Are high glycemic foods before a workout beneficial or detrimental?

For years, bodybuilders have been told to not eat high glycemic foods before exercise because it blunts fat mobilization by spiking insulin. Spiking insulin inhibits fat burning and skeletal muscle fatty-acid oxidation and these effects can persist for at least 4 hours after a high carbohydrate meal.

Previous research has reported that high glycemic exercise before moderate intensity exercise has demonstrated increased fat utilization after a low glycemic meal, but the research on high intensity exercise and glycemic index is lacking.  Basically, they wanted to know if a high intensity type exercise protocol was affected by a low or high glycemic index pre-exercise meal.  One would suspect that the low glycemic index meal would result in greater fat oxidation.

Researchers from Canada reported that a both high and low glycemic index meal before highly intense exercise increased performance compared to going to the gym on a empty stomach, but more interesting there was no effect on insulin, glucose, circulating hormones, or fat oxidation when high intensity exercise was performed.

The take home message is that if you are performing a moderate intensity exercise, glycemic index can affect fat oxidation but if you are performing high intensity exercise, glycemic index is not going to impact fat oxidation so bodybuilders should be too paranoid about consuming some carbohydrates before a really intense session, as based on the current study, its not going to impact fat oxidation.  Additionally, insulin is a potent vasodilator so one would suspect much better pumps in the gym with some carbohydrates.
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Reference:

Little JP, Chilibeck PD, Ciona D, Forbes S, Rees H, Vandenberg A, Zello GA. Effect of low- and high-glycemic-index meals on metabolism and performance during high-intensity, intermittent exercise. Int J Sport Nutr Exerc Metab. 2010 Dec;20(6):447-56

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Robbie Durand has been in the sports supplement and  bodybuilding industry for 15 years. He has contributed to many national magazines and web sites. He has an M.A. in exercise physiology from Southeastern University and a B.A. in Dietetics from Louisiana State University.