You're 45 minutes into a run when it hits. Your legs get heavy. Your pace drops. Your focus disappears. You started strong, trained consistently, and ate well before the session. But somewhere in the middle, your body ran out of fuel. This isn't a fitness problem. It's a fueling problem. And once you understand the simple research-backed rules for in-workout nutrition, it stops happening.

^{This article is backed by peer-reviewed research, see full sources and quality ratings at the end.}

This is the single most important rule. Use this as your starting point:

2. Start early, not when you're already tired

Begin taking in carbohydrates within the first 15–20 minutes of any session over 75 minutes. Once your glycogen stores are critically low, you cannot catch up fast enough to restore performance. The goal is to slow the depletion from the start, not refill an empty tank mid-session.[1]

3. Spread intake evenly through the session

Take 15–20 g every 20–30 minutes rather than a large bolus all at once. Smaller, regular doses keep blood glucose stable and reduce the risk of gastrointestinal (GI) discomfort: stomach cramping or nausea caused by overloading the gut at once.[1]

4. Practice your fueling in training: never try new products on race day

Your gut adapts to carbohydrate intake over time. Regular in-workout fueling in training increases your absorption capacity and reduces GI issues. Trying a new gel, drink, or quantity for the first time on race day is one of the most common causes of GI problems mid-race.[4]

A 2025 review (Cao et al., n = 100+ studies) confirmed a clear dose-response relationship: 30–60 g carbohydrate per hour improves performance in sessions of 60–150 minutes, and 60–90 g per hour is optimal for sessions over 150 minutes.[1]

A 2019 review (Mata et al.) found that carbohydrate supplementation improved endurance performance in 82% of controlled studies, making this one of the most replicated findings in sports nutrition.[5]

The 2016 joint position stand from three major professional bodies (the American College of Sports Medicine, the Academy of Nutrition and Dietetics, and the Dietitians of Canada), which reaches the same practical conclusions.[3]

Every time you exercise at moderate to hard intensity, your body burns glycogen: a form of sugar stored in your muscles and liver. Think of glycogen as a fuel tank. It holds roughly 400–500 g in total, enough for about 60–90 minutes of sustained effort at a challenging pace.

Once the tank empties, performance drops sharply. Your pace slows, your effort level (called rate of perceived exertion, or RPE: how hard the workout feels) spikes upward, and your ability to maintain intensity collapses. This is what athletes call "hitting the wall" or "bonking." It isn't a matter of mental toughness. It's physiology.[5]

The key insight: adding carbohydrates during exercise doesn't just give you energy. It preserves your glycogen stores by providing an external fuel source. The longer you can slow glycogen depletion, the longer you sustain performance.

For workouts under 45 minutes, stored glycogen is more than sufficient. No extra carbohydrates are needed.[2]

For sessions in the 45–75 minute range, the picture is more nuanced. Performance benefits are modest but measurable, especially at harder intensities. Interestingly, even rinsing your mouth briefly with a carbohydrate drink, without swallowing, has been shown to improve performance in sessions of around 60 minutes. Researchers believe this works through signals sent from the mouth to the brain's reward and effort centers, reducing perceived effort even without fuel being delivered.[1][2]

The maximum rate at which your gut can absorb a single carbohydrate type (such as glucose alone) is roughly 60 g per hour. That is the ceiling for single-source products.[5]

When you combine glucose with fructose (a different sugar type), the two use separate absorption pathways in the small intestine. This allows total absorption to reach up to 90 g per hour without increasing GI problems. A recommended ratio is 2:1 glucose to fructose, or 1:0.8 if your product uses that blend.[4] For most recreational athletes in sessions of 75 minutes to 2.5 hours, 30–60 g per hour from any standard gel or sports drink is sufficient. Only in sessions over 2.5 hours does the math require a mixed glucose-fructose product.

Not significantly. Multiple studies confirm that gels, sports drinks, and chews produce equivalent performance benefits when total carbohydrate intake is matched.[4] The critical variable is the amount and timing, not the delivery format. A gel taken with water is effectively equivalent to the same grams dissolved in a sports drink. Choose whichever format your gut tolerates best, and determine that in training.

A 90-minute run at moderate to hard effort sits squarely in the 30–60 g per hour range. That means 45–90 g total across the session. Three approaches that all work:

All three deliver equivalent fuel. Choose the one your stomach handles best, and find that out before you race.

1. Starting too late.

Most people wait until they feel tired before reaching for a gel or drink. By then, glycogen is already critically low and performance has dropped. For any session over 75 minutes, start fueling within the first 15–20 minutes. You are filling the tank before it empties, not trying to refill one that is already dry.[1]

2. Trying to hit 90 g/h with single-sugar products.

If you push intake above 60 g per hour using only glucose-based gels, you exceed your gut's absorption capacity and risk cramping or nausea. At doses above 60 g/h, switch to a mixed glucose-fructose product. Check the label: look for "fructose" or "maltodextrin + fructose" in the ingredients.[4][1]

3. Treating race day as the first test of your fueling strategy.

Your gut is trainable. Regular in-workout fueling in training adapts your intestinal transport capacity, increases absorption, and reduces GI symptoms over time. Using an untested product or a higher carbohydrate dose for the first time during a race is one of the most preventable causes of GI distress mid-event. Test everything in training first, including the amounts, brands, and timing.[4]

The research on in-workout carbohydrates is unusually consistent: session duration drives the decision. Under 45 minutes, water is enough. From 45–75 minutes, small amounts can help, especially in hard efforts. From 75 minutes upward, 30–60 g per hour makes a clear, measurable difference. Over 2.5 hours, target 60–90 g per hour with a glucose-fructose blend.

Start today: Check the duration of your next long session. If it runs over 75 minutes, plan your fueling now. Calculate your grams per hour, pick a product you have used before, and set a reminder to start early, within the first 20 minutes.

Red flag signs your fueling strategy isn't working:

All three are classic signs of glycogen running low. They are fixable with better fueling timing and amount.