Carbs for a marathon: how much to eat and when to start

Eating a carbohydrate-rich diet during the two or three days before a marathon, while reducing training, helps runners increase the glycogen stores they bring to the start line. It does not mean eating an enormous plate of pasta the night before: the daily amount and how it is spread across meals matter more than any single meal.

For a prolonged event such as a 42.195-kilometre marathon, sports nutrition guidelines commonly suggest around 8-12 grams of carbohydrate per kilogram of body weight per day during the final 36-72 hours. This is a guide, not an obligation to reach the top of the range: digestive tolerance, eating habits and medical circumstances matter too.

What happens to your energy stores

Dietary carbohydrates are converted, among other things, into glycogen, a way of storing energy in the muscles and liver. During a race, the body uses both this fuel and fat. The balance changes with intensity, duration, training and energy availability.

Stores are limited, and maintaining pace can become harder when they run very low. The familiar marathon wall does not have a single cause: pacing, preparation, heat and other factors also play a part. Starting with more glycogen may delay fatigue, but it does not guarantee a result or eliminate every risk.

Increasing carbohydrate intake is most useful for prolonged efforts, typically lasting more than 90 minutes. A demanding loading protocol is not usually needed for a short race. For a half marathon, its usefulness depends on the expected duration and the runner’s circumstances; there is no single rule for everyone.

When to start and how much to eat

For a Sunday race, one option is to increase carbohydrate intake from Thursday or Friday and maintain it through Saturday. That period coincides with a reduction in training volume: using less glycogen while eating more carbohydrate makes it easier to increase muscle stores.

To calculate a guide, multiply body weight by the intended grams of carbohydrate per kilogram. For a 60-kilogram person, the range is 480-720 grams a day; for a 70-kilogram person, 560-840 grams; and for an 80-kilogram person, 640-960 grams. These figures refer to carbohydrate itself, not the total weight of the food.

The amount can be surprising: 560 grams of carbohydrate provide about 2,240 kilocalories before protein and fat are included. In practice, such a high intake is not comfortable or appropriate for everyone. Staying closer to the lower end and adjusting meals to personal tolerance may be more sensible than forcing an amount that causes discomfort.

Older protocols involved several days of intense exercise and a very low-carbohydrate diet before loading. Deliberately depleting stores is not necessary: a high-carbohydrate diet combined with a training taper can increase them without adding that restrictive, potentially tiring phase.

Carbohydrate foods that are easier to digest

Rice, pasta, bread, potatoes, cereal, fruit, milk and yoghurt can all feature in pre-race meals. Honey, jam, juice and sports drinks also provide carbohydrate and can help when eating a large volume is difficult. No single product is essential; what matters is the total amount and individual tolerance.

Figures vary by preparation and brand, but offer a useful guide: 100 grams of white bread provide about 50 grams of carbohydrate; 300 grams of cooked white pasta, around 90; and a medium or large banana, roughly 25. On packaged products, the label shows the actual amount per serving.

Pasta and rice change weight when cooked because they absorb water. That means 100 grams dry is not the same as 100 grams cooked. When comparing a recipe with a label, check which weight each figure refers to: it prevents large errors in estimating intake.

Temporarily reducing fibre may help runners who often experience gastrointestinal discomfort. White bread, rice or pasta, peeled fruit and fruit purée are common options in the final 24-48 hours. There is no need to eliminate all vegetables or ban wholegrains for someone who tolerates them well; outside this context, fibre remains a valuable part of the diet.

A sample day to put the amounts in perspective

For a 70-kilogram person, the lower end of the range is about 560 grams a day. The following example shows one way to spread that amount; it is not an individual diet or a menu that must be repeated unchanged. Figures are approximate and vary with foods, brands and preparation.

At breakfast, 100 grams of white bread, 30 grams of jam, a banana and 250 millilitres of juice can provide around 118 grams of carbohydrate. Mid-morning, another 100 grams of bread with a serving of jam can add about 72 grams. This is a considerable amount of food, which is one reason to test the distribution in advance.

At lunch, 300 grams of cooked pasta and 60 grams of bread would provide approximately 120 grams. An afternoon snack of 500 millilitres of sports drink containing 30 grams of carbohydrate, a banana and 250 millilitres of juice would add another 80. Check the drink label: concentrations differ.

For dinner, 300 grams of cooked white rice and 60 grams of bread provide approximately 114 grams. A snack of 60 grams of bread with 20 grams of jam and 20 grams of honey adds close to 58. The total is around 560 grams, allowing for normal variation; protein foods and other well-tolerated accompaniments can also be included, even though they are not part of this carbohydrate calculation.

The example shows a distribution, not a requirement to consume juice, honey or sports drinks. Potatoes, cereal, fruit purée and gluten-free alternatives can also be used. Portions should reflect appetite and tolerance, without completely displacing protein, fat and other nutrients or turning a guideline into an obligation.

The day before: familiar food, not experiments

The evening meal can be simple and familiar: rice or pasta with an accompaniment the runner knows and a moderate amount of fat. There is no universal dinner time, but leaving enough time before bed may feel more comfortable than eating a large meal very late. The work of filling stores has already been spread over the preceding days.

A frequent cause of discomfort is changing too many things at once: introducing a very fatty sauce, large amounts of fibre or an unfamiliar supplement. There is no need to automatically ban every seasoned food either; a runner’s own digestive experience matters more than a rigid list of forbidden foods.

Body weight may rise during these days because glycogen is stored alongside water and because digestive contents change. A weight increase alone does not prove loading has worked, nor can the entire change be attributed to water. It is not a reason to sharply restrict food or drink before competing.

Hydration without overdrinking

Glycogen storage is associated with water, but that does not justify setting every runner a fixed target of three or four litres a day. Needs vary with body size, weather, activity and the water obtained from food. A sensible approach is to drink regularly and pay attention to thirst and environmental conditions.

On a hot race day, sweating can increase and make a practised fluid strategy more important. Conversely, drinking large amounts out of obligation, particularly all at once, can dilute blood sodium and contribute to hyponatraemia, a potentially serious condition. Adding salt to water indiscriminately is unnecessary.

Sports drinks can supply fluid and carbohydrate, as well as sodium in some formulations, but their composition varies. Account for what they provide without using them as a reason to drink beyond thirst. Very dark urine can signal insufficient fluid; aiming for completely clear urine is not a reliable goal.

Race-day breakfast

A carbohydrate-rich meal one to four hours before the start can support energy availability, including the replacement of some liver glycogen used overnight. Many runners prefer breakfast two or three hours beforehand, but the timing depends on the start time and what they have tolerated in training.

As a general guide for the pre-exercise meal, recommendations range from 1 to 4 grams of carbohydrate per kilogram of body weight within that one-to-four-hour window. This is not an invitation to eat the maximum immediately before the start: the less time available, the more important it is to match the amount to digestion and individual experience.

White bread with jam, a banana and a familiar drink is one possible combination; rice or fruit purée can also work. For someone who eats little on waking, a smaller serving may be more comfortable. Coffee can remain part of the routine if it is already familiar, but starting caffeine or greatly increasing the dose on race day introduces an unnecessary variable.

Fuel for the 42 kilometres

Starting with good stores does not replace fuel during the race. For prolonged endurance efforts, runners commonly use 30-60 grams of carbohydrate per hour; in events lasting more than two and a half hours, some trained runners tolerate up to 90 grams an hour. The appropriate amount depends on duration and what the digestive system has learned to accept.

Gels, drinks and foods provide different amounts. A gel containing 25 grams of carbohydrate, for example, provides 50 grams if two are taken per hour, provided the figure refers to carbohydrate rather than the weight of the packet. Sports drinks also count towards the total. At higher intakes, products containing different carbohydrate types, such as glucose and fructose, may help the body use them.

The approach should be practised on long runs, starting with tolerable amounts and adjusting gradually. Not everyone needs 90 grams an hour, and gels are not all taken in the same way: some require water, while others are formulated to be consumed without it. Package directions and previous experience are more useful than copying someone else’s routine.

When to adapt the approach and seek medical guidance

People with diabetes, kidney disease, digestive disorders, a history of eating disorders or another condition affecting diet need an adapted approach. In these cases, a very high carbohydrate intake or changes to fluids and medication should not be improvised: they need to be assessed with healthcare professionals.

Coeliac disease does not prevent an increase in carbohydrates. Rice, potatoes, fruit and dedicated gluten-free products are possible options; for oats, it is important to choose certified gluten-free varieties when appropriate. The same applies to other dietary restrictions: the goal can be reached through different combinations without relying on wheat pasta.

Pre-race eating should not be confused with a last-minute attempt to change race weight. Good preparation aims to bring the runner to the start with energy and without discomfort, not to compensate for food eaten by restricting intake. Eating enough throughout training matters more than treating the final days as a solution in isolation.

A strategy that begins long before the start

The potential benefit of filling stores makes most sense alongside the rest of preparation: training, rest, pacing, race-day fuel and weather conditions. No menu can prevent fatigue on its own or correct a start that is too fast. Carb loading is one piece of the whole, not a guarantee of finishing at a particular pace.

Practising meals and drinks in the preceding weeks helps runners identify practical amounts and foods they tolerate. With that information, the final days can be simpler: carbohydrate spread across meals, reduced training, hydration without excess and a familiar routine. Arriving at the start with well-supported energy stores also means arriving with a prepared stomach.

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