Electrolyte balance during exercise

Normal body function depends on a balance between water and electrolytes. When large amounts of water are lost from the body, as during exercise, the balance between water and electrolytes can be disrupted quickly. Our focus will be on the two major routes for electrolyte loss: sweating and urine production.

Electrolyte loss in sweat

Human sweat is a filtrate of blood plasma, so it contains many substances found there, including sodium(Na⁺), chloride(Cl^-), potassium(K⁺), magnesium(Mg²⁺) and calcium(Ca²⁺). Although sweat tastes salty, it contains far fewer minerals than the plasma and other body fluids. In fact, sweat is 99% water.

Sodium and chloride are the predominant ions in sweat and blood. As indicated in the table below, the concentrations of sodium and chloride in sweat are about one-third those found in plasma and five times those found in muscle. Each of these three fluids’ osmolarity, which is the ratio of solutes(such as electrolytes) to fluid, is also shown. Sweat’s electrolyte concentration can vary considerably between individuals. It is strongly influenced by genetics, the rate of sweating, the state of traiing, and the state of heat acclimatization.

Electrolyte concentrations and osmolarity in sweat, plasma, and muscle of men following 2h of exercise in the heat
Site	Na+	Cl-	K+	Mg2+	Osmolarity(mOsm/L)
Sweat	40-60	30-50	4-6	1.5-5	80-185
Plasma	140	101	4	1.5	295
Muscle	9	6	162	31	295
mEq/L = milliequivalents per liter(thousandths of 1g of solute per 1L of solvent), second column represents mEq/L of electrolytes

At the elevated rates of sweating reported during endurance events, sweat contains large amounts of sodium and chloride but little potassium, calcium and magnesium. Based on estimates of the athlete’s total body electrolyte content, such losses would lower the body’s sodium and chloride content by only about 5% to 7%. Total body levels of potassium and magnesium, two ions principally confined to the insides of cells, would decrease by about 1%. These losses probably have no measurable effect on an athlete’s performance.

As electrolytes are lost in sweat, the remaining ions are redistributed among the body tissues. Consider potassium. It diffuses from active muscle fibers as they contract, entering the extracellular fluid. This increase causes in extracellular potassium levels that it does not equal the amount of potassium that is released from active muscles, because potassium is taken up by inactive muscles and other tissues while the active muscles are losing it. During recovery, intracellular potassium levels normalize quickly. Some researchers suggest that these muscle potassium disturbances during exercise might contribute to fatique by altering the membrane potentials of neurons and muscle fibers, makin it more difficult to transmit impulses.

Electrolyte loss in urine

In addition to clearing wastes from the blood and regulating water levels, the kidneys also regulate the body’s electrolyte content. Urine production is the other major source of electrolyte loss. At rest, electrolytes are excreted in the urine as necessary to maintain homeostatic levels, and this is the primary route for electrolyte loss. But as the body’s water loss increases during exercise, urine production rate decreases considerably in an effort to conserve water. Consequently, with very little urine being produced, electrolyte loss by this avenue is minimized.

The kidneys play another role in electrolyte management. If, for example, a person eats 250 mEq of salt(NaCl), the kidneys will normally excrete 250 mEq of these electrolytes to keep the body NaCl content constant. Heavy sweating and dehydration, however, trigger the release of the hormone aldosterone from the adrenal gland. This hormone stimulates renal reabsorption of sodium. Consequently, the body retains more sodium than usual during the hours and days after a prolonged exercise bout. This elevates the body’s sodium content and increases the osmolarity of the extracellular fluids.

This increased sodium content triggers thirst, compelling the person to consume more water, which is then retained in the extracellular compartment. The increased water consumption reestablishes normal osmolarity in the extracellular fluids but leaves these fluids expanded, which dilutes the other substances present there. This expansion of the extracellular fluids has no negative effects and is temporary. In fact, this is one of the major mechanisms for the increase in plasma volume that occurs with training and with acclimatization to exercise in the heat. Fluid levels return to normal within 48 to 72h after exercise, providing there are no subsequent exercise bouts.