Marines Suffer Most Cases of 2 Life-Threatening Conditions Related to Exercise and Heat, Report Finds

Recruits run during a physical training session at Marine Corps Recruit Depot, San Diego.
Recruits with Bravo Company, 1st Recruit Training Battalion, run during a physical training session at Marine Corps Recruit Depot, San Diego, April 12, 2020. (Cpl. Brooke C. Woods/U.S. Marine Corps photo)

Cases of two dangerous exercise-related medical conditions -- breakdown of muscles and sodium imbalances -- have risen in the past four years among U.S. service members, with the Marine Corps experiencing the highest rates.

Instances of those largely preventable illnesses, called rhabdomyolysis and hyponatremia, respectively, rose from 2020 to 2023, with the former reaching a high for the period and the latter having the second-highest rate since 2008, according to Defense Health Agency reports published May 8.

The study said 529 active-duty service members experienced exertional rhabdomyolysis, commonly shortened to "rhabdo," while 153 troops developed hyponatremia last year.

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Rhabdo is a breakdown of muscle cells that can occur as a result of heat, trauma or intense workouts. The degeneration releases protein and other cell components into the bloodstream, where they can cause organ damage, including electrolyte imbalances, kidney failure and heart arrhythmia.

Hyponatremia is a sodium imbalance often caused by overconsumption of fluids such as water or sports drinks before or during vigorous exercise.

Both conditions can be life-threatening.

According to the reports, rates of rhabdo in the Marine Corps in 2023 were roughly five times the rate of the Air Force, Navy and Coast Guard. The Army had the second-highest rate, nearly three times the other three services.

Rates for rhabdo were highest among recruits, non-Hispanic Black service members and those in combat-arms specialties such as infantry, artillery, combat engineering and armor.

The Marine Corps also topped incidence rates in 2023 of exertional hyponatremia, at nearly 18 cases per 100,000 person years, with the Army and the Air Force experiencing rates roughly two-thirds that of the Corps. Hyponatremia rates were highest among women, troops older than 40 and those stationed in northeastern U.S., as well as recruits and service members in combat-arms occupations.

The researchers did not say why the Marine Corps saw the highest rates of both conditions. The service's physical fitness test requires the longest run of any armed forces training -- three miles -- and the bulk of its bases are located in warm climates where the majority of operational Marines train or work outside.

According to the reports, installations with the highest number of cases largely were in states with high heat and humidity. From 2019 to 2023, there were 288 cases of rhabdo at Fort Liberty, North Carolina; 248 cases at Marine Corps Recruit Depot Parris Island; 199 cases at Fort Moore, Georgia; and 134 cases at Camp Lejeune and Marine Corps Air Station Cherry Point, North Carolina. Camp Pendleton rounded out the top five with 99 cases during the study period.

For exertional hyponatremia, Parris Island experienced the highest number of cases across a longer study period between 2008 and 2023 -- 203. Fort Moore was second with 140, followed by Joint Base San Antonio-Lackland with 68; Fort Liberty, with 61; and Camp Lejeune and MCAS Cherry Point, with 59.

The authors said that, given that both conditions are largely preventable, leadership should continue to be vigilant, monitoring environmental conditions, troop fitness and hydration, and work and rest ratios. They said careful attention should be paid to preconditioning before strenuous training, and all who work with troops should be able to identify the symptoms of either condition.

"The most severe consequences of exer­tional rhabdomyolysis are preventable with effective mitigation measures and heightened suspicion of probability when environmental conditions favor muscu­lar injury," the authors wrote.

"Considering the characteristics of military environ­ments such as long-term military train­ing and combat operations, exertional hyponatremia may continue to pose a health risk to U.S. military personnel, sig­nificantly reducing performance and com­bat effectiveness," they wrote.

In 2019, five service members, including two Navy sailors and three airmen, died during physical training or shortly thereafter. Two of the deaths were attributed to exertional rhabdomyolysis.

In the same issue of the Medical Surveillance Monthly Report as the studies on rhabdo and hyponatremia, Armed Forces Health Surveillance Division researchers said the rates of heatstroke declined for active-duty service members from 2019 to 2023, while rates of heat exhaustion rose.

In 2023, 415 service members suffered heatstroke, while 2,263 experienced heat exhaustion. Those at high risk include the unacclimated; personnel taking medications for depression, allergies, high blood pressure or attention deficit hyperactivity disorder; and those who aren't in good physical shape.

"Leaders, as part of their risk assessments, must balance miti­gation efforts against the requirements of their operations or training activities," the researchers wrote. "The most effective countermeasures against heat illness include restricting activ­ity to early morning or evening hours when environmental heat is lower; adherence to work and rest cycles based upon current heat conditions; removal or modification of gear to facilitate heat loss; maintenance of proper hydration levels; maximized physi­cal fitness; and gradual acclimatization to the local heat environment."

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