Dehydration Prevents The Development Of Hyponatremic Encephalopathy During Exercise

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PURPOSE The aim of this study was to evaluate the possible roles of dehydration or, conversely, overhydration in the etiology of exertional hyponatremic encephalopathy. METHODS Bodyweight changes and post race serum sodium concentrations ([Na+]) were collected from 1423 athletes competing in endurance events on three continents: the 2000–2001 SA 226 km Ironman triathlon, 1997 NZ 226 km Ironman triathlon, 2001–2003 Houston marathon (USA), 2002 Christchurch marathon (NZ) and 109 km Argus Cycle Tour (SA). Data were grouped according to serum [Na+] levels into hypernatremia (> 145mmol/L), normonatremia (< 145 >135mmol/L), biochemical hyponatremia (< 134 >130mmol/L), borderline hyponatremia (<129>128mmol/L) and symptomatic hyponatremia (<128mmol/L). The degree of overhydration (> +0.00 kg weight gain), euhydration (0.01 to −2.49 kg weight loss) and dehydration (>-2.5 kg weight loss) defined overall fluid balance from pre to post race. RESULTS Almost all (95%) athletes completed these races either euhydrated (45%) or mildly dehydrated (50%) with normal (82%) or elevated (11 %) serum [Na+]. There was a significant inverse relationship between body weight gain during exercise and the post-race serum [Na+] so that increasingly more severe hyponatremia accompanied higher levels of weight gain during exercise (r = 0.40; p <0.0001). Thirty-two (2% of the sample) subjects had post-race serum [Na+] below 130mmol/L; 30 were either overhydrated (26 athletes) or euhydrated (4 athletes) and only 2 athletes with hyponatremia (0.14% of the total sample) lost more than 2.5 kg and were therefore probably dehydrated. Of the 25 subjects with symptomatic hyponatremia, 22 were overhydrated after exercise, two lost less than 800 g during exercise and one with mild hyponatremia lost 3.7 kg. CONCLUSION These data confirm that the vast majority of athletes finish prolonged competitive athletic events without either significant dehydration or overhydration and with normal serum [Na+]. Overdrinking sufficient to cause weight gain during exercise was increasingly likely to cause hyponatremia leading to hyponatremic encephalopathy in those with the greatest weight gains during exercise. In contrast, dehydration was associated with hypernatremia and protection from clinically significant hyponatremic encephalopathy. Hence dehydration and the avoidance of overdrinking protects against hyponatremic Encephalopathy