Energy Gels - How they work

Your body uses two primary sources of fuel to feed the muscles when you’re running — fat and carbohydrate. Fat is a largely abundant resource, but is broken down into usable energy slowly, making it an ineffective fuel source when running anything faster than about 60-70% of your VO2max (roughly equivalent to your aerobic threshold or marathon pace).

Therefore, your body relies on carbohydrate as its primary fuel source when racing. Generally, the faster you run, the greater the percentage of your fuel will come from carbohydrates. The problem with carbohydrate is that we can only store a limited amount in our muscles — even when you load up. Typically, we can store about 90 minutes of muscle glycogen when running at half marathon pace and about 2 hours worth when running at marathon pace. So, if you’re not an elite athlete, you’ll be running out of muscle glycogen long before you cross the finish line.

Simply speaking, energy gels are designed to replenish carbohydrate stores that are depleted when running. Sounds like energy gels are a savior, right?

Unfortunately, energy gels don’t provide a simple one-to-one replacement (something you won’t read on the label of your favorite gel) because the glycogen we ingest from gels doesn’t always make its way to the working muscles. So why use them? 

Different Types of Energy Gels

Isotonic Energy Gels Isotonic means that they have already been mixed to the correct water / electrolyte balance. Therefore, you don't need to take extra water. These are excellent if you are worried about getting the right electrolyte / water concentration. They will provide energy and also help provide optimal hydration. The drawback is that they are a little more bulky to carry around. But, it also saves carrying both gels and water. They are great for training runs for simultaneously topping up both water and energy levels.

Glucose / Fructose 2:1 Studies have suggested that a combination of glucose and fructose in a 2:1 ratio can lead to a higher uptake of carbohydrate than just relying on glucose. Glucose has a very high GI index (increases blood sugar immediately) Fructose has a lower GI index and raises your blood sugar levels more gradually. Therefore, this combination can be good for long distance running who are struggling to take on enough carbohydrate during a race.

Caffeine Some energy gels also include caffeine - a legal stimulant. Studies suggest that ingesting caffeine can boost your performance, though it can vary between individuals. In long distance races, some runners like to take a caffeine energy gel towards the end of the race when they are becoming tired. Some studies have suggested caffeine is less effective in hot weather because it can lead to dehydration. Caffeine also acts as a diuretic making you need a toilet break in a race - another reason to save for later in the race. Whether you want to use caffeine will come down to personal preference whether you want to benefit from this everyday stimulant. 

When to Use Energy Gels in Your Race Strategy

Now that you understand the different types of energy gels, the key question is how and when to use them to maximise your performance during a race. It’s not enough to just grab a gel and take it when you start feeling tired; timing and quantity matter.

Pre-Race: While it might be tempting to take an energy gel before your race even starts, doing so too early may lead to a rapid spike and then crash in blood sugar levels. Instead, focus on a solid pre-race meal rich in carbohydrates to ensure your glycogen stores are topped up. Save the gels for when they’ll count: mid-race.

Mid-Race: Most runners will start depleting their glycogen stores around the 60-90 minute mark, depending on their pace. This is when energy gels become crucial. A good strategy is to start taking gels before you hit that wall — typically around the 45-minute mark for a half marathon or 60-75 minutes into a marathon. This ensures you’re maintaining your carbohydrate levels, rather than trying to catch up once you’ve already run low.

Spacing Out Gels: Taking gels at regular intervals — about every 30-45 minutes after your first one — helps to maintain steady carbohydrate levels throughout your race. Don’t wait until you’re feeling fatigued, as by then your glycogen stores are likely already depleted.

Hydration is Key: If you’re using traditional (non-isotonic) gels, it’s essential to chase them with water. Without water, gels can sit in your stomach and cause discomfort, or they may not be absorbed effectively. Keep in mind that isotonic gels don’t require this extra hydration, which can be handy in races where water stations are scarce.

End of Race: For longer races like marathons, it can be helpful to use caffeine gels in the final stages. The caffeine boost, paired with a carbohydrate top-up, can give you that extra kick when you’re hitting the dreaded “wall.” However, remember that caffeine affects everyone differently. If you’re sensitive to caffeine, consider using gels with a lower dose or spacing out their intake to avoid jitters or dehydration.

Training with Gels

Just as you wouldn’t test out a new pair of shoes on race day, you shouldn’t experiment with gels for the first time during an important race. Incorporating them into your long training runs is vital for understanding how your body reacts to different types, how often you need them, and whether you experience any gastrointestinal issues (a common side effect for some runners).

Start by testing different types of gels during your long runs — isotonic, glucose/fructose blends, or those with caffeine — and figure out what works best for you. This is your chance to determine if certain flavours or ingredients upset your stomach or if particular brands don’t sit well with you.

The Bottom Line

Energy gels can be a game-changer for endurance runners, but they’re not a magic bullet. It’s essential to use them strategically — understanding the timing, dosage, and hydration requirements. Additionally, gels should be just one part of your broader nutrition plan, both in training and on race day.

By trialling different types and timing strategies in your training, you can fine-tune your approach to ensure you’re fuelling your body in a way that helps you cross the finish line feeling strong, rather than crawling across in a glycogen-depleted haze. After all, the best race-day strategy is the one that’s been practiced, tested, and perfected long before the starting gun goes off.