Endocrine, Skeletal, and Circadian Biomarker Profiles Across Operational Phases in Elite Special Forces Operators

ABSTRACT: Kraemer, WJ, Volek, JS, Stephenson, MD, Szivak, TK, Saenz, C, Buga, A, Lee, EC, Ratamess, NA, Post, EM, Caldwell, LK, Hagen, JA, and Maresh, CM. Endocrine, skeletal, and circadian biomarker profiles across operational phases in elite special forces operators. J Strength Cond Res XX(X): 000-000, 2026-This investigation examined multisystem biomarker responses across distinct operational phases in elite Special Forces operators and evaluated whether integrated patterns, including principal component analysis (PCA), could reveal coherent signatures of stress and recovery. Fifty-six male operators (28-42 years, 175-187 cm, 77-95 kg) participated in a longitudinal study. Resting venous blood samples were obtained at 3 visits aligned with recovery/reset (visit 1), peak training stress (visit 2), and reintegration (visit 3). Serum analytes included 25-hydroxy vitamin D, triiodothyronine (T3), β-C-terminal telopeptide (CTX), melatonin, C-reactive protein (CRP), R-β-hydroxybutyrate (R-BHB), and plasma samples for ghrelin, neuropeptide Y (NPY), and vascular endothelial growth factor (VEGF). All biomarkers were analyzed in duplicate using ELISA. Repeated-measures analysis of variance with Bonferroni correction (p ≤ 0.05) and exploratory PCA were performed. Vitamin D and T3 declined significantly from visit 1 to visit 2 (-16% and -30%; p < 0.001) and remained suppressed at visit 3. CTX decreased markedly at visit 2 (-87%; p < 0.001) but recovered by visit 3. Ghrelin increased at visit 2 (+16%; p = 0.002), whereas NPY increased only at visit 3. Melatonin doubled at visit 2 (+105%; p < 0.001) and remained elevated. R-BHB, VEGF, and CRP did not change significantly. Exploratory PCA identified 3 components representing hormonal-circadian regulation, metabolic-inflammatory strain, and neuroendocrine appetite regulation. A phase-based, multisystem biomarker panel, supported by PCA-derived physiological axes, identified coherent signatures of operational stress and recovery. These findings support the use of integrated biomarker monitoring to characterize system-level stress responses and inform phase-specific recovery and readiness strategies in elite tactical populations.