If you’ve ever heard about the glycemic index, chances are it was in relation to managing diabetes. Or maybe, more recently, you saw a headline that made you wonder if you’ve been avoiding pasta unnecessarily. So what exactly is this “index” and how useful is it for the everyday runner?
What is Glycemic Index?
At the most basic level, the glycemic index (GI) is a measure of how quickly a food causes a person’s blood sugar to rise. Foods are ranked on a scale of 0 to 100, with lower numbers indicating that foods take longer to digest and absorb, causing a slower rise in blood sugar compared to foods with a higher number. For example, white bread has a score of 71, whereas chickpeas have a score of 10. People managing diabetes use the index to help keep their blood sugar levels in check.
Why Should Runners Care About Glycemic Index?
For starters, glycemic index is one more useful way to classify most runners’ fuel of choice: carbohydrates. A GI ranking offers a quick and dirty way to identify if a carb is highly processed, because foods that are high on the GI scale are often also high in processed carbohydrates and sugars. Alternatively, foods that are lower on the scale are usually richer in fiber, protein, and/or fat.
“We want relatively stable energy levels as we exercise and live,”says Claire Shorenstein, R.D. and certified coach for Road Runners Club of America. While she doesn’t use the index when working with her clients (she usually looks at other factors, such as fiber), she speculates that since lower GI foods lead to more stable blood sugar, endurance runners might find that lower GI foods provide them more sustained energy throughout their runs.
What Does the Science Say?
A recent study in the journal Nutrients examined the potential advantages of a low glycemic index diet for runners. Scholars from Poznan University of Life Sciences and Vanderbilt University recruited 21 trained endurance runners who were randomly assigned to eat either a low GI diet or a medium-GI diet for three weeks. Both diets included the same number of calories, comprised of 60 to 64 percent carbs, with the low GI diet including carbs like whole-grain rye bread, rolled oats, brown rice, and vegetables (except for corn and potatoes, which rank higher on the index), and the medium GI diet including carbs like wheat bread, potatoes, cornflakes, and white rice. After three weeks of eating one diet, the runners took a one-week “break” during which they went back to their regular diet, and then they switched to the other study diet (either low GI or medium GI) for three more weeks.
At the beginning and end of each three-week diet, runners participated in an aerobic capacity test (pedaling a stationary bike while the resistance gradually rose until exhaustion) and an endurance test (running as far as they could in 12 minutes). After eating the low glycemic index diet, the runners lasted longer on the cycling-to-exhaustion test and covered significantly more ground on the 12-minute running test. Results also showed that the low GI diet decreased runners’ body mass, compared to the medium GI diet. The researchers therefore concluded that, “athletes and coaches might consider the GI of food in planning a healthy diet, as the LGI [low GI] diet might slightly improve performance and help to achieve desirable body composition.”
The Bottom Line
Although the study indicated that it included both professional and recreational runners, a sample size of twenty-one runners is too small to take into account every type of “trained endurance runner.” Also, three weeks is not a very meaningful amount of time to test a diet, considering how long a typical training cycle lasts (e.g., 16 to 20 weeks for a marathon), and 12 minutes is probably not what any marathon runner would consider a meaningful test of their endurance. Plus, because the researchers did not test the athletes’ blood, they’re unable to conclude the exact biological reasons why a low GI diet affected exercise performance, endurance, and body mass the way it did.
Still, studies like this one do offer food for thought. “I don’t use glycemic index with my athletes, but this research is making me wonder if I should look into it,” said Shorenstein. In the meantime, she advises athletes to focus on eating a balanced diet. “It’s not just about the carb, it’s about what you’re eating with the carb. Keep experimenting and figure out what works best with your performance.”