Carb Loading for a Half Marathon: How Much You Actually Need (and When to Skip It)

There are two ways to get carb loading wrong before a half marathon, and most runners pick one of them. The first is doing too much — three days of pasta, a stomach that feels like a kettlebell on the start line, and a kilo or two on the scale that sends you into a quiet panic. The second is doing nothing, or near enough — a normal Saturday, a small dinner, and a wall somewhere after 15km that no amount of grit can run through.

The evidence says most runners land closer to the second. An international study of endurance athletes found that on the day before competition, carbohydrate intakes of just 2.5–7.3g per kilogram were the norm — well under the 8–12g/kg the guidelines call for. People think they’re loading. The plate says otherwise. The half marathon is the distance this goes wrong most often, because runners either borrow the full marathon protocol (too much) or assume the race is too short to bother (too little). The truth sits in between, and it’s simpler than either.

Does a half marathon even need carb loading?

Start with the question almost no carb-loading article asks: do you even need to?

Your muscles and liver hold roughly 500g of carbohydrate — enough for about 75 to 90 minutes of moderate-to-hard running before the tank runs low and your body is forced to lean on fat, which burns too slowly to hold race pace. That moment is the wall. Carb loading doesn’t add a new fuel system; it tops the existing one closer to its ceiling so the wall arrives later, or never. That 90-minute figure is the entire decision.

Distance	Typical time	Load?
parkrun / 5k / 10k	Under ~50 min	No — tank’s already full.
Fast half (sub-90)	75–90 min	Marginal. Normal eating usually covers it.
Most halves	1:45–2:30	Yes — a light, one-day top-up.
Marathon	3 hr+	Yes — the fuller protocol (10–12g/kg).

A frequently cited treadmill study (Sherman and colleagues, 1981) found that loading muscle glycogen higher did not improve half-marathon performance for runners finishing inside about 90 minutes — they simply had fuel left over at the line. If you’re running 1:20, loading is mostly insurance. If you’re running 2:10, you’ll genuinely dip into your reserves, and a modest top-up earns its place. For a 5k or 10k, skip it — you’ll finish with glycogen to spare and only carry the bloat.

How much: your number

For a half marathon, the target is 8–10g of carbohydrate per kilogram of body weight, for a single day. That’s the recreational sweet spot — deliberately lighter than the 10–12g/kg the ACSM recommends for a marathon, because there’s no sense filling stores you won’t empty. Run the maths once and you never have to guess again.

Body weight	8g/kg	10g/kg
55 kg	440g	550g
60 kg	480g	600g
70 kg	560g	700g
80 kg	640g	800g
90 kg	720g	900g

Those numbers look enormous next to a normal day, and that’s the part most people miss: loading is a whole day of carb-focused eating, not one big dinner. A single pasta meal is maybe 150–200g of carbohydrate. A 70kg runner needs three to four times that across the day. You reach it by making carbohydrate the centre of every meal and snack — not by eating more of everything, but by shifting the plate so carbs make up around 70% of what you eat. For the staples that do this cheaply, our breakdown of the best value carbs for runners is a useful shopping list.

When: one day is enough

The old image of carb loading — a week out, a brutal depletion phase of low-carb training to ‘empty the tank’ first, then days of stuffing — is dead. Modern research showed you can reach the same supercompensated glycogen levels from roughly 24 to 36 hours of high-carb eating, with no depletion phase and far less misery. Trained runners respond even faster.

For a half, that means the day before is the day that matters. It lands naturally inside your taper, when training volume has dropped and you’re no longer burning through what you store. Load while you’re still running hard and you’re just topping up a leaking tank; leave it to race morning and you’re too late — a single breakfast can’t replace a day’s worth of glycogen. Done properly, muscle that sits around 150 mmol/kg when well-fed climbs toward 200 — a 30–40% jump in stored fuel. That extra is what carries you through the back third of the race.

What to put on the plate

Two rules: make it fast, and make it boring.

Fast means low-fibre, easily digested carbohydrate — white rice, white pasta, white bread, potatoes with the skin off, ripe bananas, fruit juice, oats, honey, sports drink. The wholegrain virtue you practise the other fifty-one weeks of the year works against you here: fibre fills you up and stirs the gut at exactly the wrong time. Save the brown rice and lentils for the week after.

Boring means nothing new. Race week is not the time for the restaurant you’ve been meaning to try or the gel flavour you saw online. Use foods you’ve eaten before easy runs and know your stomach tolerates. Go lighter on fat and heavy protein at the final dinner, too — both slow digestion and crowd out room for carbs.

Keep protein steady, drink to match

Loading is a shift in ratio, not a free-for-all. You’re moving the plate toward carbohydrate, not piling carbs on top of everything else — keep protein at its normal level (it protects the muscle you’ve trained) and let it ride while carbs take centre stage. Total calories rise a little; they don’t double. This is exactly how a good system handles it: carbs up, protein held, spread across the day rather than dumped into one meal.

Then drink. Every gram of glycogen your body stores is held with about 3 grams of water, which does two things. It’s why the scale jumps 1–2kg overnight — that’s stored fuel and its water, not fat, and it’s gone by the finish line. And it’s why under-drinking quietly caps your load: no water, no storage. Sip through the day and lean on a little extra sodium (sports drink, a salted meal) to help you hold the fluid.

The mistakes that backfire

Most bad race mornings trace back to one of these.

1. Borrowing the marathon protocol

Three days at 10–12g/kg for a half leaves you heavy, bloated, and carrying glycogen you’ll never burn. One day at 8–10g/kg is the dose. More isn’t better — it’s just heavier.

2. The one giant dinner

A heroic bowl of pasta the night before feels like loading and isn’t. It’s a fraction of the day’s target, eaten too late, sitting in your stomach overnight. Spread the carbs from breakfast onward instead.

3. “Healthy” high-fibre food

Wholegrain pasta, brown rice, big salads, legumes — good habits, wrong week. Fibre is the most common cause of a nervous, churning gut on the start line. Go white and simple for the last 24–36 hours.

4. Forgetting the water

Carbs without fluid don’t store well. Load but skip the drinking and you’ve done half the job — you’ll feel flat despite the effort.

5. Panicking at the scale

You will weigh more on race morning. That’s the load working, not a problem. Runners who see the number and try to ‘undo’ it with a fasted morning just arrive at the start underfuelled.

The protocol, start to finish

Loading isn’t fuelling

One last distinction, because the two get confused. Carb loading fills the tank before the gun. Race fuelling — gels, drink, chews during the run — replaces what you burn after it. For a half much over 90 minutes you want both: arrive full, then top up with 30–60g of carbohydrate per hour once you’re past the first half. Loading buys you the start; fuelling buys you the finish. We cover the in-race side in how to fuel a long run, and the everyday baseline that makes loading effortless in how many carbs runners need per day.