The effects of pre-exercise high and low glycaemic index meals on substrate metabolism in inactive middle-aged women

Background and objectives: Unhealthy diet and physical inactivity are the major risk factors for non-communicable diseases. Glycaemic index (GI) is an indicator of glucose absorption into the systemic circulation after ingestion of the same amount of carbohydrates from different foods. Given that lowered postprandial insulin after consuming low GI foods, the consumption of low GI foods has been shown to enhance fat oxidation and reduce carbohydrate oxidation compared with high GI foods during subsequent exercise. However, few studies have examined the influence of pre-exercise meal with different GIs in untrained adults performed feasible exercise. Thus, the aim of this study was to investigate the effect of high and low GI meals before exercise on substrate metabolism in inactive middle-aged woman. Methods: Ten inactive middle-aged women (aged 42±6 years, mean ± standard deviation) completed two trials, high GI and low GI, in a randomised crossover design. Each participant consumed an isocaloric high GI or low GI breakfast (1.5 g/kg body mass carbohydrate) with GI values of 73 and 41, respectively 120 minutes before a 60-minute walk at 50% of maximum oxygen uptake. After performing a 60-minute walk, participants rested for 60 minutes. Expired gas samples were collected every 30 minutes for 15 minutes prior to walking, and further samples were collected continuously throughout the walking session and the entire 60-minute post-walking period. Results: The pattern of fat oxidation response was different between trials (trial-by-time interaction, P