The effect of HMB ingestion on the IGF-I and IGF binding protein response to high intensity military training

Objective Insulin-like growth factor-I (IGF-I) is a metabolic and anabolic biomarker that has been proposed to reflect physiological adaptations resulting from multistressor environments. The bioactivity of IGF-I is regulated by seven different insulin-like growth factor binding proteins (IGFBPs) which act not only as carriers of IGF-1, but also function as a modulator of IGF-I availability and activity. Supplementing with β-hydroxy-β-methylbutyrate (HMB) has been shown to enhance physiological outcomes associated with intense training, and has been reported to augment the IGF-1 response. The purpose of this study was to examine the effect of 23 days of HMB supplementation on circulating levels of IGF-I and IGFBPs in combat soldiers during highly intense military training. Methods Thirteen male soldiers from an elite infantry unit volunteered to participate in this double-blind, parallel design study. Soldiers were provided 3 g·day^− 1 of either HMB (n = 6) or placebo (PL; n = 7). During the study soldiers performed advanced military training with periods of restricted sleep and severe environmental stressors. Blood samples were obtained prior to (PRE) and approximately 18 h following the final supplement consumption (POST). Results No significant differences were observed for circulating IGF-1 concentrations between HMB and PL (p = 0.568). In addition, no differences were seen between the groups for IGFBP-1 (p = 1.000), IGFBP-2 (p = 0.855), IGFBP-3 (p = 0.520), IGFBP-4 (p = 0.103), IGFBP-5 (p = 0.886), or IGFBP-6 (p = 0.775). A significant difference was noted between HMB (169.9 ± 23.0 ng·ml^− 1) and PL (207.2 ± 28.0 ng·ml^− 1) for IGFBP-7 at POST (p = 0.042). Conclusions Although the results of this study do not support the influence of HMB supplementation on circulating concentrations of IGF-1 or IGFBPs1–6 during high intensity military training, it does present initial evidence that it may lower circulating IGFBP-7 concentrations. This may provide some indication of a reduced stress response, but further investigation on the physiological role of IGFBP-7 and military training is needed.