Acute Incremental Exercise Decreases Plasma Ghrelin Level in Healthy Men

Introduction

Ghrelin, a 28 amino acid peptide, was originally isolated as an endogenous ligand for growth hormone (GH) secretagogue receptor (GHS-R) from human and rat stomach [1]. This peptide strongly stimulates GH secretion and food intake [1]. Plasma ghrelin levels increase following fasting and decrease after oral and intravenous glucose administration, suggesting that ghrelin is upregulated under conditions of negative energy balance and downregulated in the setting of positive energy balance [2].

The systemic circulation during acute exercise is regulated by the attenuation of vagal efferent activity, known as Bainbridge reflex, and an increase of sympathetic nerve activity. A vagal efferent pathway mediates ghrelin release [3] [4]. In addition, gastric vagal afferent serves as the major pathway conveying ghrelin's signals for starvation and growth hormone secretion to the brain [5]. Moreover, ghrelin exhibits GHS-R-independent biological activities, including a cytoprotective effect on cultured cardiomyocytes and a proliferative effect on human osteoblast [6] [7]. Thus, ghrelin plays a role in cell survival through anabolic processes. As exercise stress induces a variety of catabolic processes, involving glycolysis, lipolysis, and protein degradation [8]. We assume that ghrelin release and ghrelin action may change in response to the alternation of the autonomic activity and/or catabolic state during an acute exercise.

A number of studies have reported the relationship between plasma ghrelin levels and exercise [9] [10] [11] [12] [13] [14]. Although acute aerobic exercise such as running and cycling exercise is not likely to alter plasma ghrelin levels [9] [11] [12] [14], acute resistance exercise induces a decline in circulating ghrelin levels [10] [13]. These findings suggest that mechanical stress to muscles or the metabolic state may influence the plasma ghrelin concentrations. However, the relationship between physical exercise and changes in plasma ghrelin levels has yet to be defined. We investigated the effect of energy consumption including metabolic changes on plasma ghrelin level using incremental exercise protocol, in which each stage was set for 10 minutes for stabilizing the relationship between exercise intensity and hormonal parameters.

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1 These authors contributed equally to this work.

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