Science in Sport Isotonic Energy Gels: How to Use Them for Marathon Fueling

By Ethan Brooks

Science in Sport isotonic energy gels are designed to deliver carbohydrate quickly, with a concentration close to that of your blood, so your gut can absorb them without much delay. This guide explains what “isotonic” actually means, how to decide whether these gels suit your stomach, and how to build a practical fueling plan for long runs and race day. By the end, you will know how many to carry, when to take them, and how to avoid the most common problems marathoners run into.

What “Isotonic” Means for an Energy Gel

In sports nutrition, “isotonic” describes a drink or gel whose osmolality is roughly equal to that of human blood. In practice, manufacturers usually reach this by using a carbohydrate blend rather than a single sugar, which keeps the concentration lower than a plain glucose gel of the same calorie count. A lower concentration generally empties from the stomach more readily, which matters when you are running and your gut is already working hard to handle blood flow for your legs.

Shopping for one? See our guide: 5 Best Science in Sport GO Isotonic Energy Gels: Expert Picks Compared.

Isotonic does not mean “easier on every stomach.” It describes one physical property. Your tolerance also depends on how much fluid you drink with the gel, how fast you are running, and how well your gut has adapted to carbohydrate intake during training.

Step 1: Understand the Carbohydrate Target

Most marathon fueling guidance suggests aiming for roughly 30 to 60 grams of carbohydrate per hour during running, with some trained athletes tolerating more when they have practiced the strategy. A typical single gel sachet from this brand range contains around 20 to 25 grams of carbohydrate, though you should always check the label on the packet you are using because formulations change.

Before you plan around a number, work out your target:

1. Decide your hourly carbohydrate goal, starting near the lower end (about 30 grams per hour) if you are new to gels.
2. Divide that goal by the carbohydrate content printed on your gel to find how many gels you need per hour.
3. Subtract any carbohydrate from sports drinks or other food you plan to consume during the same hour.

If you are unsure how this fits your body, a sports dietitian or your coach can help you set a safe starting point.

Step 2: Match Gel Timing to Your Race Plan

Gels work best when you take them at regular intervals rather than in bursts when you feel tired. Many runners take the first gel somewhere between 45 and 60 minutes into a run, then repeat every 30 to 45 minutes. For a detailed walkthrough of spacing and how it changes for different distances, see When to Take Energy Gels: A Marathoner's Timing Guide.

Half marathoners usually need fewer gels, and the timing is tighter if they are racing hard. When to Take Energy Gels During Half Marathon: A Step-by-Step Timing Guide covers that scenario in more detail.

Sample Timing Table for a Marathon

Step 3: Pair Gels With the Right Amount of Fluid

Isotonic gels are designed to be taken with water, and the amount matters. Taking a gel with a large volume of plain water can cause sloshing and cramping, while taking it dry can be hard to swallow. A common approach is to take a few small sips of water, about 50 to 100 milliliters, with each gel. Adjust based on temperature and sweat rate.

If you are using a sports drink on course, factor its carbohydrate into your hourly total. Some runners find that a drink-plus-gel combination causes stomach discomfort, while a water-plus-gel approach works well. Test both in training.

Step 4: Practice in Training Before Race Day

Gut training is one of the most important steps, and it is the one most often skipped. Start with one gel per hour on long runs over 90 minutes, then build up gradually over several weeks. Keep a simple log noting the time taken, the flavor, any stomach symptoms, and the weather.

Tools and Preparation Checklist

• Several gels from the same batch you plan to race with
• A running belt or pocket setup that keeps gels easy to reach
• A watch or phone timer set to your fueling intervals
• A training log for noting gel timing and stomach response
• Water bottle or hydration pack for sip-based intake
• Race-day gels stored at moderate temperatures, away from direct heat

Common Mistakes to Avoid

• Trying a new flavor on race day. Taste fatigue is real, and unfamiliar flavors can trigger nausea. Practice with every flavor you plan to use.
• Waiting until you feel empty. Fueling late is harder to recover from. Start at a steady interval from the first 45 to 60 minutes.
• Taking gels with too much water. Large volumes can cause bloating. Stick to small sips.
• Ignoring stomach feedback. If you experience cramping or nausea in training, reduce frequency or change your fluid plan rather than pushing through.
• Stacking multiple carbohydrate sources without calculating. Gels plus a sports drink can exceed your intended hourly total quickly.

Tips for Better Results

• Keep gels in an inside pocket or on a belt so you can take them without breaking stride.
• Tear the top of the sachet before you need it, so you are not fumbling mid-race.
• If you are running in heat, plan for higher fluid intake, but keep the sip volume small.
• Carry a spare gel in case of a dropped packet or an aid station shortage.

Recommended gear

A simple running belt or pocket system helps you carry gels comfortably for the full distance. Look for one with a small zipped pocket so sachets do not bounce loose.

• Running belt with gel loops for quick access: View on Amazon
• Soft flask for measured sips of water during long runs: View on Amazon
• Stopwatch or GPS watch with interval alerts: View on Amazon

FAQ

Are Science in Sport isotonic energy gels good for beginners?

They can be, because their lower concentration is often gentler on the stomach than high-concentration gels. Beginners should still test them in training before using them on race day.

How many gels should I take in a marathon?

Many marathoners take between three and five gels across the race, depending on their carbohydrate target, pace and gut tolerance. Calculate your number from your hourly goal and the carbohydrate listed on the packet.

Do I need to drink water with an isotonic gel?

Most people take gels with a small amount of water to help swallowing and digestion. Large volumes of water can cause discomfort, so keep sips modest.

Can I take isotonic gels on an empty stomach?

You should eat a normal pre-run breakfast before long runs. Taking a gel on an empty stomach is not recommended as a substitute for a proper meal, and it is best to test your response in training first.

What if a gel upsets my stomach?

Reduce the frequency, take smaller sips of water, or switch to a different carbohydrate source. If symptoms persist, speak with a doctor or sports dietitian before continuing.

Do isotonic gels expire?

Yes. Check the best-before date on the packet and avoid using gels that are past it, damaged, or have changed texture or smell.

Should I use gels or chews for a marathon?

Both work, and the choice comes down to personal preference and how your stomach responds. Compare the two in Energy Chews vs Gels (2026): Which Is Better for Marathon Fueling?.

Conclusion

Science in Sport isotonic energy gels can be a practical way to meet your carbohydrate target during long runs and race day. Success depends on calculating your hourly intake, timing gels at regular intervals, pairing them with a modest amount of water, and practicing the full plan in training. Always test your strategy well before the start line, and consult a healthcare professional or sports dietitian if you have digestive concerns or a medical condition.

Related reading

• Top 8 Whole Foods Energy Gels Worth Considering in 2026

About Ethan Brooks

Ethan Brooks is part of our editorial team and reviews and edits our marathon & half-marathon guides, checking product details against manufacturer specifications and owner feedback.