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 23days 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 3g·day 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 18h 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±23.0ng·ml) and PL (207.2±28.0ng·ml) 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.