The purpose of this study was to identify physiologic changes in body composition and resting metabolic markers of health across 2 wk of critical training (CT) in wildland firefighters (WLFFs).

Twenty-two male and 3 female participants were recruited from 2 hotshot crews across the western United States prior to the 2022 fire season and monitored over their 80-h CT. Body weight (BW) and skinfolds were recorded before and after CT to estimate body fat (BF) and lean body weight (LBW). Blood was analyzed for changes in hematocrit, hemoglobin, plasma volume, and resting values of a lipid and metabolic panel.

The high physical demands of CT resulted in improvements in total cholesterol (−19.3 mg/dL, P<0.001), triglycerides (−34.4 mg/dL, P<0.001), low-density lipoprotein cholesterol (−18.1 mg/dL, P<0.001), very-low-density lipoprotein cholesterol (−5.2 mg/dL, P<0.001), high-density lipoprotein cholesterol (+4.0 mg/dL, P=0.002), non-high-density lipoprotein cholesterol (−19.3 mg/dL, P<0.001), and fasting glucose (−4.3 mg/dL, P=0.008) from before CT to after CT. Significant decreases in hemoglobin and hematocrit were also seen (P<0.001) with corresponding increases in estimated plasma volume (+6.1%, P<0.001). These alterations were seen despite maintenance of BW, LBW, and BF. Lower pretraining BF was associated with a greater magnitude of improvements in fasting glucose and cholesterol markers.

The observed improvements in baseline metabolic and cardiovascular markers along with plasma volume expansion suggest a positive response to the physical stress of WLFF CT. It appears that higher preseason fitness was associated with greater adaptations to the CT stressor.