The effect of HMB ingestion on the IGF-I and IGF binding protein response to high intensity military training
Introduction
Insulin-like growth factor-I (IGF-I) is a metabolic and anabolic biomarker that has been examined in relation to physiological changes resulting from multistressor environments such as inadequate rest, high volume training, and poor nutrition [1], [2], [3]. Physiologically, IGF-I is known for having both mitogenic, and myogenic properties, and has been studied for its role in muscle remodeling and hypertrophy following training [2], [4], [5]. 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, which prolong the half-life of IGF-I, but also function as a modulator of IGF-I availability and activity [6]. Circulating IGF-I concentrations have been reported to be associated with lean body mass and level of fitness [7]. Studies have also revealed a positive relationship between energy balance and IGF-I concentrations [2], [8], [9].
β-Hydroxy-β-methylbutyrate (HMB) is a metabolite of the branched-chain amino acid leucine and is produced endogenously in small amounts [10]. Supplementing with HMB has been shown to attenuate muscle loss and promote gains in strength and lean body mass by inhibiting protein degradation and stimulating protein synthesis [10], [11], [12]. HMB-induced elevation in net skeletal muscle protein balance, may be driven in part by increases in hepatic, and skeletal muscle IGF-1 [13], [14]. A recent study reported that acute HMB supplementation can augment the IGF-1 response to resistance training [15], while others have reported no change in resting IGF-1 concentrations following 7-weeks of HMB supplementation [16].
Physiological stresses associated with military training or deployment (e.g., environment, nutrition and sleep deprivation) have been well documented [17], [18], [19], [20]. Nindl et al. [18] observed a 12.6% decrement in body mass, a 6.1% decrement in fat free mass, a 20% decrement in repetitive dynamic lifts, a 16% decrease in vertical jump and a 21% decrease in peak power following 8-weeks of intense Ranger training. In addition, they reported a significantly lower muscle cross-sectional area in both upper and lower limbs. During military deployment, periods of high intensity operations are also interspersed with lower intensity military activity. A 9-month deployment has been reported to result significant decreases in body mass, aerobic capacity and upper body muscular power performance (− 1.9%, − 4.9% and − 6.6%, respectively) [20]. These stresses have also been reported to pose significant challenges on the health and performance of soldiers [19]. It has been previously suggested that IGF-1, and it's binding proteins, may serve as important biomarkers for a variety of health related outcomes; both positive (i.e., muscle hypertrophy, bone growth, and body composition) and negative (i.e., greater cancer risk and decreased longevity) [5]. Friedl et al. [3] reported a significant decline (approximately 50% decrease) in IGF-I concentrations during an 8-week United States Army Ranger training course. Although Nindl et al. [1] reported no change in IGF-I and IGFBP concentrations in male soldiers participating in a gender integrated 9-week basic combat training course, they did report that IGF-I concentrations were significantly elevated and IGFBP-2 was significantly reduced in the women soldiers. Others have reported significant decreases in IGF-I concentrations during 8-days of sustained, high intensity military training [2], [21]. Significant elevations in IGFBPs1–3 have also been reported during these high intensity training days [21].
Investigations examining HMB supplementation and the IGF-I response have primarily used recreational or young, competitive athletes [15], [16]. The physiological and psychological stresses associated with military training generally exceed that which is common among competitive athletes [17]. Thus, the potential for HMB supplementation to maintain the IGF-I system (i.e., IGF-I and IGF binding proteins) during highly intense military training may be enhanced. Therefore, the purpose of this study was to examine the effect of HMB supplementation on circulating concentrations of IGF-I and IGF binding proteins in combat soldiers during highly intense military training.
Section snippets
Participants
Twenty-seven male soldiers from an elite combat unit of the Israel Defense Forces (IDF) volunteered to participate in this double-blind, parallel design study. Following an explanation of all procedures, risks and benefits, each participant provided his informed consent to participate in the study. The Institutional Review Board of the IDF Medical Corp approved this research study (ClinicalTrials.gov identifier: NCT02503007). Participants were not permitted to use any additional dietary
Results
Participants included in the final analysis consumed 89.3 ± 6.8% of the possible servings. Among participants that began the study, no differences were found between HMB and PL in reported side effects. Gastrointestinal discomfort (cramps, bloating or and/or diarrhea) was reported in 14 of the original 27 participants. However, these complaints were reported evenly between the groups; seven of the complaints were made by participants in HMB and seven reported by PL. No other side effects were
Discussion
The main findings of this study indicated that 23 days of HMB supplementation appeared to have no significant effect on circulating concentrations of IGF-I as well as IGFBPs 1–6 in combat soldiers during highly intense military training. However, HMB supplement intervention appeared to decrease resting IGFBP-7 concentrations compared to PL. The examination of IGF-I has been previously recommended as a biomarker reflecting physiological stress in military personnel [4], [5]. In general,
Acknowledgements
The authors would like to thank Metabolic Technologies Inc. (Ames, IA) for providing support for this study. All authors declare that they have no conflict of interests to report.
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