Discussion
The present study demonstrated that consuming a low GI meal increased fat oxidation and decreased carbohydrate oxidation compared to a prior high GI meal during a 60-min walk in middle-aged women. Despite these changes in substrate oxidation during walking and postprandial alterations in glucose, insulin, and total PYY concentrations, consuming low GI meals did not appear to have a significant overall effect on homeostatic or non-homeostatic appetite assessed over time.In the present study, fat oxidation was enhanced and carbohydrate oxidation was suppressed during exercise when a low GI meal was consumed 100 min before performing low intensity exercise. These findings observed in ‘middle-aged’ women coincide with most of the previous acute studies conducted in young menTo the best of our knowledge, the present study is the first to examine the acute effects of prior GI meals on substrate oxidation during exercise in Asian (Japanese) women, as most studies examining this area have employed participants from Western countries (for a review, seeIn the present study, serum 3-hydroxybutyrate concentrations during the last hour of the postprandial period were higher in the low GI trial than in the high GI trial. This finding suggests that consuming a low GI meal could have promoted the catabolism of fatty acids because 3-hydroxybutyrate is synthesised from acetyl-coenzyme A, which is produced by the oxidation of fatty acids in the liver.Several studies have reported that the hedonic system of non-homeostatic appetite control is affected by meal-induced changes in hormone concentrations, habitual physical activity, and structured exercise.Regarding homeostatic appetite, the findings of enhanced total PYY concentrations during the postprandial period in the low GI trial compared to the high GI trial in the present study are not consistent with those of a previous study that reported no difference in total PYY concentrations between low and high GI trials in young, sedentary women.The present study had several strengths. Middle-aged women were employed, whereas most previous studies examining the effects of the different GI meals on substrate oxidation during subsequent exercise have employed young men, with few studies in middle-aged or older women (for a review of these, seeIn conclusion, the present study shows that consuming a low GI meal elicits enhanced fat oxidation during a subsequent 60-min walk in middle-aged women. However, differences in postprandial glucose and insulin concentrations induced by different GI meals did not affect subjective appetite or food reward evaluated over time in the present study.