Muscle, liver and kidney biomarkers in military training: a 37-week comparative study of dropouts and finalists

Military operational training soldiers are subjected to extreme physical and environmental stress, increasing the risk of exertional rhabdomyolysis (ER) and acute kidney injury (AKI). This study analysed muscle, liver and kidney biomarkers in Brazilian Navy marines undergoing 259 days of high-intensity training across diverse biomes, comparing biomarker behaviour between those who completed the course (finalists) and those who withdrew (dropouts).

This observational study followed 35 male marines (five finalists, 30 dropouts) during six operational missions, each lasting 7-12 days, involving intense physical exertion, controlled feeding, sleep deprivation and free hydration. Blood samples were collected at baseline and post-mission to measure creatine kinase (CK), lactate dehydrogenase (LDH), aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma-glutamyl transferase (GGT), albumin, creatinine, urea, estimated glomerular filtration rate (eGFR) and electrolytes. Statistical analyses included analysis of variance, Friedman tests and area-under-the-curve comparisons.

CK, LDH and AST levels remained elevated throughout training, with peak CK values in the 'tropical mission' (3,156 (2,397; 4,804) U/L) and the 'high-altitude mission' post-100km march (9,720 (7,956; 8,3969) U/L), indicating subclinical ER. ALT increased significantly in the 'high-altitude mission' (1,431 (902; 7,099) U/L), while GGT and albumin remained stable. Creatinine showed significant variation, decreasing after the survival phase of the 'high-altitude mission' (0.7 (0.7; 0.8) mg/dL), with eGFR reductions in the 'semi-arid mission' (92 (82; 118) mL/min/1.73 m²). No significant differences in CK, creatinine or eGFR were found between finalists and dropouts, suggesting non-physiological factors influenced dropout rates.

The 'tropical mission' and 'high-altitude mission' posed the highest risk for ER and AKI due to extreme CK elevations and creatinine fluctuations. Enhanced hydration protocols and CK reference value adjustments may be necessary to mitigate health risks in military training.