Do You Need to Fuel During a Half Marathon? A Science-Based Answer

The honest answer: it depends on how long you'll be out there.

A runner finishing in 75 minutes is doing a fundamentally different metabolic thing than someone crossing at 2:15. The fast runner is basically sprinting on a full tank. The 2:15 runner is pushing deep into glycogen depletion territory, and if they didn't bring a gel or two, they're going to feel it around mile 10.

Most half marathon advice lands in one of two camps: "you don't need anything, it's only 13 miles" or "fuel it like a marathon." Both miss the mark for most people. Your finish time is the answer.

The glycogen math

Your muscles store glycogen, which is basically carbs packed away for later. How much depends on your size, sex, and how well you ate in the days before the race. A trained athlete in the 70-80kg range stores roughly 1,800-2,400 calories' worth. A 55kg woman is closer to 1,500. Burke's research on carb loading established these ranges and they've held up well.

Here's the thing about half marathon intensity: you're working hard. Somewhere around 80-85% of VO2max for most runners, which means your body is pulling predominantly from carbs, not fat. At that effort, Hargreaves and Spriet's review puts carbohydrate oxidation at around 2-3g per minute. Do the math over 90-120 minutes and you're chewing through a massive chunk of your glycogen.

Rapoport's modeling puts it even more bluntly: without exogenous carbs, males deplete glycogen in roughly 84 minutes and females in about 71 minutes at race intensity. Those numbers aren't exact for every runner, and they shift with fitness, carb loading, and body size, but they tell you something important. If your half marathon takes longer than about 90 minutes, you're running into the red zone. And if you're a smaller or female athlete, that window is probably shorter.

So the question isn't really "do I need fuel?" It's "how long will I be running?"

Where you fall: fueling by finish time

Under 1:30

You're probably fine. Your glycogen stores can handle 90 minutes at race effort, especially if you ate well the day before and had a solid pre-race breakfast. Water at a station or two is plenty. Some runners like a caffeine gel around mile 8-9 for the mental lift (3-6mg/kg is one of the most proven performance boosters in sports science), but you don't need to eat.

1:30 to 2:00

The gray zone. You'll probably finish without fuel, but you might not finish well. This is where a lot of runners fade in the last 5K and chalk it up to fitness when it was actually glycogen. One or two gels at mile 4-5 and mile 8-9 is cheap insurance that costs you nothing and can make the difference between a strong close and a death march from mile 11.

Worth noting: if you're a lighter athlete, you're storing less glycogen and burning through it faster relative to your body's total supply. That 71-minute depletion figure from the modeling data? It was based on female runners. So if you're a 55kg woman running 1:45, you're not in the "probably fine" camp. You're in the "bring a gel" camp.

2:00 to 2:30

Yes, you need fuel. You're firmly past the glycogen window for most body sizes at this point. Plan on 1-3 gels spread across the race and take them with water, not sports drink. Jeukendrup's recommendations for events in this duration range are 30-60g of carbs per hour. You don't need Ironman-level fueling, but you do need something.

Over 2:30

Treat this like a scaled-down marathon. Carbs every 30-40 minutes starting around mile 3. Bring your own fuel instead of relying on whatever the aid stations have. The marathon nutrition guide has the full playbook for longer efforts, and most of it applies here.

Need your exact race-day numbers? Build a personalized carb, fluid, and sodium plan in about 60 seconds: Use the free EnduranceOS calculator.

What bonking actually feels like at mile 10

Marathon bonking is dramatic. You've seen the videos of runners wobbling, legs giving out, the whole catastrophe. Half marathon bonking is sneakier.

You don't collapse. You just... can't hold pace anymore. Your legs feel like they've shifted into a lower gear and no amount of willpower brings the speed back. Splits drift 30-60 seconds slower per mile. You're still running, still conscious, still moving forward, just slower than you trained to be, which honestly might be more frustrating than a full-body shutdown because you feel like you should be able to push through it.

The cruel part? A single gel at mile 5 would've prevented the whole thing.

Bonking feels like something that only happens in marathons and Ironmans. But at half marathon intensity, you're burning carbs faster per minute than in those longer races because the effort is higher. You just have fewer total minutes to burn through your supply. The margin is thinner than people realize.

A simple plan (that you'll actually follow)

Pre-race breakfast

This matters more for a half than a marathon because you don't have as many hours to fuel during the race itself. Aim for 1-4g of carbs per kg about 2-3 hours before the start. A bagel with peanut butter and a banana. Oatmeal with honey. Whatever you've practiced with in training. The goal is topping off your glycogen without sitting heavy in your stomach.

During the race

If you're in the 1:30-2:00 range: One gel at mile 5-6 with water. That's about 25g of carbs, enough to top off the tank without overcomplicating anything. Carrying a single gel isn't rocket appliances, and the downside of having it and not needing it is basically zero.

If you're in the 2:00-2:30 range: Gel at mile 4-5, second gel at mile 8-9, water at every other aid station. That gives you roughly 30-40g of carbs per hour, which lines up with Jeukendrup's recommendations for this duration. Swap the second gel for chews if you want variety.

If you're over 2:30: Gel every 30-40 minutes starting at mile 3-4, water at most stations, and consider a pre-race sodium boost if it's going to be warm. The sweat rate article has a DIY protocol you can knock out in an hour if you want your actual numbers.

Sodium

Less critical than in a marathon or Ironman, but not zero, especially in heat. Most gels have 40-80mg of sodium, which probably covers you for a sub-2-hour race on a cool day. Racing in summer heat? A SaltStick cap before the start and one at mile 6 is easy insurance.

The "it's only 13 miles" trap

This is the main reason people under-fuel halfs. Thirteen miles doesn't feel like it needs a nutrition plan. You don't carry bottles on a 10-miler, so why would a half be different?

Because the intensity changes everything. A half marathon isn't just three more miles tacked onto a 10-miler. You're racing at a sustained effort that torches carbs far faster than a training run, and you're doing it for long enough that the math catches up with most runners.

Here's something interesting: even when total glycogen isn't the limiting factor, your brain still responds to fuel. Chambers et al. (2009) showed that cyclists who swished a carbohydrate solution in their mouths, without even swallowing, completed a time trial roughly 2% faster. Brain imaging revealed that the carbs activated reward and motor control centers. A systematic review confirmed the effect across multiple studies. Your brain senses fuel is available, and it lets you push harder.

So even for the sub-90 crowd who technically have enough glycogen, there's a plausible benefit to taking in a gel. You don't have to. But for the weight of a single GU packet, you might run faster.

Your numbers, not averages

The brackets above work as general guidance. But your specific needs shift based on your body weight, pace, sex, the weather on race morning, and how well you carb-loaded. A 130lb woman racing in July heat needs a different plan than a 185lb guy running a cool October morning.

The EnduranceOS planner takes your race details and builds a personalized fueling plan with carb, fluid, and sodium targets matched to your body and conditions. Takes about 60 seconds, and it's free.