Carb loading for a marathon: the science of fuelling up

Carb loading for a marathon means eating extra carbohydrate in the final one to three days, while easing off your running, to top up muscle glycogen so you fade later in the race. Run a marathon and somewhere around 30 to 35 kilometres your legs will start writing cheques your fuel tank cannot cash. That moment, the dreaded “wall”, is in large part a glycogen problem. Your muscles run low on their stored carbohydrate, and without it your pace falls off a cliff. Topping that tank up to the brim before the gun goes is what pushes the wall later, or stops it arriving at all.

The good news is that the modern protocol is far simpler than the gruelling depletion regimes of the 1970s. This article explains what glycogen loading actually does, the one to three day method that has replaced the old approach, how many carbohydrates to aim for, why the scales go up, and exactly who should bother.

What carb loading actually does

Your body stores carbohydrate as glycogen in muscle and liver. During prolonged hard exercise, muscle glycogen is a primary fuel, and when it runs out you are forced to slow down. The foundational work here is Bergström and colleagues (1967), who used muscle biopsies to show that the amount of glycogen in the working muscle is a direct determinant of how long you can sustain heavy exercise. In their study, time to exhaustion roughly tripled, from around 59 minutes to 189 minutes, as starting glycogen rose with a carbohydrate rich diet.

Carb loading exploits a quirk of physiology called supercompensation. When the muscle has been emptied of glycogen and is then flooded with carbohydrate, it does not simply refill to baseline. It overshoots, packing in more glycogen than it normally holds. That elevated starting store is what buys you extra kilometres before fatigue.

You no longer need the depletion phase

The classic protocol, descended from that early Scandinavian research, was brutal. You ran yourself into the ground a week out, then ate almost no carbohydrate for three days to dig the glycogen hole as deep as possible, before three days of heavy carbohydrate intake to supercompensate. Runners hated it: the low carbohydrate days left you flat, irritable and prone to illness and injury right before the most important race of the season.

Sherman and colleagues (1981) demonstrated that the painful depletion phase was largely unnecessary. They compared exercise and diet regimens and found that a modified approach, tapering training while progressively raising carbohydrate intake, produced muscle glycogen levels just as high as the full depletion routine, without the misery of the carbohydrate starvation days. This modified taper and load became the standard advice for a generation.

Then it got even easier. Bussau and colleagues (2002) showed that trained athletes could reach maximal muscle glycogen in just one day. By combining physical rest with a high carbohydrate intake of about 10 g/kg, muscle glycogen jumped from a baseline of 95 to 180 mmol/kg within 24 hours and then plateaued, holding steady even with two more days of heavy carbohydrate. In other words, for most runners a single dedicated day of rest and carbohydrate is enough to fill the tank.

How many carbs: aim for 8 to 12 g/kg per day

The consensus target comes from the major sports nutrition reviews. Burke and colleagues (2011), summarising the evidence for the International Olympic Committee, recommend roughly 10 to 12 g/kg of body mass per day over the 36 to 48 hours before an event lasting longer than about 90 minutes. Many practitioners use 8 to 12 g/kg as a workable range.

Those numbers are higher than they sound. For a 70 kg runner, 10 g/kg is 700 grams of carbohydrate a day, the equivalent of a great deal of rice, pasta, bread, fruit and sports drink. Hitting it usually means deliberately swapping fat and fibre off the plate to make room, and topping up with carbohydrate rich drinks rather than relying on bulky meals alone. This is fuel loading, not a free pass to overeat: keep total energy roughly sensible and concentrate on the carbohydrate.

One important caveat concerns sex differences. Tarnopolsky and colleagues (2001) found that female athletes did not supercompensate as readily as men when carbohydrate was raised as a percentage of intake, but they did respond when total energy and absolute carbohydrate were increased together. The practical message for women is to focus on absolute grams of carbohydrate and adequate total energy, not just the percentage of the plate.

The water weight tradeoff

Do not be alarmed when the scales jump in the final days. Glycogen is stored with water, and the relationship is well quantified. Olsson and Saltin (1970) measured it directly: as subjects loaded glycogen, total body water rose in step, working out to roughly 3 to 4 grams of water bound per gram of glycogen. In their study a successful load added about 2.4 kilograms of body mass, of which 2.2 litres was water.

For a runner, that means a good carb load typically adds one to two kilograms over a couple of days. It is tempting to read that as bloat, but it is overwhelmingly stored fuel plus its accompanying water, not fat, and the extra water is genuinely useful. It helps buffer the dehydration of a long, hot race. The small weight penalty is paid back many times over by the energy it represents. Do not panic, and do not try to “cut” it before the start.

Who actually needs to carb load?

This is where a lot of well meaning effort is wasted. Carb loading only helps when glycogen depletion is the thing limiting you, which means events lasting longer than about 90 minutes. Hawley and colleagues (1997), reviewing the literature, concluded that elevating muscle glycogen above normal postpones fatigue by around 20 percent in events longer than 90 minutes, but provides little or no benefit for high intensity efforts under 5 minutes, or for moderate running of 60 to 90 minutes.

So a marathon (and almost any half marathon run hard) sits firmly in carb loading territory. A parkrun 5K, a track 10K or a brisk 10 mile race generally do not: your stores are not the bottleneck, so loading just adds water weight without improving your time. If your race is short, skip the pasta mountain and simply eat normally.

Loading is also only half of race day fuelling. Even a fully loaded tank will not see you through a marathon on its own, which is why taking carbohydrate during the race matters just as much. We cover the targets and timing in how many carbs per hour you need when running.

Practical meal guidance for the final days

The principles are simple, and the execution is mostly about avoiding self inflicted mistakes.

Keep fat and protein modest so carbohydrate has room on the plate, and keep drinking fluids: you are storing water alongside the glycogen, so steady hydration supports the process. The night before meal should be familiar, carbohydrate rich and low in fat and fibre. Pasta with a light tomato sauce, a rice dish or a baked potato all qualify. Above all, do not experiment. Race week is not the time to try a new restaurant, a novel gel or a cuisine your stomach has never met.

Finally, slot the load into your taper rather than treating it as a separate event. Your training volume is already dropping in the final week, which naturally helps glycogen accumulate. If you want the full picture of how to wind down without losing fitness, see our guide to how to taper for a race. And if you are weighing up other pre race supplements, our look at creatine for runners covers another compound that, like glycogen, draws water into muscle.

Frequently asked questions

Related reading: how many carbs per hour you need when running.