The orexigenic hormone ghrelin (GHR) has been shown to increase GER, whereas satiety hormones such as peptide tyrosin tyrosin (PYY), glucagon-like peptide-1 (GLP-1) and cholescystokinin have been shown to decrease GER. GER is influenced by a number of gut-derived hormones. To the authors knowledge, no studies have investigated the effects of fasted exercise compared with fed exercise on gastrointestinal function and the consequence this may have on metabolic responses. This could be due in part to differences in GER. A previous study has shown postprandial glucose response to be less following fasted exercise. Consequently, the influence of fasted exercise on gastrointestinal function is also of interest in postprandial metabolic responses. GER may also play a role in metabolic health as the delivery and absorption of nutrients in the small intestine is largely dependent on this process. Therefore, the effect of fasted exercise on gastric emptying rate (GER) may be an important mechanistic consideration for subsequent food and energy intake. The regulation of appetite and gastrointestinal motility appears to be intrinsically linked as the rate of gastric emptying determines the time of gastric distention, which is known to be a satiety signal.
However, the efficacy of whether exercising in the fasted state compared with the postprandial state is more beneficial for reducing energy intake and inducing a negative energy balance for weight loss remains unclear. Interest in the effect of fasted exercise has grown recently in both the scientific field and the general public. In addition, exercise alone induces health benefits, preventing the onset of metabolic diseases by modulating a range of risk factors. Intermittent fasting has been suggested as an effective method to promote health and prevent the onset of metabolic diseases by increasing insulin sensitivity and fatty-acid mobilisation, and reducing inflammation. With the prevalence of obesity reaching epidemic proportions worldwide, it remains a significant economic burden to healthcare systems globally. These findings suggest that gastrointestinal function, hunger and appetite regulatory hormones are not sensitive to low-intensity bouts of physical activity and holds positive implications for weight management practices. Insulin concentrations were higher in FED pre-exercise but higher in FASTED 1.5 h post lunch ( P < 0.05). A greater concentration of PP was observed in FED from pre-exercise to 30 min post lunch consumption (all P < 0.05). GLP-1 concentrations were greater in FED post-breakfast and pre-exercise ( P < 0.05), though no differences were observed after lunch. Minimal changes in appetite were observed post lunch ingestion with no differences in PYY or GHR observed between trials. Carbohydrate oxidation was greater at all time-points during FED exercise (all P < 0.05). NEFA concentrations were higher in FASTED at pre-exercise, post exercise and 30 min post exercise (pre-lunch) (all P < 0.05) but no differences were observed for glucose, cholesterol or triglycerides. No differences were observed for GER T ½ (FASTED 89 ± 22 vs. Substrate utilisation was measured every 30 min, and continuously throughout exercise by indirect calorimetry.
Subjective feelings of appetite were assessed at 15 min intervals throughout. Circulating concentrations of acylated ghrelin (GHR), glucagon-like peptide-1 (GLP-1), peptide tyrosine tyrosine (PYY), pancreatic polypeptide (PP), glucose, insulin, triglycerides, non-esterified fatty acids (NEFA) and cholesterol were measured. Blood samples were collected at baseline, post-breakfast period, pre-exercise, immediately post exercise, pre-lunch then every 30 min following lunch for 2 h. GER of a standardised lunch was subsequently measured for 2 h using the 13C-breath test method. Twelve healthy men completed two 45 min treadmill walks, fasted (FASTED) and followed consumption of a standardised breakfast (FED). To investigate the effect of brisk walking in the fasted versus fed state on gastric emptying rate (GER), metabolic responses and appetite hormone responses.
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