Objective: The present study examined the effects of pre-exercise different glycaemic index meals and timing on fat and carbohydrate metabolism, and appetite.

Methods: Nine postmenopausal women without exercise habits (age 59 ± 7 years, mean ± standard deviation) completed four trials in a random order. Participants consumed a high GI (HGI) or low GI (LGI) breakfast at 0900 and rested until 1000 (60 min-HGI trial or 60 min-LGI trial) or 1200 (120 min-HGI trial or 120 min-LGI trial). Thereafter, participants performed a 30-minute walk at 50% of estimated maximum oxygen uptake (1000-1100 or 1100-1200) and rested until 1300. Expired gas samples were collected continuously throughout the trials. Blood samples and ratings of subjective appetite were collected every 30 minutes in all trials.

Results: No main effect of trial was found in the cumulative fat oxidation or the carbohydrate oxidation during walking between four trials. At baseline (0900), there were no differences in fat or carbohydrate oxidation between the 60 min-HGI trial and the 60 min-LGI trial or the 120 min-HGI trial and the 120 min-LGI trial. A significant trial-by-time interaction was observed for the fat oxidation rates throughout the trial between the 60 min-HGI trial and the 60 min-LGI trial, and the 120 min-HGI trial and the 120 min-LGI trial. Subsequent post-hoc tests did not reveal between-trial differences for the fat oxidation rates. A significant trial-by-time interaction was observed for the carbohydrate oxidation rates throughout the trial between 60 min-HGI trial and the 60 min-LGI trial, and the 120 min-HGI trial and the 120 min-LGI trial. For the 60 min trials, post-hoc tests revealed that the carbohydrate oxidation rate was higher in the 60 min-LGI trial than in the 60 min-HGI trial at 0930 (P = 0.006). For the 120 min trials, post-hoc tests did not reveal between-trial differences in the carbohydrate oxidation. There were no differences in fasting values (0900) for the subjective appetite score throughout the trial between the 60 min- and 120 min-HGI trial and LGI trial. A significant trial-by-time interaction was observed for the subjective appetite score throughout the trial between the 60 min-HGI trial and the 60 min-LGI trial, and the 120 min-HGI trial and the 120 min-LGI trial. For the 60 min trials, subsequent post-hoc tests revealed that the subjective appetite scores were higher in the 60 min-HGI trial than in the 60 min-LGI trial at 1030 and 1100 (both of P ≤ 0.01). For 120 min trials, subsequent post-hoc tests revealed that the subjective appetite score was lower in the 120 min-HGI trial than in the 120 min-LGI trial at 1230.

Conclusion: Different GI values and timings of pre-exercise meals did not affect the substrate oxidation during a 30-minute walk in postmenopausal women, while the effects of meals with different GI values on appetite may change depending on the time of intake.