Reviewed and Updated by ERB member Shawn Dolan, PhD, RD, CSSD: Professor in the Kinesiology Department at California State University, Long Beach. 

Introduction: Electrolytes, the mineral salts that conduct the electrical energy of the body, perform a cellular balancing act by allowing nutrients into the cell, while helping to remove waste products. Certain elements, such as sodium, chloride, magnesium, calcium and potassium, play a primary role in cellular respiration — that of muscle contraction and nerve impulse transmission. It is in the cell membrane where these electrolytes conduct electrical currents similar to nerve impulses. Hydration is the medium which aids electrolyte transport and is crucial for both the health and performance of the cell. An athlete’s hydration state is mostly dependent upon water intake or loss thru sweat, but it is also heavily influenced by electrolyte status.

Sweat: Endurance performance is compromised more by warmer temperatures than by cooler temperatures. Here’s why: to control an excessive rise in body temperature, the blood flow to the skin increases in order to dissipate heat to the environment. This shift of blood to the skin will result in a lesser proportion of blood and, hence, oxygen being delivered to the working muscle. In some individuals the circulatory adjustments may not be adequate, and the body temperature will rise rapidly, leading to hyperthermia (excessive body heat). Individual sweat rates vary, but those that sweat early, heavily, and cake with salt tend to be more prone to muscle cramps during exercise (Burke, 2001). Evaporation of sweat in a hot environment can purge as much as 3 liters per hour. Alberto Salazar reportedly lost an average of 3.7 liters per hour of sweat during the hot and humid 1984 Olympic Marathon in LA (Armstrong et al., 1986). However, average sweat rates range from 0.5-2.0 liters per hour (ACSM, 2007). About 99% of sweat is water, with a number of major electrolytes found in varying amounts. Because sweat is derived from the extracellular fluid (fluid outside the cell), the major electrolytes found in it are sodium and chloride. The concentration of salt in sweat is variable but averages about 2.6 grams per liter of sweat loss. Potassium, magnesium, calcium, iron, copper, zinc, amino acids and some of the water-soluble vitamins can also be found in sweat.

Too much water? Hyponatremia is defined as a decrease in sodium concentration in the blood, which can have adverse effects on muscle contraction and performance. One study observed 27% of participants following a three-day cycling stage race competition were hyponatremic. Symptoms of hyponatremia include headache, nausea, muscle cramping, fatigue, and possibly death. Although there may be many causes of hyponatremia, the most common one for athletes is overhydration. Athletes tend to superhydrate in the days leading up to a race without an appropriate increase in electrolytes. In some cases, superhydrating can produce hyponatremia prior to the race ever starting. However, drinking only water during a race can also causes hyponatremic conditions because the body requires electrolytes to effectively maintain hydration status. Hyponatremia, rare in events lasting less than 4 hours, has been shown in recent medical studies of slower marathon runners and ultra-distance triathletes to be at least as problematic and dangerous…if not more so…than dehydration.

Sodium and Chloride: Sodium is one of the principle positive ions in the body’s fluid and is found primarily outside the cell (extracellular). Chloride, another extracellular electrolyte, is a negative ion and works closely with sodium in the regulation of body-water balance and electrical impulses across the cell membrane. Consuming adequate amounts of sodium and chloride, more commonly known as table salt, is crucial to maintaining the volume and balance of fluids outside the body’s cells and in the blood. Sodium is especially important because it plays a key role in transporting nutrients into cells to be used for energy production, tissue growth, and repair. Sodium also assists in muscle contraction and nerve impulse transmissions. During exercise, the body loses fluids and sodium through sweating. This causes a decrease in blood volume, thereby increasing sodium and chloride concentrations in the blood. The increased concentration of electrolytes in the blood through decreased blood volume is what triggers the thirst mechanism. By the time an athlete becomes thirsty; the electrolytes are already out of balance, so restoration of blood volume is critical for the prevention of dehydration. Water consumption is effective in increasing blood volume; however, there is a consequential dilution of sodium in the blood due to the increased blood volume and excessive sodium losses in sweat, so electrolyte replenishment is key. Drinking fluids with added electrolytes instead of just plain water is the best option, particularly when an exercise bout lasts longer than one hour and is in a hot or humid environment.

Potassium: Potassium is the main electrolyte found inside the body’s cells (intracellular) and stored in muscle fibers along with glycogen. It plays a key role by helping transport glucose into the muscle cell. Potassium also interacts with both sodium and chloride to control fluid and electrolyte balance and assists in the conduction of nerve impulses. When glycogen breaks down to supply energy for workouts, muscle cells are depleted of potassium. As a result, there is a greater concentration of potassium in blood and greater quantities are lost in the urine. Symptoms of potassium depletion include nausea, slower reflexes, irregular heartbeat, drowsiness, and muscle fatigue and weakness. Although potassium deficiencies are rare, they may occur under certain conditions — during fasting, diarrhea and when using diuretics. Replenishing lost potassium after exercise is important, but hyperkalemia (high serum potassium levels) can cause electrical impulse disturbance, irregular heart beat, and possibly death. Individuals should never take potassium supplements in large doses without the advice of a physician.

Calcium is a mineral that is often overlooked with regard to hydration. The skeleton is the major reservoir of calcium in the human body. Besides building teeth and bones, calcium is needed by many other cells to perform different functions in the body: contraction and relaxation of muscle, nerve conduction, secretion of hormones, enzymatic reactions, and blood coagulation. Calcium plays a central role in both the synthesis and breakdown of muscle glycogen and liver glycogen. Blood calcium levels are tightly regulated by hormones at the expense of bones. Many do not realize that bones are constantly being broken down and rebuilt through the processes of resorption and formation. The National Academy of Sciences (http://www.iom.edu/CMS/3788/29985/37065.aspx) recommends the following calcium intake levels for different age groups: 500mg for 1-3year olds, 800mg for 4-8 year olds, 1,300mg for those aged 9-18, 1,000mg for ages 19-50 years, and 1,200mg for those over 50 years of age. Endurance athletes may require even greater levels. Dairy products like milk, cheese and yogurt are excellent sources of dietary calcium because they are also fortified with vitamin D which is necessary for optimal absorption of calcium into the body. Low serum levels of calcium can cause a number of problems, including muscular cramping due to an imbalance of calcium in the muscle and surrounding fluids. Muscular contraction and exercise performance in active individuals is also compromised by low serum calcium. In addition to calcium intake, athletes should be aware that weight-bearing exercise is beneficial in the maintenance of a healthy skeleton. Non-weight bearing sports like bicycling and swimming have been associated with bone mass similar to or below that of normal sedentary people (Duncan, 2002; Heinonan, 1993; Warner, 2002; Taaffe, 1995 & 1999).  It is important to fit in some weight bearing exercise and consume varied sources of calcium in your diet.

Magnesium: Magnesium is an element found in every cell of the body, with the largest concentrations found in the bones, muscles, and soft tissues. Magnesium forms part of 300+ enzymes involved in nerve impulse transmission, muscle contraction, and ATP (or energy) production. Increased levels of exercise deplete your body’s stores of magnesium so it is crucial to replenish what you have lost. Investigators suggest that prolonged exercise increases the loss of magnesium from the body via urine and sweat. Signs of magnesium depletion include dizziness, muscle weakness, fatigue, irritability, and depression.

Electrolyte Chart

From Maughan and Shirreffs, 1998. Fluid and electrolyte loss and replacement in exercise. In Oxford textbook of sports medicine, 2nd Edition. Edited by Harris, Williams, Stanish, and Micheli. New York: Oxford University Press, pp. 97-113.

Dietary Sources of Electrolytes

Sweat and electrolyte losses can be prevented and replaced with meals and snacks as well as sport foods (i.e. sport drinks, gels, energy bars, etc).

Meals & Snacks

The typical American consumes approximately 6-9 grams of sodium per day through a variety of foods. However, 1.5 g or 1,500 mg is the recommended intake of sodium per day for 19-50 year olds (http://www.iom.edu/CMS/3788/3969/18495.aspx). Endurance athletes will have increased requirements, depending on their sweat rate, concentration of sweat, body mass, training environment, etc. The increased requirements offset increased losses and are typically consumed via sport foods. Sodium is readily found in tomato juice, dill pickles, canned soups, processed or packaged foods, salt added to foods, or by adding ½ tsp of table salt to 1 Liter of water. Keep in mind, when salt is added to food, 1 tsp provides 2,000 mg of sodium. Potassium is abundant in orange juice, bananas, potatoes, yogurt, halibut, milk, and cantaloupe. Approximately 1 glass of orange or tomato juice can replace potassium, magnesium, and calcium lost in 2-3 liters of sweat.

Sport Foods

Endurance athletes have different fluid and electrolyte needs particularly during longer and higher intensity training sessions and competition. The composition of standard sport drinks may not provide an adequate amount of electrolytes during activity lasting longer than 2 hours. The increased loss of sweat translates into an increased loss of electrolytes. As previously mentioned, sodium is one of the important electrolytes that needs to be replaced during exercise to prevent dehydration and hyponatraemia. Most standard sport drinks contain 50-110 mg (200-460 mg/liter) of sodium per 8 oz. Because we are limited on the amount of fluid the body can absorb by the intestines, it may be important to consume a higher amount of sodium during exercise to minimize fluid loss. The body can tolerate a higher sodium intake (closer to the amount lost in sweat) and it does not appear to negatively affect carbohydrate absorption. However, increased sodium content in sport drinks can impact taste. A more detailed comparison table of popular sport drinks and their nutrients is available at Sport Drink Comparison. The table provided below provides a general comparison of sport drinks designed specifically for different purposes.

Comparison of the Electrolyte Content between Standard Sport Drinks and Endurance Specific Sport Drinks

Practical recommendations to enhance dehydration and to minimize body water losses.
1) If your training sessions last longer than 1 hour, choose a sport drink with adequate electrolytes as part of your fluid replacement program. If training sessions last longer than 2 hours, consider drinking an endurance specific sport drink. Hydrating with water alone can lead to water intoxication and an electrolyte imbalance. 

2) Acclimatize to the heat by exercising in heat. As previously explained (second paragraph), exercising in the heat transfers blood to the skin in order to dissipate fluids. Exercising in the heat will allow your body to adapt accordingly by improving its cooling mechanism. One method of adaptation that is critical is the body’s improved ability to reabsorb sodium and potassium. This will improve your blood flow to the working muscles, effectively improving your performance in heat.

3) Test different levels of electrolytes during training in the heat. In order to do this, it is important to test different products available during different training conditions to determine what amount is appropriate for you and the type of training you do.

4) Weigh yourself before and after a long training session in the heat. Subtract the total fluids you drank from your weight difference (pre-post). The remaining difference will be your hydration deficit. Plan to rehydrate by drinking 150% of your fluid losses. In order to achieve this amount drink approximately 2 cups of fluid per pound lost (after accounting for the fluid you drank).

5) Do not make any drastic changes to your diet for the days leading up to the race and on race day. Drastic changes can adversely affect your electrolyte balance. Prior to race day, introduce an electrolyte drink by sipping on it so your body is fully prepared to accept the drink during the race.

6) During races in extreme heat, consider cooling your head and neck at aid stations where ice is available. Not only does this feel good, it also allows the oxygen carrying blood to concentrate on the working muscles, which in turn improves your exercise capacity.

7) Choose a hydration beverage to drink during exercise and recovery that contains adequate levels of all 5 electrolytes.   Focusing solely on sodium can throw your electrolyte balance off.

notes from…The Endurance Research Board

My electrolyte story…By Neal Henderson MS, CSCS
Electrolytes are lost primarily through sweat. It is extremely difficult to quantify the extent of electrolyte loss during exercise, though. Determining sweat rate is relatively easy…by measuring your body weight before and after exercise, and accounting for any fluids ingested. With mild sweating, there is an increased concentration of extracellular sodium and potassium. With sustained high sweat rates, electrolyte disturbances increase. To avoid both dehydration and hyponatremia, athletes need to determine their own rate of sweat loss, and an approximate level of electrolyte loss. This is far easier said than done. While preparing for the 2001 5430 Ironman-distance Triathlon in Boulder, Colorado I performed several sweat related training sessions. I determined that at my Ironman pace on the bike, I was sweating about 1.5 liters/hour in 80-86 degree heat. I developed my entire nutrition plan on the bike to meet this need. From previous Ironman races, I knew that I also lose a very high amount of sodium loss (I look like a white sparkling ghost at the finish of most hot races, from salt losses in my sweat). To match my needs, I decided to drink primarily a popular fluid replacement drink, and added an electrolyte tablet containing an additional 500 mg of sodium per liter of fluid. This would add up to about 1 gram of sodium/liter of fluid, which is a relatively high amount of sodium in sweat. During the race, the temperature rose to about 94-96 degrees for much of the bike, and I didn’t pace myself too well. By mile 6 of the run, the bottom was dropping out quickly and my pace gradually slowed. In the end, my marathon split was only 2 minutes faster than my bike split (5:02 vs. 5:04) and I was taken directly to the medical tent. There, I was weighed…coming in at 10.5 lbs under pre-race weight (6% weight loss), and received 2 liters of IV fluids. Even though I averaged 1.5 liters of fluid intake per hour, I was nearly 5 liters in deficit at the finish. Had I consumed 2.0 liters/hour…I may have had a better ending to the race. When considering fluid replacement needs, total volume of fluid is not the only concern. Electrolyte concentration must meet your individual loss rates, and the energy content (carbohydrates primarily) must be considered. Experimentation and experience may be most important for you in preparation for a long distance endurance race, or one that will be held in extreme heat. Remember, what works for one person may literally kill another! Perform your own experiments with an N=1, and good luck!

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From Maughan and Shirreffs, 1998. Fluid and electrolyte loss and replacement in exercise. In Oxford textbook of sports medicine, 2nd Edition. Eited by Harris, Williams, Stanish, and Micheli. New York: Oxford University Press, pp. 97-113

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