Off The Road: Glycogen Depletion – The Energy Crisis

by Russ Ebbets

It was during my second year of college when I could first start to handle upping my weekly mileage. Tuesdays were always a difficult day as we did a double workout that began with an easy 10 miles before morning classes. In the afternoon the distances ranged from eight, nine or ten miles depending on what group you wanted to tuck-in with.

For me it always seemed to happen about a mile and a half from home. The best way to describe it was that “the wheels fell off.” I could keep running but I had to drop off the pace and just trying to remain anywhere near the pack was a monumental struggle. It was like I had two flat tires. Not knowing what had or was happening I’d curse myself, set my resolve and direct any energies I could muster to forge on. It never really worked. Post workout I’d ultimately conclude that I hadn’t figured out the “secret” yet and resolve to give it a go the next week.

Glycogen depletion is a physiologic state where the body’s energy stores become exhausted. In long distance running this state has achieved mythic status and is often referred to with colorful phrases as bonking, running on empty, or hitting the wall. The reality is that the glycogen stores of the body have been spent, and just like an empty debit card there is nothing left to spend.


For most people there is enough glycogen stored energy in the body to expend around 2500 calories. Caloric expenditure is a measure of one’s effort. There are assigned values for the different activities one may engage in (Figure 1). More difficult activities (running, hiking) use more calories and can eventually lead to exhaustion. Low intensity activities (standing, sleeping) are on the other end of the spectrum and while the caloric output is less there is still a cost.

The average person has about 500 grams (g) of stored glycogen in their muscle tissue. One’s liver and blood glycogen have a lesser amount, roughly totaling 125g. From a caloric production standpoint one gram of glycogen can produce four calories of energy (1g glycogen = 4 calories). That means a body has around 2500 calories of fuel before glycogen depletion sets in (500 + 125 = 625; 625 grams of glycogen x 4 calories/gram = 2500 calories).

If one burns 5g of glycogen per minute and there are 625g available one’s energy stores will last approximately 2 hours. How do you know this? Divide 625/5 = 125 minutes = 2hours, 5 minutes. This assumes you are running at about a 65% effort, a typical marathon pace. If running a marathon requires 750g of glycogen there are at least 25 minutes of the race with no readily available fuel. When one crosses this threshold, the bottom falls out as one has “hit the wall.”

Proteins and fats can be used for energy, but they are not readily available. The other point to remember is that the above example is an average for a 2:30 marathoner. Multiple factors such as weight, sex, physical condition, pace, age and environmental conditions all can significantly affect one’s energy expenditures. This link can give a more specific measure of one’s caloric demand with different physical activities:

Hopefully, you can see that glycogen depletion is different for each runner. I would question whether Eliud Kipchoge, who recently broke the 2-hour barrier in a staged marathon even enters glycogen depletion. I base that comment on his post-race statement about feeling “fresh” at the end of that race and the fact that he covered the last 5k in close to 14 minutes or 4:30 mile pace.

So, while glycogen depletion is an inevitable reality (for most) for foot races lasting longer than two hours there are several racing strategies a runner could employ that will help delay the onset of glycogen depletion. It should be noted that these strategies should be practiced in the build-up to the marathon or ultramarathon so that one is comfortable and familiar with which strategies work (and which ones don’t work) for you. The day of your big event is not the time to experiment with something new.

An overall race plan – It seems inconceivable to me that anyone would enter a race without a well defined race plan, but I am sure that is often the case. Such simple considerations in one’s race plan should include warm-up procedures, bathroom schedule, pre-race hydration schedule, procedures for the arrival at the starting line, intermediate split times for pre-determined miles/kilometers, in-race hydration and foodstuff schedule. Peculiarities of a racecourse should also be considered such as hills, exposed areas, windy areas, location of water stations, even the prevalence of sweeping turns that would allow one to run tangents. If one is to be competitive it always helps to review the race tactics of one’s competitors. Answers to hypothetical questions can further direct focus. Do they start fast? Do they push the hills? Do they usually lead or prefer to follow? How strong is their willfulness in the finishing drive? The point here is not to “overthink” the race but rather have a Zen-like sense of calm knowing that one is “prepared for everything, expecting nothing.”

Carbohydrate loading – Over the last 30 years carbohydrate loading (glycogen comes from carbohydrates) has fallen in and out and back into favor. Carbohydrate loading can be accomplished in several ways but one of the most common is to start a week out from one’s big event with several days of high protein/low carbohydrate meals. Then a longer Tuesday workout (for a Saturday “big event”) is done such, as 12-16 x 400m at half-marathon pace to exhaust any remaining carbohydrate stores in the body. For the remainder of the week meals should be primarily complex carbohydrates (pastas, brown rice) up to race day. In essence the body super compensates or “rebounds” by hyper-storing carbohydrates that could theoretically push the reality of “hitting the wall” or glycogen depletion 10-20 minutes deeper into the race. The caveat here is NOT to make the week leading to the event one’s first attempt at carbohydrate loading. A trial run 3-4 weeks out will give a good indication as to how one’s body will react to this eating strategy.

Drafting – In longer races few runners are strong enough to push the pace the whole way, particularly for the first half of the race. The longer races then become a situation of “let it happen v. make it happen.” In the “let it happen” phase one is along for the ride with a goal of expending as little energy as possible and arriving at some pre-determined race point where the “push” to the finish can be made.

Drafting is an age old technique where a lead runner, biker or race car is used to “break the wind” for those that follow. While the effects of drafting are more dramatic for a cyclist or race car driver, they nonetheless reduce the effort of forward running. In the second half of the race for a runner there is the push to the finish with the onset of glycogen depletion and the willful struggle to get home. Drafting will play less of a role here.

Most people know that Kipchoge’s race was staged to create as ideal racing conditions as possible. To combat wind resistance, his phalanx of runners ran in a “V” shaped pattern that essentially rendered any forward wind resistance to zero. Wind tunnel experiments determined that the “V” shape pattern (Figure 2) reduced the forces the most. While that initially seemed like a good idea the lack of any breeze hitting Kipchoge’s body led to overheating problems due to the lack of cooling from surface evaporation of his sweat. Kipchoge had to periodically pull out of the pattern to allow himself the benefits of a slight breeze, cooldown and then return to the phalanx.

In race nutrition – A logical solution to the energy crisis is to eat something. It makes total sense but the ability to swallow food and hold it down while running at 6-8-10 or 12 miles per hour is something else. Most people would have difficulty eating and walking at the same time. A related problem is what are you going to eat? Bananas are a great option but who is going to carry it, peel it and handle the trash? There are any number of jelly beans, goos and gels marketed but again transport can be an issue. And how often can one ingest the goo or gel before it causes a stomachache? Two times? Three times? Remember, these are highly concentrated substances that are effective with water, but then again most water stations offer water in limited quantities of a 4-5 ounce cup. Ultimately, these are all individual questions that need to be sorted out long before race day and practiced so whatever goes in your mouth doesn’t go down the “wrong tube” and you end up on the side of the road coughing your guts out.

Running economy or cost of transport – Symmetry of motion has long been a goal with distance running due to its economical use of the body’s resources. This symmetry necessitates an awareness as to what constitutes an efficient foot strike, knee, hip and arm actions as one cycles through the step-by-step running action. As previously stated, glycogen depletion often leads to a breakdown in running form. Forward motion will become more labored and one’s ability to direct forward motion with will power underscores the ability to recognize what is happening (mindfulness) and the ability to produce symmetric actions with diminishing energy resources.

Mindfulness – Finally, longer distances offer one the opportunity to “zone out” during a race. This lack of attention can cause one to lose the sense of pace, even “get lost” within a race. A lack of a mindful sense of awareness can lead to a rude awakening once glycogen depletion sets in. Conversely, running with attention to one’s internal and external environments allows one to monitor performance levels, split times, hydration schedules, nutritive intake and in general adhere to the pre-determined race plan that can in sum ease the transition to the depleted state as one’s energy stores gradually decline.

Before we conclude it begs the question – is glycogen depletion a dangerous situation? The answer needs to be qualified. Generally speaking, glycogen depletion is a short-term condition that can be readily rectified with a nutritious post-race meal. Were one in the state for days or weeks there could be numerous consequences for the body. Glycogen depletion is essentially an early form of starvation.

A second concern is the loss of symmetric motion due to glycogen depletion. The loss of symmetric motion and a coordinated flow during glycogen depletion is replaced by actions that may be described as “grinding or grabbing.” There will also be a diminished shock absorption and increased jarring with each foot strike that no doubt contributes to the point tenderness in the leg’s muscles and joints the days following a race. Ultimately, one’s recovery from a marathon or longer event needs to be considered in terms of weeks, even months as opposed to days for the shorter middle distance races.

Two conditions that could mistakenly be confused with glycogen depletion are heat injury and hyponatremia. A dangerously high elevation of one’s core temperature, muscle cramping and a decreased sweating or even dry skin can signify the presence of heat injury. One needs to accept the fact that heat injury can be life threatening. Discretion is another form of valor. Heat injury can be the result of a poorly executed hydration plan, poor acclimatization (such as traveling from the cool Northeast to race in the hot, humid South) or the result of chronic overtraining in the weeks prior to the race.

Hyponatremia is almost the flip side of the coin. With hyponatremia one has consumed too much water, overhydrated, and essentially suffers from water intoxication. Hyponatremia (literally “low sodium”) is a condition where one has sweated out one’s electrolytes and drunken an excess amount of water diluting the electrolyte stores that remain. Early signs include swollen fingers and confused thoughts. Severe cases include brain swelling and again can be life threatening. Tell tale signs include one’s singlet becoming “crusty” with a rough, starchy texture. The skin may have a glazed feel to it. Both these conditions underscore the importance of maintaining the mindful attitude noted above.

For the well-trained athlete one, can expect glycogen depletion with a degree of certainty in any race that lasts longer than two hours. With proper preparation and adherence to a sensible race plan the onset of glycogen depletion can be delayed possibly minutes but not avoided altogether. Use of long-term preparation strategies will help as will in-race strategies that include hydration, a well thought out race plan and possibly ingestion of foodstuffs along with careful self-monitoring of signs and symptoms of mounting fatigue.

English historian Edward Gibbon is credited with the quote, “The winds and waves are on the side of the most able navigator.” There are many “winds and waves” that go into a successful competitive effort, but it is preparation, awareness and the will to execute that differentiates merely surviving glycogen depletion versus excelling in a race in spite of it.

Off The Road Archive

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The Long Trail: Running and Hiking

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Running Economy


Athletic Prehab Exercises


Multi-Lateral Development and the 7 Primal Movements


When I Was a Child

Russ Ebbets, DC is a USATF Level 3 Coach and lectures nationally on sport and health related topics. He serves as editor of Track Coach, the technical journal for USATF. He is author of the novel Supernova on the famed running program at Villanova University and the sequel Time and Chance. His most recent book, A Runner’s Guide, a collection of training tips and running ideas has been nominated for the Track and Field Writers of America Book of the Year 2019. He can be contacted at

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