Carbohydrate-Loading Before Endurance Events
The practice of “carb-loading,” or consuming high amounts of carbohydrates in the days and hours prior to a long endurance event such as a marathon, has been a staple in the pre-race regimens of many athletes. Because carbohydrates are the main source of fuel during high intensity efforts of longer than 90 minutes (Heikura et al., 2018), it is critical that athletes seek to increase availability in their muscles before a race or event (Ravindra et al., 2020). However, some research has called into question the benefit of high carbohydrate intake, specifically among females, citing that women are unable to use stored muscle glucose in the same way as men (Sims, 2022). While it is true that men are more receptive to carbohydrate loading than their female counterparts, the overwhelming evidence pointing to the benefits of the practice makes it clear that both genders can use it to enhance athletic performance.
Research on the relationship between carbohydrates (CHO) and exercise dates back over 100 years, when a 1920 study identified CHO as a necessary source of energy in exercise (Hearris et al., 2018). Additionally, research revealed that athletes who consumed a diet high in CHO leading up to a long event were more likely to delay fatigue than their low-CHO consuming counterparts. A 1923 study at the Boston Marathon measured the blood glucose levels of finishers, finding that many met the criteria for hypoglycemia, and concluding that a lack of glycogen in the muscles was related to fatigue (Hearris et al., 2018). Subsequent research, particularly that of Scandinavian scientists during the mid-1900s, “provided the platform for modern day sports nutrition practices” (Hearris et al., 2018), concluding that glycogen in muscles is consumed by the body to fuel exercise, and that storage of glycogen can be improved by a diet high in CHO prior to the event. Since then, a “wealth of studies” (Hearris et al., 2018) have corroborated these findings, and specific recommendations for athletes before, during, and after competition have been established.
In endurance events of moderate to high intensity, up to 90% of energy is derived from carbohydrates (Ravindra et al., 2020). Consuming CHO prior to exercise allows glucose in the blood to be more readily taken into the muscles, which can store roughly 400 grams of glycogen (Tiller et al., 2022). The liver also provides storage for about 100 grams of glycogen, and more recent studies have further linked liver glycogen to capacity for endurance exercise (Hearris et al., 2018). As a whole, overwhelming evidence from the past 40 years has concluded that athletes are able to increase storage of glycogen in their bodies by consuming high amounts of CHO in the 24 to 48 house prior to an event. Recommendations to maximize storage levels are 8-12 grams per kilogram of body weight daily (Hearris et al., 2018). Research has also been extended to include CHO intake during endurance events of longer than 90 minutes. Athletes regularly use a combination of CHO-based sports drinks, bars, and gels to accomplish the 30-90 grams per hour recommendation, which allows glycogen to be “spared” in the liver, causing it to be available later in the event (Hearris et al., 2018). An additional developing theory is that of the “train low compete high” model, in which athletes intentionally restrict CHO intake in training (for example, completing an early morning workout without eating beforehand, allowing glycogen in the liver to by reduced to as low as 20% capacity), while increasing consumption prior to competition. This allows for beneficial adaptations, such as activation of certain transport proteins that play a role in endurance levels (Tiller et al., 2022). However, training with consistently low glycogen levels can be detrimental, leading to consequences such as low energy availability, which can develop into Relative Energy Deficiency in Sport (RED-S), bone fractures, and decreased immune system function (Tiller et al., 2022). Thus, this method should only be implemented at certain periods in the training cycle (generally early on) with the help of a dietician.
Elite athletes have frequently been the subject of studies related to nutrition, specifically, CHO intake. Of particular interest is the diet of East African distance runners. Of the top 100 fastest marathon times in history, runners of East African origins hold 76 and 93 of these spots for men and women, respectively (Zani et al., 2022). Kenyan and Ethiopian runners are the most dominant in the sport, and, interestingly, consume a diet extremely high in carbohydrates – usually 60-80% of total intake (Burke et al., 2019). These come in the form of vegetables, rice, pasta, potatoes, flatbread, and “ugali,” a porridge made from maize meal. Workouts are generally completed after a breakfast high in CHO, and athletes maintain these diets throughout training and competition. They are, essentially, carb-loading all of the time. Of course, there are a great number of other variables that contribute to East African success, but their simple, high-CHO diets are certainly worth noting.
Despite the wide body of research pointing to the benefits of carbohydrate loading prior to endurance events, some studies have called into question its effectiveness on females. Women depend more on blood glucose and fatty acids to fuel endurance activities as opposed to glycogen stored in their muscles and liver (Sims, 2022). This means that women burn through more fat and are unable to utilize stored glycogen as effectively as their male counterparts. A Canadian study even showed that women had little response to CHO loading, however, it was later found that these subjects were not adequately meeting their energy needs in total. A subsequent study increased the subjects’ caloric intake, as well as CHO consumed, and outcomes showed increased performance (Deldicque & Francaux, 2015). Thus, it is difficult to determine if benefits can be attributed to greater total or CHO-specific intake. Either way, it is clear that female endurance athletes in particular must be cognizant of their high energy needs and consume an adequate amount of CHO as well as fuel in general.
These studies should not dissuade female athletes from increasing their carbohydrate intake prior to competition. Indeed, researchers note that while females are not as responsive to the practice, they can still gain “some benefits” from CHO-loading (Deldicque & Francaux, 2015). Additionally, it is evident that further research needs to be done to corroborate these findings. In sum, because of the overwhelming evidence supporting the practice of CHO-loading, CHO-heavy diets of dominant endurance athletes, and scientific basis of CHO use by the body during exercise, athletes of a wide range of disciplines should strive to increase their CHO intake and engage in “carb-loading” prior to and during competition.
References
Burke, L. M., Jeukendrup, A. E., Jones, A. M., & Mooses, M. (2019). Contemporary Nutrition
Strategies to Optimize Performance in Distance Runners and Race Walkers, International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 117-129. https://journals.humankinetics.com/view/journals/ijsnem/29/2/article-p117.xml
Deldicque, L., & Francaux, M. (2015). Recommendations for healthy nutrition in female
endurance runners: An update. Frontiers in Nutrition, 2(17). https://doi.org/10.3389/fnut.2015.00017
Hearris, M., Hammond, K., Fell, J., & Morton, J. (2018). Regulation of muscle glycogen
metabolism during exercise: Implications for endurance performance and training adaptations. Nutrients, 10(3), 298. https://doi.org/10.3390/nu10030298
Heikura, I. A., Stellingwerff, T., & Burke, L. M. (2018). Self-reported periodization of nutrition
in elite female and male runners and Race Walkers. Frontiers in Physiology, 9. https://doi.org/10.3389/fphys.2018.01732
Ravindra, P. V., Janhavi, P., Divyashree, S., & Muthukumar, S. P. (2020). Nutritional
interventions for improving the endurance performance in athletes. Archives of Physiology and Biochemistry, 128(4), 851–858. https://doi.org/10.1080/13813455.2020.1733025
Sims, S. (2022). Can (and Should) Women Carb Load? Dr. Stacy Sims.
Tiller, N. B., Roberts, J. D., Beasley, L., Chapman, S., Pinto, J. M., Smith, L., Wiffin, M.,
Russell, M., Sparks, S. A., Duckworth, L., O’Hara, J., Sutton, L., Antonio, J., Willoughby, D. S., Tarpey, M. D., Smith-Ryan, A. E., Ormsbee, M. J., Astorino, T. A., Kreider, R. B., … Bannock, L. (2019). International Society of Sports Nutrition Position Stand: Nutritional Considerations for single-stage ultra-marathon training and racing. Journal of the International Society of Sports Nutrition, 16(1). https://doi.org/10.1186/s12970-019-0312-9
