When to take your first gel during a marathon

Most runners carry the right number of gels. Most runners still bonk.

The reason is almost always timing — not quantity. You can consume 200g of carbohydrates during a marathon and still hit the wall at mile 19 if you front-load when you should front-load, or wait too long before you start.

Here's the physiology, and what to do with it.

Why early fueling matters more than you think

When you start running, your body is drawing from two fuel sources: glycogen (stored carbohydrates in your muscles and liver) and fat. At marathon pace for most recreational runners, you're running at an intensity where glycogen dominates — roughly 60–70% of energy from carbohydrates.

Your glycogen stores are finite. For the average person, that's approximately 90–120 minutes of hard running. After that, you're in trouble unless you've been actively fueling.

Here's the critical point: carbohydrates consumed during exercise take 20–30 minutes to reach your bloodstream and muscles. If you wait until you feel yourself slowing down to take a gel, you're already behind. The gel you take at mile 18 will help you at mile 20 — but the damage from mile 18 is already done.

The 45-minute rule — and why it's a starting point, not a rule

The common advice is to take your first gel at 45 minutes. That's reasonable for most athletes running at a comfortable steady pace. But it's not universal.

If you're running faster, you're burning more carbohydrate per minute. A 3:30 marathon runner depletes glycogen faster than a 5:00 runner — they need to start fueling earlier and fuel more frequently.

If you're heavier, you'll have more glycogen storage capacity, but also higher absolute energy demands.

If it's hot, your carbohydrate oxidation rate increases.

The 45-minute guideline is a population average. Your number depends on your pace, your size, and the conditions on race day.

How many gels do you actually need?

A standard energy gel contains approximately 22–25g of carbohydrates. The maximum carbohydrate absorption rate from a single carbohydrate source (typically glucose or maltodextrin) is around 60g per hour. Using a combination of glucose and fructose — as many modern gels do — you can push this to 90g per hour.

For most recreational marathoners running at a pace where carbohydrate is the dominant fuel:

• A 4:00 marathon runner needs approximately 55–65g of carbohydrate per hour from mile 1
• A 3:30 runner needs 65–75g per hour
• A 3:00 runner needs 75–90g per hour

That's 2–4 gels per hour depending on your pace, and starting significantly earlier than most runners manage.

The practical math: for a 4:00 marathon, you'll likely need 6–8 gels taken every 30–40 minutes from around the 30-minute mark. Most runners carry 4–5.

Why gels make some people feel sick

GI distress during marathons is common and miserable. The usual culprits:

Too much fructose taken too quickly. Gels high in fructose taken in large amounts overwhelm the gut's absorption capacity, sitting in the stomach and causing cramping and nausea. Taking gels with water (not sports drink) and spreading them out helps.

Taking gels at the wrong intensity. At very high intensities, blood is shunted away from the digestive system. Gels taken when you're surging or at lactate threshold are harder to process.

Not practising in training. Your gut is trainable. Athletes who fuel consistently in long training runs develop better tolerance for gels at race pace. Athletes who never fuel in training and then try 8 gels on race day often pay for it.

The product isn't right for you. Gel composition varies significantly. Some athletes tolerate certain carbohydrate blends better than others. This requires testing — which is exactly why you should be practising your race nutrition in your long runs.

Building your gel timing plan

The practical approach:

1. Calculate your race duration and pace. This sets your carbohydrate need per hour.
2. Choose your fueling start point. For most runners: 30–45 minutes in. For faster runners: 30 minutes or earlier.
3. Set your intervals. Every 30–40 minutes is a reasonable starting interval. Adjust based on your pace.
4. Count your gels. Work backwards from your finish time.
5. Match to aid stations. Gels are easier to take when you can wash them down immediately with water.
6. Practice every long run. Your race-day plan should not be the first time you've tested this.

Sweatr builds this plan from your training data. It calculates your carbohydrate needs based on your body weight, your target pace, and your historical activity data — then maps out a gel timing schedule that fits the actual aid stations on your race course.

The number it gives you isn't a guess. It's built from what you've actually been doing in training, adjusted for race conditions.

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Gel products referenced: SiS Beta Fuel, Maurten Gel 100, High5 Energy Gel, Precision Fuel & Hydration. Nutritional values vary between products — always check the label.