Can Changes in Body Mass and Total Body Water Accurately Predict Hyponatremia After a 161-km Running Race? Lebus DK, Casazza GA, Hoffman MD, et al (Univ of California Davis Med Ctr, Sacramento; et al) Clin J Sport Med 20:193-199, 2010 Objective.dTo relate changes in body mass, total body water (TBW), extracellular fluid (ECF), and serum sodium concentration ([Na + ]) from a 161-km ultramarathon to finish time and incidence of hyponatremia. Design.dObservational. Setting.dThe 2008 Rio Del Lago 100-Mile (161-km) Endurance Run in Granite Bay, California. Participants.dForty-five runners. Main Outcome Measurements.dPre-race and post-race body mass, TBW, ECF, and serum [Na + ]. Results.dBody mass and serum [Na + ] significantly decreased 2% to 3% (P < 0.001) from pre-race to post-race, but TBW and ECF were unchanged. Significant relationships were observed between finish time and percentage change in body mass (r ¼ 0.36; P ¼ 0.01), TBW (r ¼ 0.50; P ¼ 0.007), and ECF (r ¼ 0.61; P ¼ 0.003). No associations were found between post-race serum [Na + ] and percentage change in body mass (r ¼0.04; P ¼ 0.94) or finish time (r ¼ 0.5; P ¼ 0.77). Hypo- natremia (serum [Na + ] < 135 mmol/L) was present among 51.2% of finishers. Logistic regression prediction equation including pre-race TBW and percentage changes in TBW and ECF had an 87.5% concordance with the classification of hyponatremia. Conclusions.dHyponatremia occurred in over half of the 161-km ultra- marathon finishers but was not predicted by change in body mass. The combination of pre-race TBW and percentage changes in TBW and ECF explained 87.5% of the variation in the incidence of hyponatremia (Fig 4). : An introductory article in the 2007 Year Book of Sports Medicine 1 queried how far hyponatremia was a problem in well-managed athletic competition. The study by Lebus and associates provides pre- and postrace sodium ion concentrations for 41 runners completing an ultramarathon event. The time to cover 161 km averaged almost 26 hours. Environmental temperatures during this period ranged widely from 12.2  to 37.6  C, but no other details on weather conditions are given. Twenty-four aid stations provided various foods, including salty snacks, water, and sports drinks, but the nutritional advice given to the runners is not detailed, but probably followed the Western States Endurance Run Participants Guide. 2 None of the group required medical treat- ment for hyponatremia, although 16 of 41 athletes had what some have termed a biochemical hyponatremia (Na+ concentration of 130-135 mM/L), and 5 athletes were in the clinically hyponatremic zone (Na+ concentration <130 mM/L). 3 The study makes several interesting observations. Firstly, the average prerace sodium ion concentration (138 mM/L) was a little low, sug- gesting that some runners had hyperhydrated prior to the event. Secondly, 314 / Sports Medicine