There is a common misconception in the running world that I used to believe myself: You don’t need to fuel for a half-marathon. I used to think that fuelling was a “marathon-only” problem and that for 13.1 miles, you could just tough it out. Due to that thinking, I entered my half-marathon without a fuelling plan and hit the wall at mile 8.
However, our biology tells us the exact opposite, especially if you are a mid- to back-of-the-pack runner. If you want to finish strong rather than “fading” and slogging through those final miles, you need a strategy. This guide is your definitive, step-by-step plan to nailing your half-marathon fuelling, combining hard science with the “tough love” coaching you need to get to the finish line with a smile (and a PR).
The Science of 13.1: Why You Need a Strategy
Why is the half-marathon so physically demanding? It’s because of the intensity.
As recreational runners, we typically work at between 72% to 86% of our VO2 max during a half-marathon. This is a critical data point because, at this level of effort, your body is burning significantly more carbohydrates than fat. In fact, research shows this intensity sits well above the “Fat-Max” zone (the 47-75% range where your body is efficient at burning fat for fuel).
The 90-Minute Reality Check: Even if you start with “full” glycogen stores—the way your body stores carbohydrates in the muscles and liver—you have a finite tank. That tank generally holds enough energy for roughly 90 minutes of running.
- If you are aiming for a finish time of 1:45, 2:00, or 2:30, you will likely run out of fuel before the finish line.
- This is where you “hit the wall.” Your body tries to switch to burning fat, but because you are running at a high intensity, fat is too slow to process. This leads to that lead-like feeling in your legs and the mental “fog” that makes the final miles feel like an absolute slog.
The Half-Marathon Fuelling Formula
Everywhere you look, there are calculators for pace, finish times, and carb-loading. But there really is no calculator that can tell you how much fuel to take during the race itself. That is because mid-race fuelling is dictated by human biology, not your body weight.
The Intestine Limit: SGLT1 vs. GLUT5
When we consume sugars—whether it’s glucose, fructose, or maltodextrin—our intestines have a limited “processing speed.” Think of it like a supermarket with two checkout lanes:
- Lane 1 (The SGLT1 Transporter): This handles Glucose and Maltodextrin. It processes about 60g per hour1.
- Lane 2 (The GLUT5 Transporter): This handles Fructose. It processes about 30g per hour1.
This gives us a biological ceiling of 90g of carbohydrates per hour. While there is research into elite runners pushing toward 120g, for us recreational runners, the goal is simpler and safer: Aim for 30–60g of carbohydrates per hour for anything under 2.5 hours.
Doing the Math for Your Race
You must check the grams of carbs per sachet, not just the brand.
- High5 Gels: Contain ~23g carbs. If you take one every 30 minutes, you hit 46g/hr.
- Kendall Mint Co: Contains ~27g carbs. One every 30 mins = 54g/hr.
- Precision Fuel / Maurten: These often sit at 30g per sachet.
The Manual Formula: To hit that 30-60g target, you are generally looking at one gel every 30 minutes. I always recommend aiming for the higher end of the scale, especially if you’ll be out there for over two hours. Don’t try to “get away with the minimum.”
The Carb Load: Saturating the Tank
Carbohydrate loading is a technique used in the 36-48 hours before an event to “supercompensate” your glycogen stores.
- The Target: Aim for 8g to 12g of carbohydrates per kilogram of body weight.
- The Female Factor: Research shows that female athletes often need to increase their overall caloric intake by about 30% during the carb-load. Why? Because it is incredibly difficult to hit the required 8-12g of carbs per kg of body weight within your usual daily calories. Don’t be afraid of the extra energy; it’s what allows you to actually reach the fuel levels your muscles need.2
Read more on carb-loading here
Tough Love: Weight Loss vs. Race Week
I have been on the weight loss journey, and I know how terrifying it feels to see your calories go up. But I’m going to give you some tough love: Race week is NOT the time for a calorie deficit.
I learned this the hard way. I thought the only way to get faster was to be lighter, so I under-fuelled my first half-marathon. I hit the wall at Mile 8, and the rest of the race was miserable. In my next event, I ignored the scale, followed this strategy, and set a 10-minute personal best. The Scale Scare: When you carb load, the scales will go up. This is not fat. For every gram of glycogen stored, your body stores about 3 grams of water. This is a temporary gain that is actually a massive advantage—it means you are hydrated and fuelled. If the number on the scale stresses you out, put the scales out of sight. Don’t stand on them. They aren’t the most important thing this week; your performance is.
Want support with your training and nutrition/fuelling? Then check out my coaching program here
Managing a “Sensitive Runner’s Stomach”
When you run, your body diverts blood away from your gut and digestive tract to your working legs. This slows down digestion and can lead to nausea or the “Runner’s Trots.”
Training Your Gut
Your gut is a muscle. You wouldn’t run 13.1 miles without training your legs; you shouldn’t expect your stomach to handle 60g of carbs without practice.
- Practice in Training: Use your long runs to test different sugars. Some people handle Maltodextrin better; others prefer a “Dual-Fuel” (Glucose + Fructose) mix, which uses both “checkout lanes” in the gut and prevents a “backlog” of sugar that causes sickness.
- The Water Rule: Never take a gel without a sip of water. If you put concentrated sugar into your gut without extra fluid, it can actually pull water out of your bloodstream, increasing dehydration and GI distress.
Your Race Day Timeline
- 2–4 Hours Before: Take on 1–4g of carbs per kg of body weight. If you’re eating 4 hours out, go higher (4g). If it’s closer to the start, keep it light (1g).
- Hydration Hook: Aim for 5-10ml of fluid per kg. Tip: Use orange juice! It gives you liquid carbs and hydration at the same time—killing two birds with one stone.
- During the Race: Start your first gel 30 minutes in. Then take one every 30 minutes like clockwork.
- Safety Net: Always carry an extra gel or two. You might drop one, or the aid station might not have the brand you’ve practised with. Don’t leave it to chance.
The Caffeine Side-Note
Caffeine is a proven performance enhancer because it reduces your “perceived exertion”—basically, it makes the hard work feel a little easier.
- The Strategy: Take 3-9mg per kg of body weight before the race. Then, during the race, aim for 50mg per hour.
- Timing: Caffeine peaks in the blood about 30-60 minutes after you take it. If you know you’ll struggle in the second half, time your caffeine gel for the 60 or 90-minute mark. * Warning: Practice this! Caffeine can trigger GI issues for some, so don’t try it for the first time on race day.
Hydration & Electrolytes
For a half-marathon, the science is clear: Drink to thirst. Research shows that drinking to thirst is sufficient to avoid both dehydration and exercise-associated hyponatremia (EAH).
- The Formula: If you want to be precise, calculate your sweat rate. Weigh yourself (naked) before a 60-minute run. Weigh yourself after (again naked and towel dried). The difference in grams is your sweat rate in ml/hr, taking into account fluid taken on during the run.
- Sodium: Aim for 400-800mg of sodium per hour if you are a “salty sweater.” Most gels already have electrolytes added to help with this.
If you do intend to track your sweat rate, then remember to repeat this process multiple times in different temperatures and weather conditions.
The Recovery Window
You aren’t done when you cross the finish line. To stop the “breakdown” state and help your muscles repair:
- 0–2 Hours After: Hit 0.3g of protein per kg.
- The Refuel: Aim for 1–1.2g of carbs per kg in the 6 hours following the finish.
- Rehydration: Drink 150% of the fluid weight you lost. If you haven’t monitored it, just drink to thirst and include electrolytes.
FAQ: The Half-Marathon Fuelling Deep Dive
Do I really need to fuel for a 1:45 half-marathon? Yes. Fueling is about more than just “finishing.” It’s about performance. Studies show that runners who follow guidelines finish nearly 5% faster than those who fuel freely.
What if I can’t stomach gels? There are plenty of alternatives like chews, carbohydrate drinks, or even diluted orange juice. The key is to find the 2:1 glucose-to-fructose ratio that works for your gut through practice.
Will carb-loading make me “heavy”? Yes, temporarily. But as we discussed, it is water weight that will be burned and used for hydration as you race. It’s a tool, not a setback. Should I use the race-course fuel? Only if you have practised with it! Check the race website to see the gel provider, buy a few, and test them. If you haven’t tested them, carry your own.
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Articles Mentioned
1Jeukendrup, A., 2013. Multiple Transportable Carbohydrates and Their Benefits. Sports Science Exchange, [e-journal] 26 (108), pp.1–5. <https://www.gssiweb.org/docs/default-source/sse-docs/sse-108-jeukendrup.pdf?sfvrsn=2>
2Wismann, J. and Willoughby, D., 2006. Gender Differences in Carbohydrate Metabolism and Carbohydrate Loading. Journal of the International Society of Sports Nutrition, [e-journal] 3 (1), pp.28. 10.1186/1550-2783-3-1-28. <https://doi.org/10.1186/1550-2783-3-1-28>
Jiménez-Alfageme, R., Garrone, F.P., Rodriguez-Sanchez, N., Romero-García, D., Sospedra, I., Giménez-Monzó, D., Ayala-Guzmán, C.I. and Martínez-Sanz, J.M., 2025. Nutritional Intake and Timing of Marathon Runners: Influence of Athlete’s Characteristics and Fueling Practices on Finishing Time. Sports Medicine – Open, [e-journal] 11 (1), pp.26. 10.1186/s40798-024-00801-w. <https://doi.org/10.1186/s40798-024-00801-w>
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