Lauren was competing in her first ironman distance triathlon, a 140.6-mile race consisting of 2.4 miles of swimming, 112 miles of cycling, and 26.2 miles of running. At mile 22 of the run, approximately 16 hours after starting the race, she collapsed. After being admitted to the medical tent, Lauren complained of nausea, a headache, and general fatigue. The medical staff noted that Lauren’s face and clothing were covered in white crystals. When they weighed her and compared that value with her pre-race weight recorded at registration, they realized Lauren had gained 2 kg during the race.

The medical staff was concerned with Lauren’s large weight increase during the race. They asked her to recall what she ate and drank during the race. Lauren reported that to avoid getting dehydrated in the warm weather, she had drunk large quantities of water in addition to sports gel and sports drinks containing carbohydrates and electrolytes.

The medical staff analyzed Lauren’s blood for electrolyte concentrations. Her serum Na+concentration was 124 mEq/L. The normal range is 135–145 mEq/L. Lauren’s diagnosis was hyponatremia {hypo-, below + natri-, sodium + -emia, blood}, defined as a serum Na+concentration below 135 mEq/L. Hyponatremia induced by the consumption of large quantities of low-sodium or sodium-free fluid, which is what happened in Lauren’s case, is sometimes called dilutional hyponatremia.

During exercise in the heat, sweating rate and sweat composition are quite variable among athletes and depend partly on how acclimatized the individual is to the heat. Sweat fluid losses can range from less than 0.6 L/h to more than 2.5 L/h, and sweat Na+ concentrations can range from less than 20 mEq/L to more than 90 mEq/L. The white salt crystals noted on Lauren’s face and clothing suggest that she is a "salty sweater" who probably lost a large amount of salt during the race. Follow-up testing revealed that Lauren’s sweat Na+ concentration was 70 mEq/L.

The human body attempts to maintain fluid and sodium balance via several hormonal mechanisms. During exercise sessions, increased sympathetic output causes increased production of aldosterone and vasopressin, which promote the retention of Na+ and water by the kidneys.

In acute cases of dilutional hyponatremia such as Lauren’s, the treatment goal is to correct the body’s depleted Na+ load and raise the plasma osmolarity to reduce cerebral swelling. The physicians in the emergency medical tent started a slow intravenous drip of 3% saline and restricted Lauren’s oral fluid intake. Over the course of several hours, the combination of Na+intake and excretion of dilute urine returned Lauren’s plasma Na+ to normal levels.

Hyponatremia has numerous causes, including inappropriate secretion of antidiuretic hormone (a condition known as SIADH, which stands for syndrome of inappropriate antidiuretic hormone secretion). To learn more about medical causes of hyponatremia, Google hyponatremia. To learn more about exercise-associated hyponatremia, visit the Gatorade Sports Science Institute at www. gssiweb. com.

Which of the following directly caused Lauren's weight gain? View Available Hint(s)

A. increased absorption of water in the digestive system and an increased reabsorption of water by the collecting duct

B. decreased reabsorption of water by the collecting duct

C. increased secretion of Na at the collecting duct

D. increased sodium content of the blood plasma