Paying attention to the glycemic index may be a positive for your performance.

At first glance, using the glycemic index might sound like another nutritional phenomena that could be shelved next to the Zone Diet, Atkins or South Beach. However, before you nix this nutrition guideline and catalogue it with the list of growing diet trends and passing food faux pas, take another look at what’s on the menu and how you as an athlete can benefit.

The Facts

The glycemic index (GI) is a scale that ranks the effect of glucose on your body by measuring how fast it raises your blood sugar level and assigns it a number (0-100). While the index was originally designed for diabetics, sports nutritionists found they could gauge a food’s GI effects on athletes.

“Athletes commonly eat foods solo, a banana, a bagel,” says Boston-based Nancy Clark, MS, RD, CSSD and author of Nancy Clark’s Sports Nutrition Guidebook. “Hence exercise scientists became curious about the possibility that quick or slow carbs impact exercise performance because they affect blood glucose in different ways.”

These findings helped establish the glycemic index as a useful tool for maximizing energy levels during exercise by indicating what to consume at what time. Some foods, like white bread, are known to quickly raise GI levels, while others, like lentils, are known to maintain low blood sugar levels. Translation: your GI level could very likely affect your athletic performance. So how do you know when to eat what, and why?

The Logistics            

Eric Hörst, climbing-training expert and author of Training For Climbing and Conditioning For Climbers, says a good rule of thumb is to consume “low- and mid-GI foods before and during climbing/training, then high GI foods immediately afterwards (to speed recovery), followed by a taper back to mid- and low-GI foods” if another workout is scheduled within 24 hours.

This kind of glycogen loading is especially important for endurance sports and those with rigorous daily training schedules to maintain consistent energy long before you mount your bike, tie in for a climb or eject the start line of a race.

“The consumption of carbohydrates in the hour before exercise can help increase glycogen stores and raise blood glucose and insulin levels,” says John Ivy, exercise physiologist at the University of Texas and author of The Performance Zone, and it “will help jump-start the fueling process.”

Consuming low-GI foods prior to exercise will help you maximize this energy. Some athletes, however, might be sensitive to certain foods. Clark suggests you “go by what feels good for your stomach.”

“Continue to consume low- to mid-GI carbs during exercise,” says Hörst—like a sports drink, gel, or granola bar—to keep a steady flow of nutrients and energy and help maintain the stores. According to Ivy, by doing this, “there is less likelihood of having a rebound hypoglycemic response.”

Unlike low-GI foods, high-GI foods will spike your blood sugar; the higher the GI levels, the quicker they raise blood-sugar levels. When blood-sugar levels are spiked the athlete might at first experience a jolt of energy, but that will undoubtedly be followed by “hitting the wall” or “bonking” when energy levels drastically drop.

While it’s not advised to eat high-GI foods before a workout, at the end of a workout it’s important to replenish the muscles as quickly as possible during a 45-minute time period after intense exercise known as the “glycogen window.”

According to Ivy, intense exercise quickly uses up muscle glycogen stores, and it’s “important to replenish muscle glycogen as fast as possible.” Ivy suggests trying “a supplement of carbohydrates (high glycemic) plus protein. It will increase muscle glycogen synthesis faster than carbohydrates alone…and will limit muscle tissue breakdown that occurs with hard exercise.”

Bob Seebohar, MS, RD, CSSD, CSCS and sports dietician at the U.S. Olympic Committee, has been working with athletes for the past 10 years. He says it “somewhat depends on the sport and the specific energy systems used, but in general, having ‘slower burning’ (lower GI) fuel before a competition or training session is a benefit to almost any athlete, while ‘faster burning’ (higher GI) fuel during a competition or training session is beneficial in order to maintain blood sugar and prevent hypoglycemia.”

Is the Glycemic Index for Everyone? 

While many athletes adhere faithfully to the parameters of the glycemic index while training, some believe that what works in theory doesn’t always translate. Clark suggests that there are too many grey areas and variables in the glycemic index, and claims it’s a “false scale.” How much you eat, the food’s preparation, hot or cold, and where it was grown are only a few of the things that factor in when determining GI. What’s more, response to carbs can vary not only from person to person, but throughout the day. “It’s better to experiment with different foods,” says Clark, and then decide what works for you.

In the end, diet choices are yours. The GI rating can create a good guideline to follow, but ultimately you’re the boss of what goes into your  body.