Energy gels deliver a concentrated dose of simple carbohydrates that your body can absorb quickly and convert to fuel during prolonged exercise. Most gels pack 20 to 30 grams of carbohydrates into a small, portable packet designed to be swallowed mid-run, mid-ride, or mid-race without slowing you down. The mechanism is straightforward: sugars enter your bloodstream, reach your working muscles, and either get burned immediately or help preserve the limited fuel your body has stored.
What’s Actually Inside a Gel
The primary ingredient in most energy gels is maltodextrin, a processed carbohydrate derived from corn or potato starch. Despite being technically a complex carbohydrate by structure, maltodextrin breaks down almost as fast as pure glucose. Many gels also include fructose, which matters more than you might think. Your intestines have separate transport pathways for glucose-type sugars and fructose. By including both, a gel can push more total carbohydrate into your bloodstream per minute than either sugar could achieve alone.
The ratio between these two sugars affects how much fuel you actually absorb. A 2:1 glucose-to-fructose blend produces 1.2 to 1.5 times higher carbohydrate oxidation rates compared to glucose alone, and has been linked to roughly 8% improvements in time trial performance. Even a simple 1:1 blend of glucose and fructose produces 27% higher fuel oxidation than the same amount of glucose by itself. This is why most modern gels use a dual-sugar formula rather than relying on a single carbohydrate source.
Beyond sugars, some gels contain caffeine (typically 25 to 100 milligrams per packet), electrolytes like sodium and potassium, and occasionally amino acids. Caffeinated gels are popular for longer events because they help counteract both mental and physical fatigue. The general performance recommendation for caffeine during endurance events is 3 to 6 milligrams per kilogram of body weight, spread across pre-race and mid-race doses. For a 70-kilogram (150-pound) athlete, that works out to roughly 200 to 400 milligrams total.
How Your Body Turns a Gel Into Fuel
When you squeeze a gel into your mouth and wash it down with water, the sugars travel to your stomach and then into your small intestine, where absorption happens. Glucose and maltodextrin cross the intestinal wall through one type of transporter protein, while fructose uses a different one. Once in the bloodstream, these sugars reach your muscles, where they stimulate glucose uptake and get burned for energy.
This incoming fuel partially offsets the drain on your body’s stored energy. Your muscles and liver hold a limited supply of glycogen, the stored form of carbohydrate that powers intense exercise. A meta-analysis published in the Journal of Applied Physiology found that consuming carbohydrates during exercise spares roughly 24 mmol per kilogram of dry muscle weight over about 100 minutes of effort. That’s a modest but meaningful buffer. Separately, glucose or sucrose ingestion during exercise has been shown to prevent liver glycogen depletion, even when muscle stores still decline. Keeping your liver topped off matters because the liver is responsible for maintaining blood sugar levels, and when it runs dry, you’re much closer to the classic “bonk” or “hitting the wall.”
How Many Grams You Actually Need
The right amount of carbohydrate depends on how long you’re exercising. Current sports nutrition guidelines break it down simply:
- 1 to 2 hours: about 30 grams of carbohydrate per hour
- 2 to 3 hours: about 60 grams per hour
- 3 hours or more: up to 90 grams per hour
A single gel typically contains 20 to 30 grams, so for a two-hour effort you’d need roughly two gels per hour. For an ultra or Ironman lasting well beyond three hours, you’d be looking at three or more gels per hour, often supplemented with other carbohydrate sources like sports drinks or chews. Hitting 90 grams per hour from gels alone is difficult, which is why many endurance athletes mix their fuel sources.
Timing matters too. Most athletes start taking gels 30 to 45 minutes into exercise, before they feel depleted, then continue at regular intervals. Waiting until you feel empty means you’re already behind on fueling, and catching up mid-race is harder than staying ahead of it.
Why Gels Sometimes Cause Stomach Problems
Stomach cramps, nausea, and bloating are the most common complaints about energy gels, and the science behind it comes down to concentration. Most gels are extremely concentrated sugar solutions. A study in the International Journal of Sport Nutrition and Exercise Metabolism tested a range of commercial gels and found that only 15% had an osmolality (a measure of how concentrated a solution is) below 1,000 mmol/kg. Just one gel in the entire sample was isotonic, meaning close to the concentration of your body’s own fluids. The average across all gels was approximately 4,500 mmol/kg, and 70% exceeded 2,000 mmol/kg.
This matters because highly concentrated solutions empty from your stomach slowly. They draw water into the gut to dilute themselves, which can cause distension and cramping. The thicker and stickier the gel, the worse this tends to be, especially if you don’t drink enough water alongside it. On the other hand, drinking too much water with a low-calorie gel can dilute your stomach contents to the point where they pass through quickly but deliver very little energy.
The sweet spot is a gel that’s relatively close to isotonic, consumed with a moderate amount of water. A common recommendation is to drink about 14 ounces of water between each gel, not necessarily all at once, but spread across the interval before your next one. Training your gut in practice sessions is equally important. Your intestines can adapt to processing fuel during exercise, but only if you practice regularly. Race day is not the time to try a new gel brand or a higher intake rate for the first time.
Isotonic Gels vs. Standard Gels
Some newer gel products market themselves as “isotonic,” meaning they’re already diluted to a concentration similar to your body’s fluids and don’t require extra water. These gels are thinner and more liquid, which allows them to empty from the stomach faster and reduces the risk of GI distress. The trade-off is that they’re bulkier to carry because the same amount of carbohydrate comes in a larger packet.
Standard concentrated gels are more compact and calorie-dense, making them easier to stash in a pocket or belt. But they rely on you drinking enough water at the right time. If you’re running a course with predictable water stations, standard gels paired with deliberate hydration can work perfectly well. If water access is unpredictable, or if you’ve struggled with stomach issues in the past, isotonic gels may be a better fit.
What Gels Can and Can’t Do
Energy gels are effective at delivering fast carbohydrates during sustained exercise, but they’re not a substitute for starting well-fueled. If you begin a race with depleted glycogen from poor nutrition the day before, gels won’t fully compensate. They supplement your stored fuel; they don’t replace it. The glycogen-sparing effect of mid-exercise carbohydrates is real but small, roughly equivalent to delaying the point of depletion rather than preventing it entirely.
For efforts under 60 to 75 minutes, most people don’t need gels at all. Your muscle glycogen stores are sufficient for about 90 minutes of moderate to hard exercise, and topping off with a gel during a 45-minute tempo run provides no meaningful benefit. Gels become genuinely useful once you cross the one-hour threshold, and increasingly important the longer you go. For marathon runners, century cyclists, and triathletes, they’re a core part of the fueling strategy rather than an optional extra.