You’re fifteen miles into your first half marathon and your legs feel like they’ve been filled with wet sand. Someone hands you a small foil packet. You tear it open, squeeze a viscous blob of something that tastes like overly sweet cough medicine into your mouth, and try to swallow it while running. Five minutes later, your legs feel lighter and your pace picks back up. That’s an energy gel doing exactly what it’s designed to do — and there’s a reason every runner you see at parkrun, half marathons, and ultras has a few stuffed into their shorts pocket.
In This Article
- What Are Energy Gels?
- How Energy Gels Work: The Science Made Simple
- When to Use Energy Gels
- How to Take Energy Gels Properly
- Types of Energy Gel: Thick, Isotonic, and Caffeinated
- Popular Energy Gel Brands in the UK
- Energy Gels vs Other Fuelling Options
- Common Energy Gel Mistakes
- Training Your Gut: Why Practice Matters
- How Many Gels to Carry for Different Distances
- Frequently Asked Questions
What Are Energy Gels?
Energy gels are concentrated carbohydrate supplements in small, portable sachets designed to give you quick energy during endurance exercise. Most contain 20-30g of carbohydrates — roughly the same as a banana — in a thick, syrupy form that you can swallow without chewing.
They exist because your body can only store enough glycogen (the carbohydrate fuel stored in your muscles and liver) for roughly 60-90 minutes of moderate-to-hard running. After that, your glycogen stores deplete, your blood sugar drops, and you hit the wall. Gels provide fast-absorbing carbohydrates to top up your fuel levels without needing to eat solid food while running — which, if you’ve ever tried to eat a flapjack at race pace, you’ll know is a miserable experience.
What’s Actually in Them?
Most energy gels contain a combination of:
- Maltodextrin — a fast-absorbing carbohydrate derived from corn or wheat starch. This is the primary fuel source in almost every gel
- Fructose — fruit sugar, often combined with maltodextrin because your gut can absorb more total carbohydrate when you use both types simultaneously (the dual transport theory)
- Sodium and electrolytes — to replace salts lost through sweat
- Caffeine (in some variants) — typically 25-100mg per gel, roughly equivalent to a quarter to a full cup of coffee
- Water (in isotonic gels) — these are thinner and don’t require you to drink water alongside them
The British Dietetic Association’s sports nutrition guidance confirms that consuming 30-60g of carbohydrate per hour during endurance exercise lasting over 60 minutes improves performance.
How Energy Gels Work: The Science Made Simple
When you run, your muscles burn glycogen for fuel. The harder you run, the faster you burn it. A typical runner has enough stored glycogen for about 90 minutes of steady running — roughly 15-20 miles depending on pace and fitness.
The Dual Transport Theory
Your small intestine has two main types of carbohydrate transporters:
- SGLT1 — absorbs glucose and maltodextrin. Maxes out at about 60g per hour
- GLUT5 — absorbs fructose. Can handle an additional 30g per hour
By consuming a gel that contains both maltodextrin and fructose (a 2:1 ratio is the research consensus), you can absorb up to 90g of carbohydrate per hour — far more than maltodextrin alone. This is why most modern gels use a maltodextrin-fructose blend rather than pure glucose.
How Fast Do They Work?
From swallowing to reaching your muscles, a gel takes roughly 5-15 minutes. This varies based on:
- How empty your stomach is (faster on an emptier stomach)
- Whether it’s an isotonic gel (faster absorption, no water needed)
- Your hydration level (dehydrated = slower absorption)
- How hard you’re running (blood flow diverts away from digestion at high intensities)
This delay is why you need to take gels before you feel you need them, not after. By the time you feel depleted, it’s too late — you’re already bonking, and a gel will take fifteen minutes to help.
When to Use Energy Gels
Not every run needs a gel. In fact, most of your running should be done without them.
You Don’t Need Gels For:
- Runs under 60 minutes — your glycogen stores are sufficient
- Easy recovery runs — low intensity burns more fat and less glycogen
- Parkrun — 25 minutes of running doesn’t deplete glycogen meaningfully, even at race pace
You Should Consider Gels For:
- Runs over 75-90 minutes — this is where glycogen depletion becomes a factor
- Half marathons — most runners take 1-2 gels during a half
- Marathons — typically 4-6 gels depending on pace and personal tolerance
- Training long runs — practise your race-day fuelling strategy in training, not on race day
- High-intensity sessions over 60 minutes — tempo runs, intervals, and race-pace sessions burn glycogen faster than easy running
I made the mistake during my first half marathon of not taking any gels because I thought 13 miles wasn’t far enough to need them. Around mile 10, I could barely lift my feet. I’ve never made that mistake again.
How to Take Energy Gels Properly
There’s a right way and several wrong ways to take a gel. Getting this wrong is the difference between a smooth energy boost and spending the next three miles looking for a toilet.
The Basic Method
- Tear the top off the sachet (practise this — some gel packets are surprisingly hard to open with sweaty hands)
- Squeeze the entire contents into your mouth — don’t sip it slowly
- Chase it with 150-200ml of water (unless it’s an isotonic gel, in which case no water is needed)
- Don’t take a gel with an energy drink — the combined sugar concentration can cause stomach distress
Timing
- First gel: Take at 45-60 minutes into your run (or at mile 5-6 in a half marathon, mile 5-7 in a marathon)
- Subsequent gels: Every 30-45 minutes after the first
- Before the finish: Don’t take a gel in the last 15 minutes — it won’t absorb fast enough to help
The Water Rule
Standard (non-isotonic) gels are concentrated — they draw water into your stomach to dilute them before absorption. If you don’t drink water with them, this water comes from your body, which can cause bloating, cramping, and nausea. Always take standard gels at a water station.
Isotonic gels (like SiS GO Isotonic, Maurten, and HIGH5 Isotonic) are already diluted to the right concentration and don’t need water. This makes them easier on the stomach and more convenient — you don’t have to time them with water stations.
Types of Energy Gel: Thick, Isotonic, and Caffeinated
Standard (Thick) Gels
The original format. Thick, concentrated, require water. Brands: GU, Torq, PowerBar.
- Pros: Small packet, high energy density, cheap
- Cons: Thick texture some people hate, must take with water, can cause stomach issues
Isotonic Gels
Thinner, drinkable consistency, no water needed. The most popular format in UK running right now.
- Pros: No water needed, gentler on the stomach, easy to consume while running
- Cons: Larger packet (more to carry), lower calorie density per ml, slightly more expensive
- Best-known: SiS GO Isotonic (about £1.20-1.50 each from Wiggle or directly from SiS) — this is probably the most commonly used gel at UK races. You’ll see them everywhere
Caffeinated Gels
Available in both standard and isotonic formats, with added caffeine for a mental and physical boost.
- Pros: Proven performance benefit (caffeine reduces perceived effort), useful for the second half of long races
- Cons: Can cause jitters, anxiety, or GI issues in some people. Don’t mix with pre-race coffee without knowing your tolerance
- Typical dose: 75mg caffeine per gel (SiS caffeine gels). Use sparingly — one or two caffeine gels per race, not every gel
Hydrogel Technology
Maurten pioneered hydrogel energy products. The gel forms a gel-like structure in your stomach, allowing the carbohydrates to pass through to your intestine more smoothly. Used by elite athletes including Eliud Kipchoge.
- Pros: Extremely easy on the stomach, no water needed, very high carbohydrate delivery
- Cons: Expensive — about £3-3.50 per gel from Maurten directly. Bland taste (deliberately)
Popular Energy Gel Brands in the UK
SiS (Science in Sport)
British company, headquartered in Nelson, Lancashire. The GO Isotonic range is the UK’s best-selling energy gel. Available everywhere — Wiggle, Amazon, Decathlon, supermarkets, and at most race expos.
- GO Isotonic Gel: ~22g carbs, isotonic, no water needed. About £1-1.50 each
- Beta Fuel Gel: ~40g carbs, dual-source (maltodextrin + fructose), for elite-level carbohydrate intake. About £2.50 each
Maurten
Swedish brand used by top endurance athletes. Premium pricing but exceptional stomach tolerance.
- Gel 100: 25g carbs, hydrogel technology. About £3 each
- Gel 100 Caf 100: Same but with 100mg caffeine. About £3.50 each
HIGH5
Another British brand. Good value, widely available at Decathlon and online.
- Energy Gel: ~23g carbs, standard format. About £0.80-1 each
- Isotonic Gel: ~23g carbs, isotonic. About £1-1.20 each
Torq
British brand focused on natural ingredients. More palatable than most gels.
- Energy Gel: ~30g carbs. About £1.50 each. Uses natural fruit flavours that taste genuinely better than most competitors
GU
American brand, very popular globally. Widely available in UK running shops and online.
- GU Energy Gel: ~22g carbs. About £1.20-1.50 each. Huge flavour range
I’ve tried most of these over the years. SiS GO Isotonic is my regular training gel — it’s cheap, doesn’t need water, and never upsets my stomach. For races, I use Maurten for the first half and add a SiS caffeine gel at mile 18-20 in marathons. Everyone’s stomach is different though — you need to experiment in training.
Energy Gels vs Other Fuelling Options
Gels aren’t the only way to fuel during runs. Here’s how they compare to the alternatives.
Energy Chews (e.g., Clif Bloks, SiS Beta Fuel Chews)
- Easier to portion out — take one or two chews at a time
- Require more chewing, which some people find difficult at race pace
- Slightly slower absorption than gels
- Good option for ultra runners or those who dislike the gel texture
Energy Bars
- More substantial — feel like eating actual food
- Require significant chewing — not ideal at high intensities
- Better for ultra-distance or long hikes where pace is slower
- Our nutrition and hydration guide covers when bars make more sense than gels
Real Food
Some runners fuel with bananas, dates, salted potatoes, rice cakes, or sweets (jelly babies are a classic at UK marathons). These work fine, especially for slower-paced efforts, but they’re less convenient and less precisely dosed than gels.
Sports Drinks
Carbohydrate sports drinks (SiS GO Electrolyte, HIGH5 Energy Drink) provide fuel and hydration simultaneously. Good for shorter efforts or as a supplement alongside gels. Don’t use them with standard gels though — the combined carbohydrate concentration can overwhelm your stomach.
Common Energy Gel Mistakes
Using Them for the First Time on Race Day
The single biggest mistake. Your stomach hasn’t adapted to processing concentrated carbohydrates while running. Train with the exact brand and flavour you’ll use in the race. I know runners who’ve had excellent half-marathon training blocks ruined on race day by a gel flavour they’d never tried — one was fine with orange but discovered the hard way that tropical made them nauseous at mile 9.
Taking Too Many
More is not better. Your gut has a maximum absorption rate. Exceeding 60-90g of carbohydrate per hour will cause bloating, cramping, and potential GI distress. Space your gels out and don’t double up.
Not Drinking Water With Standard Gels
Already covered above, but it bears repeating. Standard gels without water = stomach problems.
Taking Gels With Energy Drinks
Energy drink plus gel means too much sugar hitting your stomach at once. Gel with water. Energy drink on its own. Never together.
Waiting Until You’re Depleted
Take gels proactively, on a schedule. Don’t wait until you feel tired — by then your glycogen is already too low, and it takes 10-15 minutes for the gel to kick in. Set a timer on your GPS watch to remind you.
Training Your Gut: Why Practice Matters
Your gut is trainable. Regular exposure to carbohydrates during exercise improves your ability to absorb and tolerate them. This is why professional athletes can consume 90-120g of carbohydrate per hour without issues — they’ve spent months training their digestive system alongside their muscles.
How to Train Your Gut
- Start by taking one gel during your weekly long run
- For the first few weeks, use it around the 60-minute mark
- Gradually increase to two gels during longer runs — one at 45 minutes, one at 90 minutes
- Over 4-6 weeks, your stomach should adapt and any initial bloating or discomfort should decrease
- In the final weeks before a race, practise your exact race-day fuelling plan during a dress rehearsal long run
Signs Your Gut Is Adapting
- Less bloating after taking gels
- No cramping or nausea
- You can maintain pace after taking a gel without feeling heavy
- Your long runs feel consistently better fuelled
If you’re still having stomach problems after 4-6 weeks of consistent practice, try switching to isotonic gels or Maurten hydrogel — both are much easier on the stomach than standard gels.
How Many Gels to Carry for Different Distances
10K (over 60 minutes)
If your 10K takes over 60 minutes, one gel at the 45-minute mark can help. If you’re finishing in under 50 minutes, you don’t need one. Our beginner’s running guide has more on building up to 10K.
Half Marathon
Most runners take 1-3 gels:
- Sub 1:30 runner: 1 gel at mile 8
- 1:30-2:00 runner: 2 gels — mile 5-6 and mile 10
- Over 2:00 runner: 2-3 gels — mile 4, mile 8, mile 11
Marathon
Plan for 4-6 gels:
- First gel at mile 5-7
- Then every 4-5 miles (roughly every 30-40 minutes)
- Include 1-2 caffeinated gels in the second half (from mile 16 onwards)
- Carry one spare — you might drop one, or need an extra boost
Ultra Marathon (50K+)
Gels alone won’t cut it for ultras. You’ll need a mix of gels, real food, and drinks. Plan for 60-90g of carbs per hour from all sources combined. Many ultra runners find gels become unpalatable after several hours and switch to savoury foods (crisps, sandwiches, soup at aid stations).
Frequently Asked Questions
Do energy gels actually work? Yes — the science is clear. Consuming carbohydrates during exercise lasting over 60 minutes improves endurance performance. Multiple studies, including the British Journal of Sports Medicine consensus, support this. The debate isn’t whether gels work, but which type suits your stomach best.
Can energy gels cause stomach problems? They can, especially if you’re not used to them. Common issues include bloating, nausea, and cramping. These almost always improve with gut training (regularly practising gel intake during training runs). Switching to isotonic gels or hydrogel formulas also helps.
Are energy gels bad for your teeth? The sugar content isn’t great for teeth, particularly if consumed regularly during training. Rinse your mouth with water after taking a gel, and don’t let the residue sit on your teeth. Some runners use a neutral pH mouthwash after long runs.
What’s the best energy gel for beginners? SiS GO Isotonic is the most commonly recommended starter gel in UK running communities. It’s isotonic (no water needed), widely available, relatively cheap, and has a mild flavour. Start with one during your long run and see how your stomach handles it.
Can I make my own energy gels? You can make a homemade version using maltodextrin powder (about £8 per kg from Amazon), water, a pinch of salt, and flavouring. Mix 30g maltodextrin with 40-50ml water for a DIY gel. It won’t taste as good as commercial gels, but it’s much cheaper for high-volume training.