A guide to energy gels for athletes

Most runners hit the wall at some point during a long race, and when they do, the conversation almost always circles back to fuel. Specifically, how much they took, when they took it, and what form it came in. Energy gels have been a staple in endurance sports for years, tucked into jersey pockets and waistband pouches from 10Ks to ultramarathons. But a surprising number of athletes still use them wrong. They take them too early, too late, without water, or in combinations that turn their stomachs inside out by mile 18. The information below covers how gels work, when to use them, how to pair them with other fuel, and how to train your gut so race day goes according to plan.

How Energy Gels Actually Work

Energy gels are concentrated packets of carbohydrates in a semi-liquid form. Most contain between 20 and 30 grams of carbs per serving, sourced from a combination of glucose and fructose. The body absorbs glucose through one intestinal pathway and fructose through another, which means using both sugars together allows a higher total rate of absorption than glucose alone.

The current recommendation from sports nutrition research puts the optimal glucose-to-fructose ratio at 1:0.8. Older formulas often used a 2:1 ratio, but the newer balance appears to reduce stomach problems and allow the body to process more carbohydrate per hour. Athletes aiming for 60 to 90 grams of carbs per hour during prolonged activity will need to take a gel roughly every 20 to 30 minutes, depending on the size of the gel and the rest of their fueling plan.

What Goes in the Bottle Versus the Packet

Most athletes carry a mix of fuel sources during long efforts. Chews, drink mixes, and energy gels all deliver carbohydrates, but they absorb at different rates and demand different amounts of water. A runner relying on drink mix alone may struggle to hit 60 to 90 grams of carbs per hour without overloading their stomach with fluid. Gels paired with 4 to 6 oz of water solve that problem, while isotonic formulas skip the water requirement entirely.

Picking one source and ignoring the rest rarely works well. Combining gels with a low-concentration drink mix lets athletes fine-tune their intake without guessing.

When to Take Your First Gel

A common mistake is tearing open a gel within the first 30 minutes of a race. The body has stored glycogen in the muscles and liver, and those reserves will carry you for a while. Taking a gel too early wastes fuel you could use later and adds unnecessary volume to your stomach before you need it.

A better approach is to take the first gel somewhere between 60 and 90 minutes into the effort. After that initial intake, continue with a gel every 30 to 45 minutes for the remainder of the race. The exact frequency depends on the intensity, the gel’s carb content, and what else you are drinking or eating. Some athletes prefer smaller, more frequent doses. Others do better with larger servings spaced further apart. Both approaches can work as long as the hourly carb total stays within range.

Isotonic Gels and the Water Question

Standard gels are thick and concentrated. They pull water into the gut to dilute the sugar before it can be absorbed, and that process can cause cramping, bloating, or nausea if there is not enough fluid available. Taking 4 to 6 oz of water with each gel helps the body process it without complaint.

Isotonic gels work differently. Their concentration already matches the body’s own fluid balance, so they absorb faster and do not require water alongside them. For athletes who race in conditions where water is hard to carry or aid stations are spaced far apart, isotonic gels remove a variable from the equation. The tradeoff is that they tend to come in larger, heavier packets.

Caffeine in Gels and How to Use It

Some gels contain caffeine, typically in the range of 25 to 100 mg per serving. Caffeine reduces the feeling of effort during prolonged activity and improves focus and alertness when fatigue starts to set in. That makes caffeinated gels more useful in the second half of a race than the first.

Saving your caffeinated gels for mile 16 of a marathon or the final 2 hours of an ultra is a practical approach. Using them too early means the effect may wear off before you need it most. Athletes who are sensitive to caffeine should test their tolerance well ahead of any race.

Gut Training Is Not Optional

The gastrointestinal system can be trained to tolerate higher carbohydrate loads during exercise, but this takes time. Starting at least 6 to 8 weeks before race day, athletes should practice taking gels during their long training program sessions at race pace or near it. Begin with smaller amounts and work up to the full race-day intake over several weeks.

Skipping this step and then loading up on gels during a race is one of the fastest ways to end up in a porta-potty at mile 20. The gut needs repeated exposure to adapt. There is no shortcut.

Real-Time Monitoring With Glucose Sensors

Some athletes have started wearing continuous glucose monitors during  training to see how their blood sugar responds to different fueling strategies. These small sensors, worn on the arm, provide real-time data on glucose levels throughout a session. The information can help fine-tune when and how much to eat during a race, turning a guessing game into something more precise.

This approach is still relatively new in recreational endurance sports, but it has been used in clinical settings for years. For athletes who have trouble finding a fueling plan that works, the data from a glucose monitor can point to patterns that are hard to detect by feel alone.

Putting It All Together

Your fueling plan should be built in training, not invented on race morning. Start by figuring out how many grams of carbs per hour you need based on the length and intensity of your event. Choose a gel with a glucose-to-fructose ratio close to 1:0.8, decide if you want isotonic or standard formulas, and determine how caffeine fits into the second half of your race. Then practice. Every long run is a test of the plan. By the time you line up on race day, swallowing a gel at mile 12 should feel like second nature because you have done it 15 times before in training.