Carb Loading Before a Marathon: Expert Fuel Guide

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RunFuel quick guide

Increase carbohydrate deliberately during the final 36–48 hours while training tapers.
Spread the increase across regular meals, snacks and drinks instead of one large dinner.
A full performance protocol commonly uses 10–12 g/kg each day, but tolerance and individual needs matter.
Carb loading, race-morning breakfast and in-race fuelling each have a separate job.

Carb loading means deliberately increasing your carbohydrate intake during the final 36–48 hours before a marathon, while your training tapers. Spreading that increase across regular meals, snacks and drinks is usually more practical than one large dinner.

A full performance protocol typically provides 10–12 g of carbohydrate per kilogram of body weight each day. For a 70 kg runner, that is 700–840 g per day, so it takes planning and may benefit from individual guidance.

Carb loading prepares the fuel stored in your muscles. Your race-morning breakfast and in-race fuelling plan still have their own jobs to do.

When	Main focus
48 hours before	Begin deliberately increasing carbohydrate across the day
24 hours before	Use regular meals, manageable snacks and lower-volume options
Evening before	Eat a familiar carbohydrate-led dinner relatively early
Race morning	Eat your practised breakfast to replenish liver glycogen
During the marathon	Continue with the carbohydrate plan tested in training

How does carb loading help before a marathon?

Carbohydrate is stored in the muscles and liver as glycogen. During a marathon, working muscles draw heavily on that limited supply. Starting with more muscle glycogen can help delay fatigue, although pacing, fitness, heat, hydration and race fuel still shape how the marathon unfolds.

Older protocols used hard glycogen-depleting exercise and low-carbohydrate eating before the loading phase. That depletion stage is unnecessary. Modern guidance combines reduced training with high-carbohydrate eating over the final 36–48 hours.

When should you start carb loading?

Most of the deliberate increase belongs in the final two days, once training has eased. Starting early enough lets you spread food across regular meals and snacks instead of leaving the whole job to Saturday dinner.

The final 36–48 hours

For a Sunday marathon, begin increasing carbohydrate on Friday morning or lunchtime and continue through Saturday. Your training load should already be lower because of the taper, allowing more of what you eat to be stored as glycogen.

Prepare earlier by checking food availability. Before one or two key long runs, test the foods, meal pattern and temporary fibre adjustments you intend to use. You rarely need to reproduce the full calculated target for an ordinary long run.

Is one pasta dinner enough?

One dinner is not a complete carb load. Muscle glycogen builds through repeated carbohydrate intake, so distribution across two days is more effective and usually more comfortable than asking one late meal to do everything.

An enormous pasta dinner can leave you full, refluxy and awake when you need to sleep. Eat a useful lunch, add snacks and make dinner familiar rather than theatrical.

How much carbohydrate do you need before a marathon?

The calculation shows the scale of a full carb load, but cannot tell you what your stomach will tolerate. Use it as a planning tool, not a target to reach at any cost.

Understanding the guideline

The calculation uses 10–12 g of carbohydrate for every kilogram of body weight on each of the final two days. This guidance is designed to maximise glycogen availability in trained athletes. Suitability still depends on your training status, usual intake, appetite and gut tolerance.

A worked calculation

70 kg × 10 g = 700 g carbohydrate per day
70 kg × 12 g = 840 g carbohydrate per day

Seven hundred grams of carbohydrate does not mean 700 g of pasta or 700 g of food. It means the carbohydrate contained across everything eaten and drunk that day. For example, 300 g of cooked pasta generally supplies around 85–95 g of carbohydrate, depending on the product and preparation.

What if the target feels unrealistic?

Body-weight equations are broad sports-nutrition tools. If the calculated amount feels extreme for your appetite, body size, usual intake or health needs, do not force food simply to meet it. Start with a tolerable increase, spread food across the day and use lower-volume options such as white rice, bread, bagels or cereal where these suit you.

The higher numbers can be difficult for runners with a smaller appetite, a lower usual carbohydrate intake or gastrointestinal symptoms. A qualified sports dietitian or nutritionist can adapt the plan without turning the final two days into an eating challenge.

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A practical 48-hour marathon carb-loading plan

Decide what you will eat before race week, then organise the final two days around regular meals and snacks. The outline shows where extra carbohydrate can fit, with an optional example for a high target.

Two days before the marathon

Begin at breakfast or lunch rather than waiting until the evening. Add a clear carbohydrate food to each meal, use two or three snacks and make drinks contribute where that has worked in training.

If bulky food tends to leave you uncomfortable, begin shifting towards lower-fibre choices you already tolerate. This is a short race-week adjustment, not a judgement on wholegrains, vegetables or pulses as everyday foods.

The day before the marathon

Spread intake across breakfast, lunch, dinner and several snacks. A substantial lunch prevents dinner from carrying most of the day's target. Eat dinner relatively early, then prepare race breakfast and backup food before winding down.

Keep the day calm. A large amount of walking around an expo or unfamiliar city still uses energy and can make regular eating harder to organise.

How a 700 g target might be distributed

For a 70 kg runner, 700 g represents the lower end of the 10–12 g/kg/day range. This only demonstrates how that amount could be divided and is not a prescribed menu.

Eating occasion	Illustrative carbohydrate share
Breakfast	Approximately 140 g
Morning snack	Approximately 85 g
Lunch	Approximately 145 g
Afternoon snack	Approximately 85 g
Dinner	Approximately 155 g
Evening snack	Approximately 90 g
Daily total	Approximately 700 g

The purpose is to show why one large dinner cannot do the whole job. Build the actual day from foods and serving sizes that suit you, using packet labels where you want a closer estimate.

Carbohydrate-rich foods for your race-week plan

Choose carbohydrate-rich foods that you find easy to eat and digest. Everyday staples work well, while lower-fibre versions can make a high intake less bulky during this short period.

Food and serving	Approximate carbohydrate
300 g cooked white rice	80–90 g
300 g cooked white pasta	85–95 g
One plain bagel	45–55 g
Two crumpets	35–40 g
Three plain rice cakes	20–25 g
Two slices of malt loaf	25–35 g
One medium or large banana	20–30 g
150 ml fruit juice	12–15 g
500 ml sports drink	25–35 g

These are broad examples, so check the packet when a closer estimate matters. Jam, honey, syrup and lower-fat sauces can add carbohydrate without much volume. Juice or sports drink may help when more solid food feels difficult, although sweet drinks can trouble some stomachs.

General NHS guidance limits juice and smoothies to 150 ml a day and recommends having them with a meal. During a short loading period, keep sweet drinks around meals where practical, rinse with water afterwards and use solid foods for much of the plan.

Pasta is optional. Rice, potatoes, bread, noodles, cereal, crumpets and fruit can build the same plan. If you need gluten-free food, use rice, potatoes and rice noodles alongside certified gluten-free bread, bagels or cereal, checking labels for your needs.

Read carbs for runners for carbohydrate across normal training rather than race-week loading alone.

What to eat the night before a marathon

By dinner, most of the day's carbohydrate should already be in place. Choose a meal built around a generous carbohydrate serving, moderate protein and enough sauce or seasoning to make it enjoyable.

That might be pasta with tomato sauce and chicken or tofu; white rice with curry; noodles with eggs or prawns; or a large jacket potato with tuna or cottage cheese, plus bread and a carbohydrate snack or dessert. Use portions and ingredients tested before long runs.

Eating relatively early gives the meal time to settle and leaves room for a smaller evening snack if needed. The aim is to arrive at bedtime comfortable.

Keeping your stomach comfortable

Temporarily choosing lower-fibre bread, cereal, rice or pasta can reduce food bulk and may help runners who are sensitive before races. You do not need to remove every vegetable or avoid wholegrains forever. These are tactical choices for a short race-week strategy.

Very fatty meals can slow digestion, increase fullness and leave less room for carbohydrate. Keep the fats you normally enjoy, while limiting very rich or fried meals if they have caused problems before.

Glycogen is stored with water, and more food and fluid may remain in the gut, so body weight often rises during a carb load. A short-term increase does not automatically represent body-fat gain; the stored fuel and associated water are part of the purpose.

IBS or a sensitive stomach

Use foods, portions and combinations you tested in training. Do not begin a restrictive low-FODMAP diet in race week unless it has been recommended and practised with qualified support.

If bloating, diarrhoea, reflux or pain regularly disrupts long runs and races, seek help from a qualified professional with gastrointestinal and endurance-sport experience.

Carb-loading mistakes to avoid

Beginning with one enormous dinner
Confusing grams of carbohydrate with the weight of the food
Loading for an entire week
Introducing new foods, powders or supplements
Eating so much fibre or fat that carbohydrate becomes difficult to fit in
Restricting normal intake because tapering has reduced your appetite
Panicking about temporary scale weight
Forcing excessive fluid alongside carbohydrate

Hydration matters, but drinking beyond your needs is not part of carb loading. The running hydration guide explains how to arrive hydrated without overdrinking.

What happens after the carb load?

The final two days improve the fuel available at the start, but breakfast and in-race carbohydrate still have separate jobs. Rehearse both parts during training.

Marathon breakfast

Carb loading focuses mainly on muscle glycogen. Overnight fasting reduces liver glycogen, so race-morning breakfast still matters. Eat the carbohydrate-led breakfast tested in training, allowing enough time for digestion.

Use what to eat before a marathon for breakfast timing, morning hydration and final top-ups.

During-race carbohydrate

Stored glycogen remains finite, so carbohydrate during a marathon still matters, particularly as finish time increases. Use gels, sports drink, chews or another tolerated option, following the schedule tested on long runs.

The energy gels for running guide covers timing, water, caffeine and stomach comfort. After the race, read what to eat after a marathon for finish-line food, rehydration and the first 72 hours.

Who should get individual advice?

Get personalised support before attempting an ambitious carbohydrate target if you have diabetes, IBS or another gastrointestinal condition, coeliac disease, significant food allergies, pregnancy, REDs concerns or a history of disordered eating. Individual advice is also valuable if tracking food creates anxiety or the required volume feels physically or mentally difficult.

A GP or your usual clinical team should guide changes that interact with a medical condition or medication. For performance planning, look for an HCPC-registered dietitian or SENR-registered sport and exercise nutritionist.

Evidence behind this guide

This guide is informed by established marathon and sports-nutrition research. Food values are approximate; check product labels when a closer estimate matters.