Exercise-Induced Hyponatremia in Ultradistance Triathletes Is Caused By Inappropriate Fluid Retention *Dale B. Speedy, MBChB, MSc, †Ian R. Rogers, MBBS, ‡Timothy D. Noakes, MBChB, MD, §Susan Wright, MBChB, §John M. D. Thompson, PhD, ¶Robert Campbell, MBChB, ¶Ien Hellemans, MSc,  Nicholas E. Kimber, MSc, **D. Ross Boswell, MBChB, PhD, ††Jonathan A. Kuttner, MBChB, and ‡‡Shameem Safih, MBChB *Department of General Practice and Primary Care, University of Auckland, Auckland, New Zealand; †Emergency Department, Sir Charles Gairdner Hospital, Nedlands, Perth, Western Australia, Australia; ‡Sports Science Institute of South Africa, University of Capetown, Capetown, South Africa; §University of Auckland, Auckland; ¶Sportsmed, Christchurch;  Lincoln University, Lincoln; **Diagnostic Laboratory, Auckland; ††Waiuku Medical Practice, Waiuku; and ‡‡Auckland Hospital Emergency Department, Auckland, New Zealand Objective: To study fluid and sodium balance during over- night recovery following an ultradistance triathlon in hypona- tremic athletes compared with normonatremic controls. Case Control Study: Prospective descriptive study. Setting: 1997 New Zealand Ironman Triathlon (3.8 Km swim, 180 Km cycle, 42.2 Km run). Participants: Seven athletes (“subjects”) hospitalized with hyponatremia (median sodium [Na]  128 mmol L -1 ). Data were compared with measurements from 11 normonatremic race finishers (“controls”) (median sodium  141 mmol L -1 ). Interventions: None. Main Outcome Measures: Athletes were weighed prior to, immediately after, and on the morning after, the race. Blood was drawn for sodium, hemoglobin, and hematocrit immedi- ately after the race and the following morning. Plasma concen- trations of arginine-vasopressin (AVP) were also measured post race. Results: Subjects were significantly smaller than controls (62.5 vs. 72.0 Kg) and lost less weight during the race than controls (median -0.5% vs. -3.9%, p  0.002) but more weight than controls during recovery (-4.4% vs. -0.8%, p  0.002). Subjects excreted a median fluid excess during recov- ery (1,346 ml); controls had a median fluid deficit (521 ml) (p  0.009). Estimated median sodium deficit was the same in subjects and controls (88 vs. 38 mmol L -1 ,p  0.25). Median AVP was significantly lower in subjects than in controls. Plasma volume fell during recovery in subjects (-5.9%, p  0.016) but rose in controls (0.76%, p  NS). Conclusions: Triathletes with symptomatic hyponatremia following very prolonged exercise have abnormal fluid reten- tion including an increased extracellular volume, but without evidence for large sodium losses. Such fluid retention is not associated with elevated plasma AVP concentrations. Key Words: Ultra-distance—Triathlon—Arginine-vaso- pressin—Ironman triathlon—Sodium. Clin J Sport Med 2000;10:272–278. INTRODUCTION Hyponatremia (plasma sodium concentration <135 mmol L -1 ) is a relatively recently described complication of ultradistance running and triathlon events. First de- scribed in 1985 1 and 1986, 2 the condition may occur more frequently than previously appreciated. Thus hy- ponatremia of varying degrees of severity has been re- ported to occur in up to 29% of athletes in the Hawaiian Ironman Triathlon, 3,4 in 18% of finishers in the New Zealand Ironman Triathlon, 5 and in 0.3–10% of competi- tors in ultradistance running events. 6–8 Despite the frequency of this condition, its pathogen- esis remains uncertain. The bulk of current evidence sug- gests that athletes with this condition ingest fluid at un- usually high rates during prolonged exercise and retain fluid inappropriately. 1,9–13 However most of the evi- dence for this hypothesis comes from retrospective stud- ies involving relatively small subject numbers. 1,2,9,12–16 Indeed there have been only three prospective studies of the relationship between exercise-related changes in plasma sodium concentrations and in body weight, the latter an indirect measure of fluid balance, in competitive ultradistance sporting events. 5,11,12 All have found an inverse relationship between changes in body weight and in plasma sodium concentrations after Ironman triath- lons, further suggesting that the hyponatremia of exercise is caused by inappropriate fluid retention. In contrast, dehydration causes hyper-, not hyponatremia, as incor- rectly proposed by other researchers. 17 Received December 20, 1999; accepted August 28, 2000. Address correspondence and reprint requests to Dale Speedy, MBChB, 179A Hill Rd., Manurewa, New Zealand. E-mail: dalespeedy@e3.net.nz Clinical Journal of Sport Medicine, 10:272–278 © 2000 Lippincott Williams & Wilkins, Inc., Philadelphia 272