Running gels are small, concentrated packets of carbohydrates designed to fuel your body during long runs. Most contain 20 to 25 grams of carbohydrates per serving, roughly the energy equivalent of a banana but in a form you can swallow mid-stride without chewing. They exist because your body can only store enough glycogen (its preferred fuel during exercise) to power roughly 90 minutes of continuous running. After that, you need to take in carbohydrates from outside sources, and gels are one of the most portable, efficient ways to do it.
What’s Inside a Running Gel
The primary ingredient in most gels is maltodextrin, a fast-digesting carbohydrate derived from starch. Many gels pair maltodextrin with fructose, often in a 2:1 ratio. This combination matters because your gut absorbs glucose and fructose through separate pathways. Using both pathways at once lets you absorb more total carbohydrate per hour than either sugar alone, which translates to more available energy for your muscles.
Beyond the carbohydrates, gels typically include electrolytes like sodium, potassium, and magnesium to replace what you lose through sweat. Some contain amino acids that may help reduce muscle breakdown during prolonged efforts. And many offer caffeine, usually ranging from 20 to 100 milligrams per packet. For reference, a small cup of coffee has about 95 milligrams. Caffeinated gels sharpen focus and reduce the perception of fatigue, making them popular choices for the later stages of a race.
How They Fuel Your Muscles
Your muscles run on glycogen, a stored form of glucose packed into muscle fibers and the liver. During a run, your body breaks down glycogen into glucose and burns it to produce the energy needed for muscle contraction. Blood glucose also enters muscle cells directly during exercise, even without the insulin signal your body normally requires at rest.
The problem is that glycogen stores are finite. Once they run low, your pace slows dramatically. Runners call this “hitting the wall.” Taking in carbohydrates during exercise, through gels or other sources, provides glucose from the bloodstream so your muscles don’t have to rely entirely on their internal stores. Research shows that carbohydrate ingestion during exercise helps maintain liver glycogen and can spare glycogen in fast-twitch muscle fibers, the ones you recruit when you’re pushing hard or surging late in a race.
How Much Carbohydrate You Actually Need
The amount depends on how long you’re running. For runs lasting one to two hours, aim for about 30 grams of carbohydrate per hour. For two to three hours, that jumps to 60 grams per hour. Ultramarathon efforts of three hours or more call for up to 90 grams per hour.
That 60-gram threshold is important for choosing gel types. Below 60 grams per hour, a gel with a single carbohydrate source (maltodextrin alone) works fine. Above 60 grams, you benefit from a glucose-fructose blend because your intestine can only absorb about 60 grams of glucose-type carbohydrates per hour through one transporter. Adding fructose opens a second absorption channel. At very high intake rates, around 120 grams per hour (common among elite athletes), a closer-to-equal ratio of glucose to fructose appears to improve how efficiently your body oxidizes the fuel. Any carbohydrate that sits in your gut unabsorbed isn’t doing useful work and is more likely to cause stomach problems.
Standard Gels vs. Isotonic Gels
Standard gels are thick, syrupy, and concentrated. They need water to be properly absorbed. A general guideline is about 250 milliliters (roughly 8 ounces) of water per gel. Skip the water and the concentrated sugar sitting in your stomach can pull fluid from surrounding tissues, slowing absorption and increasing the chance of nausea or cramping.
Isotonic gels are formulated to match your body’s natural fluid concentration, so they can be taken without extra water. They tend to be thinner and larger in volume. The tradeoff is that you’re carrying more weight per serving, but if you struggle with stomach issues or don’t want to coordinate gel timing with aid station water cups, isotonic gels simplify the process considerably.
When and How Often to Take Gels
Most runners start their first gel 45 to 60 minutes into a race, before glycogen stores drop critically low. After that, take one every 30 to 45 minutes, which works out to about two to three gels per hour depending on the product’s carbohydrate content and your target intake rate.
For a half marathon, faster runners might need only one or two gels total, while those on the course for two hours or more may need three or four. Marathon fueling is more involved. A three-hour marathoner typically needs 6 to 9 gels across the race. A five-hour marathoner may need 10 to 15. These numbers sound like a lot, and they underscore why practicing your fueling strategy during training is essential. Your gut adapts to processing carbohydrates during exercise, so race day should never be the first time you try a gel.
If you plan to use caffeinated gels, save them for the second half of your race when fatigue is building. Starting with non-caffeinated gels and switching later gives you a noticeable lift when you need it most.
Why Gels Cause Stomach Problems
Gut distress is the most common complaint about running gels. In a study comparing gel versus liquid carbohydrate intake during simulated long-distance triathlon, seven out of the gel users reported gastrointestinal discomfort compared to zero in the liquid group. Performance between the two groups was identical, but the stomach issues are a real drawback.
Several factors contribute. During running, blood flow diverts away from your digestive system to your working muscles, which slows digestion. Concentrated gels compound the problem by creating a high sugar load in an already sluggish gut. Taking gels without enough water, consuming too many too quickly, or trying a new brand on race day all increase the risk of cramping, bloating, or worse.
You can reduce these issues by training your gut during long runs, always taking standard gels with water, spacing your intake evenly rather than doubling up, and sticking with products you’ve tested in training. Isotonic gels and liquid carbohydrate sources (like sports drinks) are alternatives if gels consistently bother your stomach. Some runners combine gels with sports drinks to hit their carbohydrate targets, but be careful not to overdo the total intake, since the sports drink itself contributes carbohydrates.
Choosing the Right Gel
With dozens of brands available, the practical differences come down to a few variables: carbohydrate content per packet, whether it uses a glucose-fructose blend, caffeine content, consistency, and taste. A gel with 20 to 25 grams of carbohydrate is standard. If you’re targeting higher intake rates for a marathon or ultra, look for products with a maltodextrin-fructose blend so you can absorb more per hour without overwhelming your gut.
Texture varies widely. Some gels are thick and require a solid squeeze to get out of the packet. Others are almost liquid. Personal preference matters here, because a gel you find unpleasant to eat at mile 18 is a gel you’ll skip, and skipping fuel late in a long race has real consequences. Buy a few different brands during training and find what sits well in your stomach and doesn’t make you gag when you’re breathing hard. That’s the right gel for you.