Glycemic Index: Does it matter what you eat before you exercise?

The glycemic index (GI) ranks carbohydrates according to their ability to increase blood glucose. This concept was initially developed to help people with diabetes stabilize their blood glucose levels. The concept has transferred to sports, raising the question: Does eating slowly released carbs (with a low glycemic response) enhance endurance performance by providing sustained energy? Research shows mixed results regarding whether or not it matters if you eat low or high glycemic foods before you exercise. Yet, many of the studies have used pre-exercise meals that are atypical of what an athlete might actually eat (lentils, instant mashed potato with eggs whites and ketchup). 

    A recent study has helped shine new light on this topic. In this study, soccer players ate a pre-exercise lunch at 11:30 a.m., with about 870 to 890 calories of either a—

• high glycemic meal (GI 80) consisting of sports drink (245 cals), askash rice (200 cals), 3.5 oz. chicken breast (160 cals), and tomato sauce (260 cals)

• low glycemic meal (GI 40) consisting of similar foods, but swapping the high glycemic sports drink and askash rice for lower glycemic apple juice and basmati rice. Both meals had similar amounts of carbs, protein, fat, and calories. 

 Three and a half hours later, the subjects performed soccer-specific drills followed by a 1 km time trial (to deplete whatever was “left in the tank”).

In both trials, the athletes showed similar glucose and insulin responses and burned similar amounts of fat and carbohydrate. Yet, 5 of the 8 athletes were faster in the 1 km time trial after the low glycemic meal. 

Although the 5 s-second improvement was not “statistically significant”, a competitive athlete consider it physiologically significant. The authors concluded the type of carbohydrate ingested in a pre-match meal has no significant impact on performance or metabolic responses during 90 minutes of intermittent high intensity exercise. Yet, they agree the question of whether or not to choose low GI pre-exercise foods warrants more research using protocols that match the pre-exercise foods athletes typically eat.

In situations where the athletes are fueling during the exercise, whether or not to consume a low GI meal becomes a moot question because fueling during exercise will offer benefits that trump the pre-exercise glycemic effect of the food.

Source

Hulton AT, W Gregson, D Maclaren, D Doran. Effect of GI Meals on Intermittent Exercise. Int’l J Sports Med. Published online 2012. 

For more information: Nancy Clark’s Sports Nutrition Guidebook