Carbohydrate is a vital energy source during aerobic and anaerobic exercise. A 24-hour fast or low carbohydrate-normal calorie diet can nearly deplete resting glycogen reserves (McArdle, Katch, & Katch, 2015). Whereas a two to three-day carbohydrate-rich diet can increase glycogen store two times compared to a moderate carbohydrate diet (McArdle, Katch, & Katch, 2015). Any significant reduction in the body’s carbohydrate supply during exercise will result in fatigue and decreased performance.
Glycogen serves as the body’s storage form of carbohydrate
The liver contains about 100 grams of glycogen which is much more concentrated than skeletal muscle. In well-fed individuals’ intramuscular glycogen stores can reach 400 grams in absolute terms, greater than the liver. In total, the human body can only store about 2,000 kcals of glycogen for eventual use for energy. At rest, the liver produces blood glucose which may account for up to 30% of the total energy needed by exercising muscles (McArdle, Katch, & Katch, 2013; McArdle, Katch, & Katch, 2015). Skeletal muscle via the hormones glucagon and insulin help regulate blood glucose and tissue glycogen in a normal range during both resting states and exercise (McArdle, Katch, & Katch, 2013; McArdle, Katch, & Katch, 2015).
Maximizing glycogen stores is important to the recreational and trained athlete
Carbohydrate is the preferred source of energy during intense aerobic exercise and the sole supply of energy of anaerobic efforts (McArdle, Katch, & Katch, 2013). In fact, carbohydrate provides the only macronutrient substrate that forms ATP without the presence of oxygen (McArdle, Katch, & Katch, 2015). During long duration – low/moderate intensity exercise fats predominate, but exercise performance suffers due to the lower efficiency of fatty acid metabolism.
Carbohydrates serve several important functions:
- a main source of energy for the central nervous system
- an energy source
- primer for fat catabolism
- protein sparing
During intense exercise the body releases hormones, among them, being glucagon (with a decrease in insulin) to increase glycogen synthesis in the liver and skeletal muscles. Within the first few seconds to minutes of exercise, an oxygen deficit requires the use of stored muscle glycogen via the rapid glycolysis energy system (McArdle, Katch, & Katch, 2015). As exercise duration increases blood glucose from the liver adds to the metabolic energy. These two energy sources are directly dependent on initial stores from carbohydrate intake (Campbell & Spano, eds., 2011).
As exercise progresses toward high-intensity aerobic activity the contribution of carbohydrates approaches 70% of total energy metabolism primarily via slow glycolysis (McArdle, Katch, & Katch, 2015). The selective dependence on carbohydrates serves two purposes: (1) it is two times faster than fat or protein metabolism, (2) it generates 6% more energy per unit of oxygen consumed (McArdle, Katch, & Katch, 2015). In other words, carbohydrate metabolism is more efficient and faster than lipids as an exercise-related energy source.
As muscle glycogen levels deplete the liver supplies a major contribution of blood glucose. However, by this point fat oxidation is providing an increasing amount of energy. Exercise intensity begins to fall. As intense aerobic exercise reaches 90 minutes it is not improbable that blood glucose levels can approach hypoglycemic levels (McArdle, Katch, & Katch, 2015) with associated symptoms including irregular heart rhythm, fatigue, pallor, shakiness, anxiety, excessive sweating, confusion, and blurred vision (Fisk, 2017).
Problems with carb depleting diets
As reviewed above one can see the dilemma of depleting glycogen and glucose sources even with proper feeding. The carbohydrate depleting diet accelerates this process as it leads to premature glycogen and blood glucose depletion. With a depleting carbohydrate diet, there are direct impacts on short-term anaerobic and high-intensity aerobic performance due to a lack of glycogen and low blood glucose and increased reliance on lipid as an energy source. In fact, with carbohydrate depleting diets, during the cross-over effect, there would not be enough muscle glycogen and blood glucose to continue the ability to increase exercise intensity.
Without adequate muscle glycogen and blood glucose, we are left with the rate-limiting impacts of fat metabolism and blunted exercise intensity (McArdle, Katch, & Katch, 2013). In addition, carbohydrates role in central nervous system function is also critical to exercise performance. When glycogen and blood glucose levels fall, the individual may experience neural symptoms such as fatigue, shakiness, confusion, vision problems, problems with coordination, and anxiety (Fisk, 2017). The deficits in nervous system function may increase the likelihood of injury (McArdle, Katch, & Katch, 2015).
Finally, maintaining a low carbohydrate diet hurts recovery. First is the ability to restore any lost glycogen stores is diminished which is one of the main goals of post-exercise nutrition. Carbohydrates are protein sparing and even with intense aerobic exercise protein catabolic processes are at play (Lunn et al., 2012; Nieman, 2010). Given this, for both strength training and intense aerobic training adequate carbohydrate (and protein) consumption may help with protein synthesis, reducing protein breakdown, favorably influencing glycogen storage and promoting faster recovery (Beelen et al., 2008; Lunn et al., 2012). An incomplete recovery will dampen future exercise sessions whether it be from continued glycogen depletion or continued protein catabolism.
Beelen, M., Tieland, M., Gijsen, A. P., Vandereyt, H., Kies, A. K., Kuipers, H., . . . Loon, L. J. (2008). Coingestion of Carbohydrate and Protein Hydrolysate Stimulates Muscle Protein Synthesis during Exercise in Young Men, with No Further Increase during Subsequent Overnight Recovery. The Journal of Nutrition, 138(11), 2198-2204. doi:10.3945/jn.108.092924
Campbell, B. I., & Spano, M. A. (Eds.). (2011). NSCA’s guide to sport and exercise nutrition (Science of Strength and Conditioning). Champaign, IL: Human Kinetics.
Fisk, M. (2017, October 03). Exercise-induced non-diabetic hypoglycemia. Retrieved from https://www.livestrong.com/article/326019-exercise-induced-non-diabetic-hypoglycemia/
Lunn, W. R., Pasiakos, S. M., Colletto, M. R., Karfonta, K. E., Carbone, J. W., Anderson, J. M., & Rodriguez, N. R. (2012). Chocolate Milk and Endurance Exercise Recovery. Medicine & Science in Sports & Exercise, 44(4), 682-691. doi:10.1249/mss.0b013e3182364162
McArdle, W. D., Katch, F. I., & Katch, V. L. (2013). Sports and exercise nutrition (4th ed.). Philadelphia, PA: Lippincott Williams and Wilkins.
McArdle, W. D., Katch, F. I., & Katch, V. L. (2015). Exercise physiology: Nutrition, energy, and human performance (8th ed.). Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins.
Nieman, D. (2010). Coingestion of protein with carbohydrate during recovery from endurance exercise stimulates skeletal muscle protein synthesis in humans. Yearbook of Sports Medicine, 2010, 163-165. doi:10.1016/s0162-0908(09)79509-1