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Carbohydrate (muscle glycogen) loading for endurance athletes

by Elaine Gonya, LAT, MSed

For many years, athletes have attempted to enhance their endurance-event performance through improved training protocols and carbohydrate (muscle glycogen) loading. And, while it is well documented that training regimens have increased in intensity, duration and complexity, research also indicates that increased carbohydrate intake before and during aerobic exercise improves muscle glycogen stores and allows endurance athletes to compete for greater time periods.

A host of studies have investigated effects of endurance training, pre-event carbohydrate loading protocols (that range from one to seven days pre-event), and carbohydrate supplementation (gel shots, etc.) during an endurance event.


One of the most prominent nutrition-exercise modifications used by endurance athletes to increase the body's glycogen reserves involves carbohydrate loading or glycogen supercompensation. The process of glycogen supercompensation has been shown to elevate glycogen storage levels by three to four times that of a high-carbohydrate diet alone (1)

Glycogen – stored in both the liver and active muscle – supplies the majority of energy required for aerobic exercise. As the aerobic exercise duration increases, depletion of the body's glycogen reserves occurs. High-intensity exercise depletes muscle glycogen very quickly resulting in fatigue at a much faster rate.

However, if an endurance athlete is able to gradually use glycogen stored within the muscles (by using a submaximal race pace), the body will turn to fat stores to supply necessary energy to the working muscles, thus sparing valuable glycogen.

When glycogen reserves from muscles are depleted, the fatigue that occurs is irreversible. Advanced stages of that fatigue can quickly progress into what many endurance athletes refer to as "hitting the wall."

With the nearly endless fat stores our bodies have, it is clearly advantageous to tap into these storage areas versus using glycogen supplies that cannot be adequately restored during endurance activities. Research continues to examine the roles that training variations and dietary alterations play in both increasing glycogen (carbohydrate) stores before endurance events and sparing valuable glycogen by focusing on burning more fat during the event (2).

It is important to note that benefits of carbohydrate loading and glycogen supercompensation only apply to intense aerobic activities lasting longer than 60 minutes; this technique is ineffective for anaerobic activities and maximal effort exercises under 60 minutes in duration (1). It has been suggested that ideal "loading" can be a result of training combined with:

  • One to three day carbohydrate loading protocol,
  • Six day carbohydrate loading protocol, and/or
  • Carbohydrate supplementation on the day of the endurance event.

One to six day carbohydrate loading

Some published research suggests muscle glycogen must be reduced or depleted prior to pre-event supercompensation efforts, while other studies regarding carbohydrate-loading regimens recognize the benefits that can occur even without a glycogen depletion period in as short as one to three days, provided that training during those carbohydrate loading days does not deplete already stored glycogen (3, 4).

There are two well-studied six-day supercompensation techniques that have been investigated. Both are somewhat complicated in that they require muscle glycogen depletion before the loading process, accompanied by very specific protocols for the loading process thereafter. However, they may not be any more beneficial than shorter, less complicated techniques (1, 5).

Carbohydrate supplementation, date of competition

Numerous studies have been conducted to evaluate the effectiveness of carbohydrate loading and supplementation on the day of competition (1, 6). Most have indicated that prolonged, moderate-intensity exercise performance can be improved by maintaining blood glucose availability during (especially in the later stages) of exercise.

Some studies reported benefits in performance with carbohydrate supplementation during activities at one-hour intervals, with increased concentrations of carbohydrate solutions being consumed late in exercise. That same study also supported use of carbohydrate supplementation three to six hours prior to the event, but not within 30-60 minutes prior to competition (6). Several studies have indicated that carbohydrate supplementation during endurance activities has been shown to be beneficial, but are quick to caution readers that a carbohydrate supplement has been ineffective in restoring levels of glycogen stores, if muscular fatigue is present (1, 3, 6).


Carbohydrate loading prior to endurance events has been demonstrated in literature to be a valuable tool for athletes competing over longer time periods and greater distances. Techniques range from improved training methods, one to six days of carbohydrate loading, to carbohydrate supplementation during endurance events. No matter which technique is used, it's important to know that when muscle glycogen levels are depleted, the fatigue that sets in is irreversible during the event.

For additional questions on carbohydrate loading, other sports medicine topics or to schedule a FREE Injury Evaluation, call the Aurora Sports Medicine Hotline™ at 414-219-7776 or 800-219-7776.


  1. McArdle, W.D., Katch, F.I., & Katch, V.L. (2001). Exercise physiology: energy, nutrition, and human performance (5th edition). Baltimore: Lippincott, Williams & Wilkins.
  2. Gollnick, P.D. & Matoba, H. (1984). Role of carbohydrate in exercise. Clinics in sports medicine, 3 (3), 583-593
  3. Fairchild, T.J., Fletcher, S., Steele, P., Goodman, C. Dawson, B., & Fournier, P.A. (2002). Rapid carbohydrate loading after a short bout of near maximal-intensity exercise. Medicine and science in sports and exercise, 34 (6), 980-986.
  4. Bussau, V.A., Fairchild, T.J., Rao, A., Steele, P., & Fournier, P.A. (2002). Carbohydrate loading in human muscle: an improved 1-day protocol. European journal of applied physiology, 87 (3), 290-295.
  5. Housh, T.J., deVries, H.A., Johnson, G.O., Evans, S.A., Tharp, G.D., Housh, D.J., & Hughes, R.J. (1990). The effect of glycogen depletion and supercompensation on the physical working capacity at the fatigue threshold. European journal of applied physiology and occupational physiology, 60 (5), 391-394.
  6. Coggan, A.R. & Swanson, S.C. Nutritional manipulations before and during endurance exercise: effects on performance. Medicine and science in sports and exercise, 24 (9 supplement), S331-S335.