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.
Overview
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).
Conclusion
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.
References:
McArdle, W.D., Katch, F.I., & Katch, V.L. (2001). Exercise
physiology: energy, nutrition, and human performance (5th edition).
Baltimore: Lippincott, Williams & Wilkins.
Gollnick, P.D. & Matoba, H. (1984). Role of carbohydrate in
exercise. Clinics in sports medicine, 3 (3), 583-593
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.
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.
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.
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.
