Why do runners eat GU? The science behind the gel

December 20, 2025 •

4 min read

Over 30 years ago, GU Energy Gels were first created for ultra-trail runners pushing the boundaries of the sport, and today, they are a staple for endurance athletes of all levels. Runners eat GU primarily to replenish their glycogen stores during prolonged, high-intensity exercise, preventing the dreaded "wall".

Quick Summary

Energy gels like GU provide concentrated, fast-acting carbohydrates to replenish muscle glycogen during long runs. This helps sustain energy levels, delays fatigue, and is easily portable for mid-exercise fueling. The specialized formulation supports performance when the body's natural fuel stores are depleted.

Key Points

Replenish Glycogen: GU provides concentrated carbohydrates to replenish energy stores and prevent "hitting the wall" during endurance activities.
Fast and Sustained Energy: A dual blend of maltodextrin and fructose ensures a quick energy boost followed by a more sustained release.
Optimized for Digestion: The carbohydrate ratio is designed to maximize absorption and minimize stomach upset during intense running.
Includes Electrolytes and Amino Acids: Key ingredients like sodium and BCAAs help maintain hydration, reduce muscle damage, and delay fatigue.
Portable and Convenient: The small, durable packaging is easy to carry and consume quickly without breaking stride during a run.
Proactive Fueling: Optimal use involves consuming a gel before you feel tired, typically every 30-45 minutes for efforts over an hour.
Practice During Training: It is essential to test your fueling strategy with GU during training runs to determine what works best for your body.

The science of fueling your run with GU

For activities lasting over 60–90 minutes, a runner’s primary fuel source, stored glycogen, begins to run low. When these stores are depleted, performance can rapidly decline, leading to fatigue and diminished physical and mental capacity. This is where a concentrated carbohydrate source like GU Energy Gel becomes essential. GU helps to maintain a steady supply of energy to your working muscles and brain, allowing you to push harder and longer.

How GU delivers energy to your muscles

GU Energy Gels are formulated with a precise blend of simple and complex carbohydrates, along with other key ingredients, to provide both quick and sustained energy.

Key ingredients in GU Energy Gel:

Maltodextrin and Fructose: This dual-carbohydrate blend is crucial for maximizing absorption. The body uses different pathways to process these two sugars, which allows for faster energy delivery to the bloodstream while minimizing the risk of stomach upset. Maltodextrin provides quick energy, while fructose is absorbed slightly slower for more sustained fuel.
Sodium: The primary electrolyte lost through sweat, sodium helps maintain water balance and aids in hydration. This is vital for preventing cramping and dehydration during prolonged exercise.
Branched-Chain Amino Acids (BCAAs): Including leucine, valine, and isoleucine, BCAAs help reduce mental fatigue and decrease muscle damage during long runs. This can help improve recovery time and support muscle repair post-exercise.
Caffeine: In some flavors, caffeine is added to increase alertness, improve focus, and provide an extra energy boost. This can be particularly beneficial in the later stages of a race or intense workout.

Types of GU for different runners

GU Energy Labs offers a variety of products tailored to different needs, from daily training to intense, long-duration events.

Original Energy Gel: The standard formulation suitable for most endurance activities over an hour. It provides a blend of carbs, electrolytes, and amino acids.
Roctane Energy Gel: Designed for higher intensity and longer duration efforts, Roctane gels contain more sodium and BCAAs than the original formula.
Liquid Energy Gel: A thinner, more drinkable option for those who prefer a less dense texture. It delivers the same fast-absorbing carbohydrates as the original gel.
Energy Chews: A solid, chewable alternative to gels, providing the same nutritional benefits in a different format for varied consumption needs.

When and how to use GU gels effectively

To get the most out of your GU, timing and hydration are key. Experimenting during training is vital to finding the right strategy for your body.

Proper usage guidelines:

Pre-Run: Take one gel 5–15 minutes before a long run to top off your energy stores, especially if your pre-run meal was a while ago.
During the Run: Consume one gel every 30–45 minutes during prolonged activity to maintain consistent energy levels and prevent hitting the wall.
Hydrate: Always consume traditional GU gels with water to aid absorption and prevent gastrointestinal distress. Isotonic gels, which are thinner and already contain water, are the exception, though drinking water is still recommended.
Don't Wait: Begin fueling proactively before fatigue sets in. Waiting until you feel a crash can be too late to recover effectively.

Table: GU vs. Other Fuel Sources


Feature	GU Energy Gel	Whole Foods (e.g., Banana, Dates)	Isotonic Gels (e.g., SIS)
Carbohydrate Absorption	Rapid due to concentrated maltodextrin/fructose blend.	Variable; slower digestion due to fiber, but provides sustained energy.	Rapid due to liquid, water-based formulation.
Convenience	Highly portable, easy to carry and consume in a small packet.	Requires carrying and potential messiness; can be more cumbersome.	Easy to consume, no additional water needed for consumption.
Digestibility	Designed to minimize stomach distress, but some individuals may have sensitivity to ingredients.	Generally gentle on the stomach, especially for those sensitive to processed ingredients.	Often gentler on the stomach due to thinner, water-based formula.
Electrolyte Content	Contains sodium and other key electrolytes.	Naturally contains some electrolytes, like potassium in bananas.	Specifically formulated with a balanced electrolyte profile.
Taste Fatigue	Wide variety of flavors helps reduce taste fatigue during long efforts.	Simple, natural tastes can be preferable over long distances.	Limited flavor options; can cause palate fatigue over prolonged use.

Alternatives to consider

While GU is a popular choice, many alternatives exist depending on personal preference and dietary needs. These include other commercial gels like Maurten or Huma, energy chews and blocks, and natural whole foods such as dried fruit or honey packets. Experimenting with these options during training is the best way to find what works for your body.

Conclusion

Runners eat GU because it offers a highly effective and convenient source of concentrated energy during long-distance or high-intensity exercise. By providing a dual-carbohydrate blend for both immediate and sustained fuel, along with electrolytes and amino acids, GU helps prevent energy depletion and optimize performance. Consistent, tested fueling during training is crucial for success on race day, and products like GU are engineered to support that strategy from start to finish.

Visit the GU Energy Labs website for detailed product information

Frequently Asked Questions

For activities over an hour, take your first gel 60–90 minutes into your workout. For shorter, more intense efforts, or if you haven't eaten recently, you can take one 5–15 minutes before you start.

A runner should consume one GU Energy Gel every 30 to 45 minutes during prolonged exercise to maintain consistent energy levels.

Yes, it is recommended to consume traditional GU gels with water to help with digestion and absorption. Isotonic gels are the exception, but proper hydration is always important.

For most runners, gels are not necessary for runs under an hour, as the body's stored glycogen is usually sufficient. However, for a high-intensity 5k or if running fasted, a gel beforehand might provide a beneficial boost.

GU Roctane is designed for longer, more intense efforts. It contains higher levels of sodium and branched-chain amino acids (BCAAs) compared to the original GU Energy Gel.

Alternatives include other commercial energy gels (e.g., Maurten, SIS), energy chews and blocks, and natural whole foods like dates, bananas, honey, or applesauce.

If you experience gastrointestinal distress, try smaller, more frequent doses or switch to a thinner, isotonic gel. It is also crucial to test your fueling strategy during training, not on race day.