduce muscle cramps but there is no real evidence to support this. Some magnesium is lost in sweat and the average is 0.4 mmol/L (10 mg/L) ranging from 2.4 to 18 mg/L (Lukaski, 2000). The RDA for men is 300 mg/L and for women, 270 mg/L. Most people meet these levels although physically active women might benefit from a supplement of dietary magnesium (Lukaski, 2000).
At the time the EFSA claims were for those products containing 15%RDA:
• Electrolyte balance – magnesium is necessary for electrolyte balance.
• Energy metabolism/Normal cellular energy supply – Magnesium is essential for the use of energy by the body.
• Muscle function – Magnesium is necessary for muscle function. About 6 to 10 mg of magnesium per kg body weight per day, at least 15%RDA and a warning with those containing >400mg:
• Necessary to support optimal exercise performance – dietary magnesium supports optimal exercise performance. Without sufficient dietary magnesium, strength gains may be compromised; Necessary to support optimal strength gains in combination with resistance exercise.
Also, 150 mg minimum daily was suggested in food supplements:
• Energy / Muscular function – Magnesium is needed for normal energy metabolism, including nerve and muscle function.
The final electrolyte, calcium plays an important role in general bone and dental health, nerve and muscle function and is a critical for maintaining an active lifestyle. We lose an average of 20 mg/L and a maximum upto 40 mg/L. The RDA in the UK is 700 mg and so a 15% RDA health claim would be 105 mg present in a product. In terms of sports drinks, there are no specific EFSA claims although a number were submitted relating to other important health functions.
American College Sports Medicine Position paper: Exercise and fluid replacement. (2007) Med. Sci. Sports Exer. . Position paper
Brouns F, Saris W, Schneider H. (1992) Rationale for upper limits of electrolyte replacement during exercise. Int. J. Sport Nutr. 2 pp. 229-238
Cunningham JJ. (1997) Is potassium needed in sports drinks for fluid replacement during exercise? Int. J. Sport Nutr. 7 pp. 154-159
Gisolfi, C, V., Duchman, S. M., (1992) Guidelines for optimal replacement beverages for different athletic events. Medicine & Science in Sports & Exercise. 24(6) pp. 679-687
Jeukendrup, A., Gleeson, M. (2004) Water requirements and fluid balance. In: Sport Nutrition, Leeds: Human Kinetics, pp. 169 – 196
Jeukendrup, A.E., Jentjens, R.L., Moseley, L. (2005) Nutritional considerations in triathlon. Sports Medicine (Auckland, NZ)35 (2) pp. 163-81
Lukasi, HC. (2000) Magnesium, zinc and chromium nutriture and physical activity. Am. J Clin. Nutr. 72 pp. 585S-593S
Maughan RJ, Leiper JB. (1995) Sodium intake and post-exercise rehydration in man. Eur. J. App Phys. 71 pp. 311-319
Shirreffs, S.M., Armstrong, L.E., Cheuvront, S.N. (2004) Fluid and electrolyte needs for preparation and recovery from training and competition. In: Maughan RJ, Burke LM, Coyle EF. Food, Nutrition and Sports Performance II, Oxon: Routledge, pp.92 – 103