Crossing the finish line, Sarah should have been elated. An experienced runner with many races and several half marathons to her credit, she trained diligently for 6 months and just successf

Hyponatremia in a Marathon Runner
Crossing the finish line, Sarah should have been elated. An experienced runner with many races and several half marathons to her credit, she trained diligently for 6 months and just successfully finished her first full marathon. Instead of enjoying her accomplishment, Sarah felt nauseous and week, had a crushing headache, and went to the medical tent for treatment. After being assessed with dehydration and heat exhaustion, she received an intravenous infusion of 2 liters of normal saline and was discharged shortly after. After her symptoms worsened while on her way home, she vomited, became disoriented and confused, and was taken to the hospital emergency room. Initial blood tests in the ER revealed that her serum sodium concentration was 116 mEq/L (normal range 135-146 mEq/L) and serum osmolality was 235 mosm/kg (normal range 255-295 mosm/kg) and she was diagnosed with hyponatremia.
At the time of her first marathon, Sarah was 41 years old, weighed 126 lb (57 kg), and was 64 in (163 cm) tall. A distance runner with years of experience, she had successfully completed races up to the 13.1 mile (21 km) half marathon distance without any issues and completed long runs up to 22 miles (35 km) in preparation for her marathon attempt. The morning of the race she ate a light breakfast and drank 1.5 liters of water. Environmental conditions during the race were not severe, averaging 61? F (15.9? C) and 67% humidity. Concerned about dehydration, however, Sarah drank approximately 4.5 liters of water during the race, which she completed in about 5 hours. She also consumed 11 energy gel packets containing a total of 300 grams of carbohydrate, 375 mg of caffeine, and 565 mg of sodium.
Sarah’s hyponatremia was likely due to overconsumption of water immediately before and during the race. Despite her relatively small stature, she drank approximately 6 liters of water while consuming only 565 mg of sodium, leading to a dilution of the sodium levels in her blood. Her condition was likely made worse by the infusion of more fluids in the race medical tent, which was done as a default treatment for heat exhaustion without assessing her body temperature or blood electrolyte levels. Once properly diagnosed in the emergency room, Sarah was administered two 100 mL infusions of hypertonic saline, and her blood sodium levels returned to normal over the next 48 hours. She remained in the hospital for another 2 days for observation and was then discharged having shown no further complications. After resting and recovering for 3 weeks, Sarah was fortunately able to gradually resume her training and racing without any further complications.
Reference: Heled, Y., Yarom, Y., & Epstein, Y. (2019). Hyponatremia Following a Marathon, A Multifactorial Case with over Infusion of Fluids. Current Sports Medicine Reports. 18(4):115-117. doi: 10.1249/JSR.0000000000000580. PMID: 30969235.
1. What is the recommendation for sodium intake for athletes competing in prolonged endurance events (for example, lasting more than 4 hours) when fluid and electrolyte loss is anticipated? How does Sarah’s sodium intake per liter of water consumed compare to this recommendation?

2. Normal saline contains sodium – why did the infusion of 2 liters of normal saline in the medical tent likely make her situation worse?

3. After completely recovering with no further symptoms, Sarah began planning to run another marathon. How would you devise a specific fluid and electrolyte replacement plan for Sarah to follow during her next marathon to prevent a reoccurrence of hyponatremia?

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