Nausea and high serum osmolality during a simulated ultraendurance adventure race: a case-control study

A 29-year-old elite adventure-race athlete presented with a 10-month history of nausea appearing during or after ultraendurance races. The athlete noted recent worsening of symptoms, including lightheadedness, dark rings under the eyes, and weakness as nausea became worse, and was unable to complete races. Possible diagnoses included dehydration or overhydration, renal damage, and gastrointestinal malfunction.

The subject (S) and a case-control athlete (C) performed an 11-hour simulated race (field test) ending in the laboratory. Blood samples were drawn, and body mass and food and drink ingested were noted at regular intervals.

Symptoms were replicated in S, whose vomitus contained undigested solids consumed during exercise. Over 11 hours, fluid loss was similar in S (9.6 L) and C (10.3 L), but fluid intake for S was 4.8 L versus 9.9 L for C. Body mass decreased 6% in S and 1.8% in C. S presented with elevated serum urea and creatinine before and after and elevated osmolality after the field test. A week after the field test, creatinine clearance was low in S but not C (83 vs 160 mmol per 24-hour urine, respectively). S was instructed to increase fluid intake throughout the day, to match fluid lost during training, and to refrain from long races. After 2 months, his serum urea, creatinine, and osmolality were normal, and creatinine clearance improved to 133 mmol per 24-hour urine. He repeated the field test and experienced no nausea.

Nausea in ultraendurance athletes might be an early symptom of chronic but reversible renal strain or insensitivity to high osmolality.