Adipose Lipolysis Unchanged by Preexercise Carbohydrate Regardless of Glycemic Index

This study aimed to determine the effect of preexercise carbohydrate of different glycemic indices on subcutaneous abdominal adipose tissue (SCAAT) metabolism and running performance.

Ten trained male runners completed three experimental trials consisting of 30 min at 60% maximal oxygen consumption, 30 min at 75% maximal oxygen consumption, and a 5-km time trial. Thirty minutes before exercise, participants consumed one of three beverages: 1) 75 g low glycemic index modified starch supplement (UCAN), 2) 75 g high glycemic index glucose-based supplement (G), or 3) a flavor-matched noncaloric placebo (PL). SCAAT lipolysis was assessed via microdialysis.

Before exercise, blood glucose and insulin were elevated with G versus PL (+53.0 ± 21.3 mg·dL (mean ± SD), P < 0.0001; +33.9 ± 11.0 μU·mL, P < 0.0001) and G versus UCAN (+36.6 ± 24.9 mg·dL, P < 0.0001; +25.2 ± 11.0 μU·mL, P < 0.0001), respectively. Fat oxidation was attenuated, and carbohydrate oxidation increased before exercise with G versus PL (-0.06 ± 0.06 g·min, P = 0.005; +0.18 ± 0.07 g·min, P < 0.0001) and G versus UCAN (-0.06 ± 0.05 g·min, P = 0.004; +0.18 ± 0.14 g·min, P < 0.0001). However, there were no differences in SCAAT lipolysis at rest or during running at either exercise intensity. Also, there was no effect of treatment on running performance.

Preexercise carbohydrate lowers fat oxidation and increases carbohydrate oxidation, and these effects are attenuated with low glycemic index carbohydrate. However, these changes are not the result of alterations in SCAAT lipolysis, nor do they affect running performance.