By Bobbi Barbarich, MSc RD
You’re throwing your race shorts in the laundry and you notice a white film on the waistband – the remnants of your sweat after the water evaporated. That fine dust is sodium, potassium, calcium, magnesium and chloride – tiny molecules that help determine whether your heart will beat again, or if you’ll be able to take another step toward the finish line.
Sweat is almost entirely water mixed with a number of electrolytes – mineral salts that conduct electrical energy. Involved in fluid balance, electrolytes manage cellular waste and nutrient passage into and out of cells, and play primary roles in nerve impulse transmission and muscle contraction. Your hydration status is mostly dependent on your fluid intake, but that fluid balance – in constant flux while you run – is determined by your electrolyte status.
Sodium and chloride are sweat’s major electrolytes, followed by intracellular potassium. magnesium, calcium, iron, copper, zinc, amino acids and some B vitamins. Sweat rates can range from 300 mL to 2.5 litres per hour, though it’s possible to lose far more. American Alberto Salazar reportedly lost nearly four litres per hour during the 1984 Olympic Marathon in Los Angeles.
Electrolyte imbalances look and feel a lot like dehydration and they’re often inseparable. A common symptom is muscle or abdominal cramping, along with light-headedness, nausea, confusion and muscle spasms. To avoid a potential pain in your side en route to your goal, consider a carbohydrate-electrolyte drink. Glucose increases the absorption of both water and sodium. Sodium enhances fluid retention and stimulates thirst, a sensation often masked when you’re powering through a training session. Health professionals and most drink companies agree that water, glucose and salt with chloride are enough to avoid cramps and dehydration.
If you’re eating a well-balanced diet – including lots of fruits and vegetables for potassium, dairy products for calcium and nuts, seeds and whole grains for magnesium – you probably don’t need further electrolyte supplements beyond an average sports drink. But as temperature, intensity and the time you spend pounding the pavement increase, the electrolytes excreted as you dump water to cool your core will increase, too. And since we all sweat a little differently, you should carefully consider which ones are putting that stitch in your side.
Sodium and Chloride
Sodium is a principle positive ion in the body’s fluid, primarily found outside the cell. Chloride, another extracellular elec- trolyte, is a negative ion and works with sodium to regulate fluid balance and electrical impulses across the cell membrane. Although individual concentrations will vary widely, one litre of sweat contains approximately 800-1400 mg of sodium. Where water consumption increases blood volume, water dilutes blood sodium due to increased blood volume and excessive sodium losses in sweat.
For events lasting longer than four hours, especially in hot weather, hyponatremia (dangerously low sodium) is the most common concern. This especially applies to slower-running marathoners. In runs lasting three hours or more, or in higher-than-usual temperatures or inten- sities, aim for 800 mg sodium per hour. If you’re still cramping, go for 1,000 mg per hour.
Potassium is the main electrolyte found inside cells. Stored in muscle fibres with glycogen, potassium helps transport glucose into the muscle cell. Potassium also works with sodium and chloride to control fluid and electrolyte balance, and assists in the conduction of nerve impulses. As you break down glycogen, potassium leaves muscle cells, increasing potassium concentration in your blood and urine. Though you must replace lost potassium during and after exercise, hyperkalemia (high serum potassium levels) can disrupt electrical impulses, possibly causing irreg- ular heartbeat and even death. Never take potassium supplements in large doses.
One litre of sweat contains approximately 150–200 mg of potassium. If you lost two kilograms of weight during a run, aim for 400 mg before, during and after a run to replace it. As much as 150 mg per hour during activity is tolerable, but too much supplemental potassium can cause cardiac arrest, so use it carefully.
Beyond teeth and bones, calcium is needed for muscle contraction and relaxation, nerve conduction, hormone secretion, enzymatic reactions and blood coagulation. Calcium also plays a central role in glycogen synthesis and breakdown. The skeleton is a huge calcium reservoir, constantly working to keep serum calcium in tight control. Due to its integral impor- tance, serum calcium is highly regulated and hypocalcemia is rare, even in extreme conditions.
Calcium is a big molecule to digest. It’s best to aim for dietary sources of 1,200– 1,500 mg per day. If your potassium and sodium intakes are on target, you’re getting adequate calcium from your diet or supplements and you’re still cramping, then consider around up to 300 mg/litre of electrolyte fluid in runs lasting more than three to four hours.