How to Fuel For a Marathon: Your Guide to Carb-Loading

How well you can run 26.2 miles doesn’t just come down to how many miles you’ve logged in training - it’s also affected by how well you’ve fuelled your body in the days leading up to race day.

Whether it’s your first marathon or your fifth, understanding how to execute a smart carb-loading strategy can be the difference between hitting the wall and hitting a personal best. 

Why You Need to Carb Load

During a marathon, your body relies heavily on glycogen as fuel. This is the stored form of carbohydrate in your muscles and liver.  But, here’s the thing: your body can only store a limited amount and exhaustion usually coincides with low levels.

As such, it’s important that you maximise your glycogen stores before competition.  The liver can only store a small amount compared to the muscles; approximately 100-120 grams, which translates to approximately 400-480 calories. In comparison, the muscles can store a significant amount of glycogen, ranging from 350 to 700 grams (between 1,400 and 2,800 calories).

Your storage capacity ranges based on your training status, diet, sex, genetics, muscle fibre composition, and body size. By executing an adequate carb loading protocol in the days leading up to a marathon, as well as appropriately tapering your training, you can ensure that your glycogen stores are topped up prior to race day. 

As a result, you’ll experience delayed fatigue, have a reduced risk of ‘hitting the wall’ and greater energy levels. In fact, a research review found that endurance performance can be enhanced by up to 2-3% by carbohydrate loading, a finding which could translate to minutes in a marathon (Hawley et al., 1997). 

How to Carb Load 

Now you understand the benefit of carb-loading for a marathon, how do you execute a carbohydrate loading protocol correctly? Firstly, to simplify the concept; ‘carbohydrate loading’ can, in other words, be described as consuming a high carbohydrate diet. 

48-72 Hours Before the Race 

The optimal timeframe to begin your loading protocol is 2–3 days before the event. There are a few reasons for this, the first being simply that your body needs time (between 48 and 72 hours) to digest, absorb and store these additional carbohydrates as glycogen. 

By starting the process a couple of days out from competition day, you can also gradually load up, rather than cramming a carb-heavy meal the night before, which can cause bloating or gastrointestinal (GI) issues during the race. 

Additionally, the final days before your run typically coincides with your tapering phase, where training volume is reduced, meaning less glycogen is being used and more can be stored effectively when you couple this with high-carb food choices.  

When it comes to what to eat and how much of it, there is also clear guidance. Research has shown that an intake of 8-12 grams of carbohydrate per kilogram of body weight per day (g/kg/BW/day) is best for maximising glycogen stores (ISSN, 2017).

For a 70 kg runner, this would mean eating between 560 and 840 grams of carbohydrate daily.  This is certainly a high intake and it should be noted that if you are new to a high carbohydrate intake then a slightly lower intake between 6 and 8 g/kg/BW/day can also be effective.

This is still a significant carbohydrate consumption and, ingested alongside your taper, likely to still result in greater glycogen storage.

How to Hit These Numbers 

Meeting this high carbohydrate intake does not mean gorging yourself on food. Instead, you can achieve this by strategically increasing the proportion of carbohydrates you eat in your meals and snacks, at the cost of some fat and protein. 

This is not only important from the perspective of fullness - whereby if you don’t reduce the fat and protein content of the meals, and simply increase the carbs significantly, you’ll feel over-full, bloated and potentially unable to finish your meals - but also because protein and fat slow digestion and too much in the days prior can cause GI issues on race day. 

Fibre should also be kept moderate to low for similar reasons and should be gradually reduced as race day approaches. Do this by opting for white carbohydrate options, not wholemeal or brown, picking refined carbs over wholegrain, and choosing lighter vegetable options like cucumbers, tomatoes, and light salads over bulkier alternatives like broccoli and cauliflower. 

Practical takeaway: At each main meal, fill ½ to ⅔ of your plate with carb-rich foods. 

Table 1. Examples of high-carbohydrate food options to include during your carb-loading phase.

In addition to building your main meals with the right proportion of carbohydrates, what you choose to snack on in the 2-3 days prior to competition is also meaningful to your overall carb intake.  Below are some simple examples of how you can introduce high-carb snacks into your day and meet the above recommendations:

• Add a 250ml glass of fruit juice alongside your breakfast
• Eat an energy bar and/or a medium banana mid-morning
• Sip on a carb drink mix throughout the day
• Enjoy a dessert (e.g. low-fat rice pudding with jam) or hot chocolate in the evening

Morning of the Race

Having successfully executed two to three days of eating a high-carbohydrate diet, your muscle glycogen levels will be topped up. That said, the pre-race fuelling isn’t finished yet. 

It’s important for optimising performance that you aim to eat a high-carbohydrate, low-fibre, low-fat breakfast in the 1-4 hours before the marathon starts.  This is to replenish your liver glycogen stores which, unlike muscle glycogen stores, deplete overnight (Jeukendrup & Gleeson, 2010).

Restoring liver glycogen levels supports optimal blood glucose levels during the race and enhances both cognitive function and endurance. The general recommendation is to consume 1–4 grams of carbohydrate per kilogram of body weight. 

It follows that the closer you are eating to the race start, the fewer grams of carbs you should consume (e.g. if you have two hours to your race start you should target 2 g/kg/BW vs. eating four hours before, when 4 g/kg/BW would be more appropriate). 

How to Hit These Numbers 

Most runners hit the sweet spot around 2–3 hours before the race as this gives the body enough time to digest the meal without hunger kicking in on the start line.

It is extremely important that you trial this timing beforehand in training sessions. If your race is early, practice waking and fuelling at this time. In particular, fructose-containing foods and drinks (fructose being a type of simple sugar naturally found in some fruits, fruit juices and honey, as well as dual-source sports nutrition products) are ideal choices since they more readily replenish liver glycogen than glucose

Example pre-race breakfasts: 

• 1–2 white bagels topped with a generous drizzle of honey and medium banana, plus 250ml of fruit juice (~120–170g carbs)
• 400g low-fat rice pudding topped with 2 tbsp of jam and a 45g dual-source carb drink mix (~100–110g carbs)
• 2–3 slices sourdough toast spread with a generous spread of jam or honey and 250ml of freshly squeezed orange juice (~100–130g carbs)

In addition to your pre-race breakfast, some athletes also favour a small carbohydrate-rich snack (~20-30 grams) with ~30-60 minutes to go. This helps maintain blood glucose levels and provides some extra fuel right before the start.

Popular choices include:

• 1 x Energy gel
• ~5–6 jelly babies
• Energy chew/s (check packet for carb content)

That all said, if you really struggle to eat on race morning due to nerves, don’t worry.

Firstly, if you’ve fuelled well with a high-carb diet for 2-3 days, you’ll have done most of the job. Secondly, you could opt for a light option like a dual-source carb drink (~80-90g carbs) and a small snack just to suppress hunger. Either way, don’t stress over it and force a large breakfast if you find that it doesn’t work for you. 

Final Thoughts

By knowing when and how to increase your carbohydrate intake in race week, you stand a greater chance of starting the marathon with a full fuel tank, which just like a car, takes you further, faster!  Start experimenting early on in your training with different pre-training options and find what works for you. Then, trust the process when the big day comes - remember, nothing new on race day!

Download our free eBook 

Nutrition Fundamentals guide

Download Now