Study suggests carbs are not the main culprit.
Symptoms of gastrointestinal distress, including nausea and flatulence, are relatively common during endurance races such as marathons and Ironman triathlons. Athletes commonly assume that GI distress is caused by overconsumption of carbohydrate (sports drinks, gels, and so forth). However, a study published in the journal Medicine & Science in Sports & Exercise suggests that this is not the case.
Researchers at four European universities recruited 221 athletes who participated collectively in two Ironman triathlons, one Ironman 70.3 event, a long cycling time trial, a cycling stage race, and a marathon and invited them to fill out questionnaires after completing their events. The athletes were asked to recall their nutrition intake during their individual races and rate the severity of 12 different symptoms of GI distress on a 0-9 scale.
Analyzing the data on nutrition intake, the researchers determined the average rate of carbohydrate intake in each type of race. Triathletes consumed the most carbs, at rates of 62 to 71 grams per hour in the three races included. Cyclists consumed somewhat less carbohydrate (53 g/h), while marathon runners took in the least—just 35 g/h.
Serious GI symptoms were reported in 31 percent of Ironman triathletes, 14 percent of Ironman 70.3 racers, 7 percent of cycling stage-race participants, 4 percent of cycling time-trial competitors, and 4 percent of marathoners. As you can see, there was a clear correlation between the average rate of carbohydrate intake in each type of race and the risk of serious GI symptoms.
Before we blame race GI problems entirely on overconsumption of carbohydrate, however, we need to dig a little deeper. If carbs were truly the main culprit then we would expect to see that, within each type of race, those athletes who consumed the most carbs had the highest risk for GI distress. But this association was not seen in runners, cyclists, or Ironman 70.3 racers. Only Ironman racers faced a greater risk of GI distress when they consumed more carbs.
It also bears noting that the rate of serious GI symptoms was more than twice as high in Ironman racers than in Ironman 70.3 competitors despite the fact that the rates of carbohydrate intake were the same. It’s interesting as well that the rates of GI distress were the same in marathon runners and cycling stage racers despite the fact that the cyclists consumed 50 percent more carbs, on average.
These numbers suggest that the particular nature of the race contributes to the risk of GI distress more than the rate of carbohydrate intake does. It seems likely that the risk of GI distress in the two Ironman events was more than two times greater than in any other race simply because it lasted twice as long as any other race for most participants. I think you’d find that the risk of all kinds of things—heat illness, bonking, etc.—was more than two times greater in the Ironman. It’s just more stressful all around.
Yet the incidence of GI distress in an Ironman 70.3 was twice as high as the risk in a cycling stage race despite taking no longer to complete. This tells us that not only the length of a race but also the transition from cycling to running contributes to the risk of GI distress. It would be very interesting to know exactly when symptoms of GI distress most commonly appeared in the triathlons studied. I’d be willing to bet it was within the first 10K of the marathon start.
There’s a reason the runners in this study reported consuming only 35 grams of carbohydrate per hour while racing a marathon, whereas cyclists reported taking in 53 g/h. The GI system cannot tolerate as much nutrition consumption during running as it can on the bike. In triathlons, racers commonly consume carbs at a rate that is tolerable on the bike, only to find that the amount or concentration of calories in their guts becomes intolerable once they start running.
Besides the particular nature of the stress that a given type of race doles out, there is another factor that also appears to be more important than the rate of carb intake in relation to the risk of GI distress: past history of such problems. The researchers found that the correlation between past GI problems in races and GI problems in the races included in this study was stronger than the correlation between the rate of carbohydrate intake in the studied races and GI distress. In other words, athletes with a history of GI problems tended to have GI problems in these races even at lower levels of carb intake, while those without such histories tended not to have problems even at higher rates of carb intake.
A final note: Even though higher rates of carb intake were linked to higher risk of GI issues in Ironman races, those athletes who consumed the most carbs also tended to finish the race faster! Just because you experience some nausea and flatulence during an Ironman does not automatically mean your race is ruined. More often than not, these things are just a price you pay for doing an Ironman and for taking in enough fuel to finish with the quickest time possible–and they’re a price worth paying.
About The Author:
Matt Fitzgerald is the author of Iron War: Dave Scott, Mark Allen & The Greatest Race Ever Run (VeloPress 2011) and a Coach and Training Intelligence Specialist for PEAR Sports. Find out more at mattfizgerald.org.