Noakes T.D., Sharwood K., Speedy D., Hew T., Reid S., Dugas J., Almond C., Wharam P., Weschler L.
University of Cape Town, Department of Human Biology, Sports Science Institute of South Africa, Newlands, 7700, South Africa; Department of General Practice and Primary Care, University of Auckland, Aukland 1020, New Zealand; Sports Medicine Practice, St. Helen's Hospital, Hobart 7216, Tasmania, Australia; Department of Cardiology, Children's Hospital, Boston, MA 02115, United States; 161 Richdale Road, Colts Neck, NJ 07722, United States; University of Cape Town, Department of Human Biology, Sports Science Institute of South Africa, Boundary Road, Newlands, 7700, South Africa
Noakes, T.D., University of Cape Town, Department of Human Biology, Sports Science Institute of South Africa, Newlands, 7700, South Africa, University of Cape Town, Department of Human Biology, Sports Science Institute of South Africa, Boundary Road, Newlands, 7700, South Africa; Sharwood, K., University of Cape Town, Department of Human Biology, Sports Science Institute of South Africa, Newlands, 7700, South Africa; Speedy, D., Department of General Practice and Primary Care, University of Auckland, Aukland 1020, New Zealand; Hew, T., University of Cape Town, Department of Human Biology, Sports Science Institute of South Africa, Newlands, 7700, South Africa; Reid, S., Sports Medicine Practice, St. Helen's Hospital, Hobart 7216, Tasmania, Australia; Dugas, J., University of Cape Town, Department of Human Biology, Sports Science Institute of South Africa, Newlands, 7700, South Africa; Almond, C., Department of Cardiology, Children's Hospital, Boston, MA 02115, United States; Wharam, P., Department of General Practice and Primary Care, University of Auckland, Aukland 1020, New Zealand; Weschler, L., 161 Richdale Road, Colts Neck, NJ 07722, United States
To evaluate the role of fluid and Na+ balance in the development of exercise-associated hyponatremia (EAH), changes in serum Na+ concentrations ([Na+]) and in body weight were analyzed in 2,135 athletes in endurance events. Eighty-nine percent of athletes completed these events either euhydrated (39%) or with weight loss (50%) and with normal (80%) or elevated (13%) serum [Na+]. Of 231 (11%) athletes who gained weight during exercise, 70% were normonatremic or hypernatremic, 19% had a serum [Na+] between 129-135 mmol/liter, and 11% a serum [Na+] of <129 mmol/liter. Serum [Na+] after racing was a linear function with a negative slope of the body weight change during exercise. The final serum [Na+] in a subset of 18 subjects was predicted from the amount of Na+ that remained osmotically inactive at the completion of the trial. Weight gain consequent to excessive fluid consumption was the principal cause of a reduced serum [Na+] after exercise, yet most (70%) subjects who gained weight maintained or increased serum [Na+], requiring the addition of significant amounts of Na+ (>500 mmol) into an expanded volume of total body water. This Na+ likely originated from osmotically inactive, exchangeable stores. Thus, EAH occurs in athletes who (i) drink to excess during exercise, (ii) retain excess fluid because of inadequate suppression of antidiuretic hormone secretion, and (iii) osmotically inactivate circulating Na+ or fail to mobilize osmotically inactive sodium from internal stores. EAH can be prevented by insuring that athletes do not drink to excess during exercise, which has been known since 1985. © 2005 by The National Academy of Sciences of the USA.