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The Importance of Carbohydrates and Glycogen for Athletes

BY Iñigo San Millán, PhD

Dr. Iñigo San Millán uses his insights from working with Tour de France teams and Grand Tour podium riders to remind us that as athletes, carbohydrates and glycogen are still our "gold."

Nutrition is a key part of the training regime for any athlete. Not eating enough calories (Kcalories-Kcal) can result in a lack of important macro and micronutrients. This is especially true when it comes to carbohydrates (CHO).

Modern societies tend to “demonize” carbohydrates. There are multiple nutritional “gurus,” books, and diets out there claiming that high-protein and/or high-fat diets with heavy restrictions on carbs are the best way to lose weight and eat healthy. However, from what I have seen, these books and diets lack substantial scientific evidence – especially when it comes to athletic performance. This is especially true for athletes who restrict their carbohydrate intake, as a massive amount of scientific evidence from the past 50 years clearly shows that a good carbohydrate diet is crucial to maintain performance.

Not only is science telling us this, but also the real-life experiences of athletes performing at the highest level. We can find a great example of this in Kenyan runners who are by far the best endurance runners ever in history, dominating all top events internationally for decades. The percentage of carbohydrates in their diet is 76.5%, with about 20% of their total daily caloric intake coming from sugar! Therefore, it seems clear the importance of carbohydrates is clear.

Unfortunately, there are many recreational and competitive athletes who either aren’t aware of the importance of carbohydrates and glycogen storage for proper training and performance. Or who just simply and deliberately restrict carbohydrates from their diet because some training buddy told them to or they read it somewhere on the internet. In a recent study done in our laboratory with 99 competitive cyclists looking at indirect parameters of glycogen depletion, we found that about 30% of all cyclists had sub-optimal glycogen levels, and none of them knew about it.

Why Do Athletes Need Carbohydrates? A Little Bit of Bioenergetics

The rate of ATP synthesis is parallel to the exercise intensity, which determines the substrate demands of skeletal muscle to generate ATP. During exercise, skeletal muscles primarily use fat and CHO for energy purposes, and at low exercise intensities, fat is the preferred substrate (although there is always some glucose utilization).

At higher exercise intensities of about 50- 60% of VO2max, ATP synthesis demand increases, and since fat cannot entirely meet the rate of ATP synthesis, glucose oxidation increases. Fat cannot synthesize ATP fast enough for the contractile demands of skeletal muscle fibers at higher intensities. Although the utilization of fat yields a much higher amount of ATP, glucose utilization is much faster and, therefore, necessary for ATP synthesis. This is why carbohydrates are of great importance during high-intensity exercises and competition. Fat simply cannot provide the energy needed for ATP synthesis.

In our laboratory, we observe the fat and carbohydrate oxidation rates (“burning”) at different exercise intensities and over time. At a typical cycling or running “race pace,” carbohydrate utilization is in the range of 2-3g/minute. Even at low exercise intensities, carbohydrates are always used, despite claims that fat is the sole fuel being used. Thus it is crucial to have good glycogen stores as well as to have the proper carbohydrate intake during exercises lasting more than two hours.

What is Glycogen?

Glycogen is the storage form of glucose and carbohydrates (CHO). Carbohydrates are a very limited source of energy, accounting for only about 1-2% of total bodily energy stores. Furthermore, about 80% of total carbohydrate is stored in skeletal muscle, about 14% is stored in the liver, and about 6% in the blood in the form of glucose. This would represent about 300-400g of glycogen stored in muscle and about 70-100g stored in the liver.

As we can see, while glycogen is “gold” to athletes, we have a very limited and low capacity to store it. At rest, skeletal muscle accounts for 15-20% of peripheral glucose utilization, while during an exercise intensity of 55-60% VO2 max, glucose utilization by skeletal muscle could account for as much as 80-85% of whole-body disposal (or even more at higher exercise intensities). It’s safe to say muscle glycogen is crucial for ATP resynthesis during exercise.

How Low Carbohydrate Diets Can Affect Performance

Muscle Glycogen Chart vs Time

Multiple studies show that fatigue and a decrease in performance are associated with low carbohydrate diets that cause glycogen depletion. Studies also show that low glycogen levels may cause overtraining.

Since our glycogen storage capacity is so limited, many high-performance athletes may find it difficult to even keep up with sufficient CHO intake and, therefore, have some patterns of glycogen depletion. When glycogen levels are low or depleted, muscles increase the utilization of protein and amino acids to produce glucose, acting as gluconeogenic precursors.

Since protein and amino acids are the building blocks of muscle, the latter may enter a catabolic situation (muscle breakdown). Essentially, the muscle “eats itself to feed itself” by increasing the amount of protein and amino acids used for energy purposes. This situation may lead to muscle damage and chronic overtraining.

It has been shown that muscle damage limits and interferes with glycogen storage and synthesis, so even with a high carbohydrate diet, it’s difficult to maintain glycogen storage. If this happens, you could enter a vicious cycle leading to overtraining and a decrease in performance.

From my experience working with all kinds of athletes, I see that a “catabolic” situation elicited by low glycogen storage is probably the number one cause of overtraining in athletes.

Carbohydrate Recommendations for Athletes

Carbohydrate intake should be based on the rates of glycogen depletion and the physical activity you’re doing. Low intensities involve lower CHO needs. However, low-intensity workouts during long periods of time will certainly require a higher daily intake of carbs. High-intensity workouts rely on glucose almost exclusively – there is always a high degree of glycogen depletion and, therefore, a higher carbohydrate intake.

The guidelines for carbohydrate consumption during competition call for 30-60 g/hour. However, it’s possible that these recommendations fall short when it comes to endurance and ultra-endurance events. With data from our lab, we believe that the right target for carbohydrates should be in the range of 80-100 g/hour for events lasting more than four hours.

We tried these new recommendations in 2010 with the Garmin Pro cycling team, and it worked really well. This method was proven during the Tour de France, with no GI disturbances from the athletes. We have since tried these recommendations with many athletes of different sports and competition levels with great success and performance results.

For such a high carbohydrate load, it’s essential to mix carbohydrates of different glycemic indexes and rates of absorption. A mixture of simple and complex carbohydrates is the most efficient way to go. Dr. Asker Jeukendrup, one of the top experts in the world of sports nutrition, has also observed similar findings. The graph below shows how Dr. Jeukendreup and his group observed that higher carbohydrate intake was associated with faster finish times at the Triathlon World Championships in Kona.

Carbohydrate Intake Grams Per Hour versus Finish Time

Daily Carbohydrate Needs for Athletes

Daily intake of CHO varies depending on the level and duration of activity. An excessive carbohydrate diet without the right amount of exercise would lead to an increase in body fat due to the conversion of CHO to fat. Therefore a recreational athlete who trains one hour/day should by no means eat as many carbohydrates per day as a professional cyclist or high-performance triathlete.


Onywera, V.O., Kiplamai, F.K., Tuitoek, P.J., Boit, M.K., and Y.P. Pitsiladis. Food and macronutrient intake of elite Kenyan distance runners. Int. J. Sport. Nutr. Exerc. Metab. 14: 709-719, 2004.  Retrieved from

San Millán I, González-Haro C, Hill J. Indirect Assessment of Glycogen Status in Competitive Athletes. Med Sci Sports Exerc. 2011; 43: S660  Retrieved from

Goodman, MN. Amino acid and protein metabolism. In Exercise, nutrition and energy metabolism,eds. E.S. Horton, R.L. TErtujn, 89-99. New York: Macmillan. 

Sherman WM. Metabolism of sugars and physical performance. Am J Clin Nutr. 62(1 Suppl):228S-241S. 1995. Retrieved from

Kjaer M, Kiens B, Hargreaves M, Richter EA. Influence of active muscle mass on glucose homeostasis during exercise in humans. 71(2):552-7. 1991. Retrieved from

Katz A, Broberg S, Sahlin K, Wahren J. Leg glucose uptake during maximal dynamic exercise in humans. Am J Physiol. 251(1 Pt 1):E65-70. 1986. Retrieved from

Costill DL, Hargreaves M. Carbohydrate nutrition and fatigue. 1992 Feb;13(2):86-92. Retrieved from

Hermansen L, Hultman E, Saltin B. Muscle glycogen during prolonged severe exercise. Acta Physiol Scand.71: 129-139, 1967.  Retrieved from

Coyle EF, Coggan AR, Hemmert MK, Ivy JL. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J Appl Physiol. 61: 165-72, 1986.  Retrieved from

Coyle EF, Hagberg JM, Hurley BF, Martin WH, Ehsani AA, Holloszy JO. Carbohydrate feeding during prolonged strenuous exercise can delay fatigue. J Appl Physiol. 55: 230-235, 1983. Retrieved from

Coggan AR, Khort WM, Spina RJ, Bier DM, Holloszy JO. Endurance training decreases plasma glucose turnover and oxidation during moderate intensity in men J Appl Physiol. 68: 990-996, 1990. Retrieved from

Coggan AR, Coyle EF. Reversal of fatigue during prolonged exercise by carbohydrate infusion or ingestion. J Appl Physiol. 63: 2388-2395, 1987. Retrieved from

Maughan RJ, Greenhaff PL, Leiper JB, Ball D, Lambert CP, Gleeson M. Diet composition and the performance of high-intensity exercise. J Sports Sci. 15: 265-275, 1997. Retrieved from

Sahlin K, Katz A, Broberg S. Tricarboxylic acid cycle intermediates in human muscle during prolonged exercise. Am J Physiol. 259: C834-C841, 1990. Retrieved from

McConell G, Snow RJ, Proietto J, Hargreaves M. Muscle metabolism during prolonged exercise in humans: influence of carbohydrate availability. J Appl Physiol. 87: 1083-1086, 1999. Retrieved from

Sherman WM, Wimer GS. Insufficient dietary carbohydrate during training: does it impair athletic performance?. Int J Sport Nutr. 1: 28-44, 1991. Retrieved from

Sherman WM. Metabolism of sugars and physical performance. Am J Clin Nutr. 62: S228-S241, 1995. Retrieved from

Snyder AC, Kuipers H, Cheng B, Servais R, Fransen E. Overtraining following intensified training with normal muscle glycogen. Med Sci Sports Exerc. 27: 1063-1070, 1995. Retrieved from

Costill DL, Flynn MG, Kirwan JP, Houmard JA, Mitchell JB, Thomas R, Park SH. Effects of repeated days of intensified training on muscle glycogen and swimming performance. Med Sci Sports Exerc. 20: 249-254, 1988. Retrieved from

Kirwan JP, Costill DL, Flynn MG, Mitchell JB, Fink WJ, Neufer PD, Houmard JA. Physiological responses to successive days of intense training in competitive swimmers. Med Sci Sports Exerc. 20: 255-259, 1988. Retrieved from

Lemon PWR, Mullin JP. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol. 48: 624-629, 1980.  =Retrieved from

Tarnopolsky MA, Atkinson SA, Phillips SM, MacDougall JD. Carbohydrate loading and metabolism during exercise in men and women. J Appl Physiol. 78: 1360-1368, 1995. Retrieved from

O’Reilly KP, Warhol MJ, Fielding RA, Frontera WR, Meredith CN, Evans WJ. Eccentric exercise-induced muscle damage impairs muscle glycogen repletion. J Appl Physiol. 63: 252-256, 1987. Retrieved from

Costill DL, Pascoe DD, Fink WJ, Robergs RA, Barr SI, Pearson D. Impaired muscle glycogen resynthesis after eccentric exercise. J Appl Physiol. 69: 46-50, 1990. Retrieved from

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About Iñigo San Millán, PhD

Dr. Iñigo San Millán, Ph.D., is the Director of the Exercise Physiology and Human Performance Lab at the University of Colorado School of Medicine and also Assistant Professor of Family Medicine and Sports Medicine Departments at the University of Colorado School of Medicine.’Dr. San Millán is considered one of the most experienced applied physiologists in the world. He has worked with many elite athletes and teams in sports including track and field, running, triathlon, rowing, basketball and cycling; including eight professional cycling teams. Follow Iñigo on Twitter.

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