Exertional hyponatremia among active component members of the U.S. Armed Forces, 2009-2024.

Exertional hyponatremia, or hyponatremia associated with exercise, occurs within 24 hours after physical activity due to a serum, plasma, or blood sodium concentration ([Na+]) below the normal reference range of 135 mEq/L. Hyponatremia can be fatal if not detected early and managed properly. From 2009 to 2024, 1,829 cases of exertional hyponatremia were diagnosed among U.S. active component service members (ACSMs), with an overall incidence rate of 8.4 cases per 100,000 person-years (p-yrs). In 2024, 134 cases of exertional hyponatremia were diagnosed among ACSMs, resulting in a crude incidence rate of 10.4 per 100,000 p-yrs. Female service members, those older than 40 years, non-Hispanic White service members, Marine Corps members, recruits, those in health care occupations, and ACSMs stationed in the Midwest U.S. had higher incidence rates of diagnosis for exertional hyponatremia than their respective counterparts. From 2009 to 2024, annual rates of incident exertional hyponatremia diagnoses peaked in 2010 (12.8 per 100,000 p-yrs) and then decreased to a low of 5.3 cases per 100,000 p-yrs in 2013. The incidence rate has fluctuated since then, rising from 6.1 per 100,000 p-yrs in 2017 to the second-highest level (11.2 per 100,000 p-yrs) in 2023 before decreasing to 10.4 per 100,000 p-yrs in 2024. Service members and their supervisors must be aware of the dangers of excessive consumption of water and the prescribed limits of water intake during prolonged physical activity, including field training exercises, personal fitness training, as well as recreational activities, particularly in hot, humid weather. Incidence rates of exertional hyponatremia changed from 2023 to 2024, with the overall incidence rate decreasing from 11.2 to 10.4 per 100,000 p-yrs. Rates increased, however, in the 25-29 years age group and in the Coast Guard, while decreasing sharply among non-Hispanic Black individuals and recruits. The highest incidence rates were observed in non-Hispanic White individuals and health care personnel.