You’re 30 kms into your marathon, feeling strong, but as you push toward the 35km mark, sudden fatigue hits. Your legs grow heavy, your energy dips, muscles start cramping and your pace begins to suffer. You’ve hit the infamous “wall.” The culprit? Depleted glycogen stores.
But what if you could delay that wall — maybe even avoid it altogether? Enter glycogen sparing. This physiological trick can help you go farther, faster, and finish your marathon feeling strong.
When you run, your body burns two primary fuel sources: glycogen (stored carbohydrates) and fat. Glycogen is the body’s preferred energy source for high-intensity efforts, while fat is a slower-burning fuel used during lower-intensity exercise.
But here’s the catch: your body has a limited supply of glycogen, typically enough for about 90 minutes to two hours of moderate-to-intense exercise. Once those glycogen stores are used up, fatigue sets in—hence, the “wall”.
Glycogen sparing is the process by which your body conserves glycogen by relying more on fat as fuel, particularly during the early stages of long-distance runs. The more fat your body can burn, the less glycogen you’ll need, allowing you to push harder in the final miles when you really need that extra burst of energy.
When you run, your body gradually increases its reliance on fat, especially as your heart rate stays in the aerobic zone (typically around 60-70% of your maximum heart rate). In this zone, you’re burning a higher percentage of fat compared to glycogen.
However, as your pace increases, so does the demand for glycogen. But with proper training, you can shift the balance in favor of fat oxidation, which in turn helps spare your glycogen stores. This is why experienced marathoners seem to cruise through without hitting the wall. They’ve trained their bodies to be metabolically efficient.
Long Runs at a Comfortable Pace: Focus on running these at a conversational pace, about 60-70% of your maximum effort. This encourages fat utilization and conserves glycogen. These runs should last two to three hours, building up as you get closer to your marathon. Do check out our podcast episode on Long Run
Fasted Runs: While you don’t want to make this a daily habit, incorporating one or two fasted runs (running in the morning before breakfast) each month can improve your body’s fat-burning efficiency. Please don’t overdo these, and keep these at a moderate and well below race pace to avoid fatigue.
Carbohydrate Periodization: On days when you’re not running long, reduce your carbohydrate intake slightly to teach your body to use fat as fuel. Save your carb-loading for harder workouts, tune-ups and race week, when you’ll need those glycogen stores topped up.
Interval Training: Incorporate speed sessions or tempo runs into your training plan. While these workouts primarily burn glycogen, they increase your overall metabolic efficiency and improve your ability to handle the higher-intensity demands of race day.
When race day arrives, your primary focus should be on preserving glycogen for the latter part of the race.
Start slow: Keep your pace controlled in the first half of the race, even if it feels easy. You want to conserve your glycogen for when the race really begins after 32K — in the final 10K.
Fuel Smart: Plan your race-day fueling strategy to include carbohydrates in the form of gels, or drinks every 30-45 minutes. This will help keep your glycogen stores topped off while your fat metabolism continues humming along.
Trust Your Training: If you’ve trained well, your body will be primed to switch to fat-burning early, preserving glycogen for when you need it most. Stay calm, stick to your plan, and save that finishing kick for the last few miles.
The Final Miles: As you approach the final stretch of your marathon, this is where glycogen sparing pays off. While other runners may hit the wall, you’ll be tapping into the glycogen reserves.
Now we are treading the slippery slope of unproven supplements. Please tread caution, but it is worth mentioning.
When carbohydrate intake is low (such as during fasting, prolonged exercise, or a ketogenic diet), the body begins to rely on fat stores for energy. Fat is broken down into fatty acids, which are transported to the liver. In the liver, these fatty acids are converted into ketone bodies (mainly beta-hydroxybutyrate, acetoacetate, and acetone) through a process called ketogenesis.
Ketones as an Alternative Fuel: Once produced, ketones can be used as an energy source by the brain, muscles, and other tissues. This makes ketones especially valuable in prolonged endurance activities where glycogen (stored carbohydrates) becomes depleted.
Taking ketones orally have gained attention as a potential alternative fuel source for endurance athletes. Theoretically, they could spare glycogen and reduce the need for carbohydrate consumption during exercise, which might improve endurance performance.
Some studies show they can help delay fatigue. However, research results are mixed. While they may benefit low-to-moderate intensity efforts, their effectiveness in high-intensity events like marathons is less clear. Do listen to this discussion on exogenous ketones in this recent episode of ‘The Real Science of Sport’ podcast, featuring David Roche who won the Leadville 100 miler with a new record.
This article is compiled by Team GeeksOnFeet for the love of running. If you have any questions or suggestions, feel free to reach out to us at [email protected].