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ADA 2021: For Athletes To Optimize Their Performance and Glycemia, Should They Cut Carbs or Carb Load?

Jul 17, 2021
Editor: David L. Joffe, BSPharm, CDE, FACA

Author: Ashley Ball, PharmD Candidate, LECOM-School of Pharmacy

The internet has many different opinions of what an optimal diet is for athletes, including whether they should cut carbs or carb load; but what do the experts say? 

With exclusive commentary from Sheri Colberg, PhD. 


Since the late 1960s, there has been a general understanding that exercise capacity is significantly improved when greater glycogen stores are available during more extended athletic events like marathons and triathlons. In most cases, a higher carbohydrate intake results in higher glycogen stores. By way of example, many elite cyclists “carb load” during main competitions. In contrast, the popular ketogenic diet is associated with fat oxidation at two to three times higher than persons who have a regular diet with no carbohydrate (CHO) restrictions. Many people with overweight lose pounds quickly when starting the ketogenic diet. An additional benefit associated with a ketogenic diet is found in the population of people who have seizures that have not had successful first-line therapy. Those people have a reduced frequency of attacks when placed on a ketogenic (in children) and low carbohydrate (adult) diet. 

There are questions to consider when seeking the best diet for athletes. Such questions are, what foods should be consumed before, during, and after physical activity? Do these foods change between the final competition versus the training sessions leading up to it? Does the intensity of exercise affect what to consume, and does a diet that optimizes glycemic variability also maximize health? 

Research supports that elite endurance athletes need CHO as fuel for their moderate to high-intensity exercise. To maintain a high-level CHO oxidation, elite athletes need to focus on what is eaten before, during, and after training. Research reveals that what is eaten is frequently dependent on the training level for that day and the next day. For instance, a high-intensity training day may have a high CHO intake for the pre-training meal, during training, and for the post-training dinner. In contrast, the evening meal may be low in CHO if training the next day will consist of an intense workout. 

Research establishes that high carbohydrate intake is optimal for elite athletes during the competition, such as for the “Tour de France.” However, low CHO consumption may have value during prior training sessions. On the other hand, a low CHO diet is beneficial because it triggers hallmark adaptations for endurance training. Furthermore, in patients with diabetes, a long-term ketogenic diet is associated with hypoglycemic resilience. 

In conclusion, athletes should fuel for the work required. If they have to exert a high amount of energy or exercise with moderate power over a longer duration, there are benefits to a higher CHO diet. If the physical activity requires little energy or is for a shorter time, then a lower CHO may suffice. While much was covered, experts agree that there is still much to learn and many unanswered questions. For instance, ischronic CHO restriction a necessity? When attempting to optimize a diet for performance and glycemia, does gender make a difference? Is a dietary pattern that is optimal for performance also optimal for health? While experts agree that a diet that manages glycemic variability is optimal for patients with either T1DM or T2DM, they question if it is optimal for the health of those in the general population. These questions provide room for many future studies. 

Some limitations of the presentation include that the experts had the potential for bias, in that they are associated with commercial products. The high CHO benefits were heavily focused on high endurance athletes, so there needs to be more research for generalizability to everyday people. As mentioned previously, reducing insulin with a ketogenic diet leads to a decrease in glycolysis, which may reduce lactate. More research needs to be done to determine if this benefits the recovery period following the physical exertion. 

Practice Pearls: 

  • Exercise capacity during more extended events and training is generally improved with a higher carbohydrate intake. 
  • Athletes should fuel for the work required in a meal-by-meal and day-by-day approach. 


Morton, James. “High/Normal Carbohydrate Intake Optimizes Performance and Glycemia.” American Diabetes Association (ADA) Virtual 81st Scientific Sessions. June 25. 2021, https://www.ada2021.org (Requires ADA Symposium login.) 

D’Agostino, Dominic. “Low Carbohydrate Intake Optimizes Performance and Glycemia.” American Diabetes Association (ADA) Virtual 81st Scientific Sessions. June 25. 2021, https://www.ada2021.org (Requires ADA Symposium login.) 


Ashley Ball, PharmD Candidate, LECOM-School of Pharmacy 


Comments by Dr. Sheri Colberg:

One vital point to keep in mind about this debate between low-carb and high-carb intakes with athletics is that it is ongoing in active individuals without diabetes. However, some facts are irrefutable, such as: 

  • Higher intensity training and events rely almost exclusively on carbohydrates as a fuel during the activity due to differences in metabolic fuel efficiency. Carbs are converted into energy (ATP) more quickly and with less oxygen required than fat. 
  • The body can adapt to a lesser intake of carbs and increase its use of fat as a fuel during exercise. However, this adaptation can take weeks to accomplish, and performance can be negatively impacted if a low-carb intake is too short before an event. 
  • Some events, such as sprinting and powerlifting, may be negatively impacted by a chronic low-carb intake. In longer (endurance) events, performance may be maintained on sustained lower-carb diets, but it is likely not improved. 

Some other interesting ideas were introduced during this symposium, such as the fact that varying carb intake can be beneficial. For example, athletes and recreational exercisers may benefit from training with a lower carb intake to increase their ability to oxidize fat. However, they will still benefit from taking in more carbs leading up to any events and during events to maximize their muscle and liver glycogen stores and carb availability during events. An example of this would be to train low-carb and compete for high-carb. 

As for glycemic management, a similar approach may be beneficial. Carbs are utilized with very little insulin during athletic endeavors, and active individuals with diabetes may benefit from taking in extra carbs during events with very little insulin needed. When attempting to carb load for even a day leading up to events, individuals should make every effort to match carb intake with insulin coverage to maintain normal blood glucose levels to maximize glycogen storage.