115 Furthermore, exercise fat oxidation was not affected when an LGI or HGI meal was provided the evening before116 and 117; this suggests that the “second meal effect” does not apply to fat oxidation. In the only study, we are aware of, to investigate GI and substrate oxidation in young people, Zakrzewski et al.65 examined the effect of HGI and LGI mixed-breakfast meals
on fat oxidation in overweight and non-overweight girls. They focused on the 2-h postprandial rest period and a subsequent 30-min walk at 50% V̇O2peak. Although breakfast GI did this website not affect postprandial fat oxidation during rest or exercise in either group of girls, it is noteworthy that LGI breakfast consumption resulted in 12% higher exercise fat oxidation (adjusted for fat free mass (FFM)) in both groups, a finding that may have meaningful health-related
implications if experienced regularly over an extended period.102 The similar insulin response between HGI and LGI reported in this study may have underpinned the similarity in fat oxidation.106 Furthermore, fructose has a lower GI than glucose, but results in higher blood lactate concentrations.127 It is possible that higher lactate concentrations compromised fat oxidation following the LGI breakfast through direct inhibition Akt inhibitor ic50 of adipose tissue FFA release.128 Indeed, resting fat oxidation was lower FAD after high fructose compared with high glucose meals in obese adults, despite lower glucose and insulin responses to the high fructose
meal.119 It is also possible that the 1.5 g CHO/kg body mass breakfast, 2-h postprandial period, and 30-min exercise duration at 50% V̇O2peak was a sub-optimal combination to induce differences in fat oxidation between HGI and LGI. However, higher exercise fat oxidation following LGI breakfasts has been reported 45 min to 3 h85 and 120 following breakfasts containing 1–2.5 g CHO/kg body mass during exercise lasting 60 or 30 min at 50%–71% V̇O2peak in adults.85 and 114 It is, therefore, difficult to ascertain which factors contribute specifically to the higher fat oxidation following LGI breakfasts in some adult studies. Furthermore, differences in fat metabolism between adolescents and adults129 may have resulted in discrepancies between this study and some of the adult literature. Consequently, these results require confirmation with larger independent samples of young people. It has been suggested that the reduced-fat oxidation following HGI breakfasts is largely due to the higher insulin response, which increases muscle glycogen stores and utilisation, resulting in higher CHO and lower fat oxidation.114 Indeed, Wee et al.