Fuelling for a marathon: How to get it right

How many gels should you take during a marathon? And what should you eat in the days before the race? We’ve shared all you need to know about fueling for 42.2 kilometres.


If you’re one of the many people preparing to tackle 42.2 kilometres, you’ve probably been thinking about how you’re going to fuel your race. During your long training runs, you’ve hopefully experimented with various energy gels, chews, sports drinks or even real food to find out what works for you – and you should have trialled some different pre-run breakfast and dinner options, too.

If you haven’t already, now is the time to work out exactly how many gels you’ll need to consume during your marathon to keep your tank topped up with fuel. You should also start to consider how much water you’ll need to drink per hour – and the amount of electrolytes you’ll need to take on – to ensure you stay hydrated. And with our guide below, you’ll be able to calculate exactly that.

What, and how much, you eat and drink during a marathon can make or break your race. Fail to fuel properly and you’ll find yourself hitting the wall – the point of complete physical exhaustion when your body running out of energy. And if you fail to drink enough (or take on enough electrolytes), you may find yourself experiencing symptoms like gastric distress, nausea and dizziness due to dehydration – which also puts your health at risk.

However, if you’re diligent about your marathon nutrition and hydration strategy – planning it carefully and executing it well – you’ll have a positive running experience all the way to the finish line.

Let’s get to work…

How many gels should I take during a marathon?

Carbohydrate is the body’s preferred fuel source during exercise. It’s broken down into glucose and stored as glycogen in your liver and muscles, and used by the body to provide energy. ‘It takes around 500g of carbohydrate to fill your glycogen stores and, at most, this will last you for 60-90 minutes of running at 55-75 percent of your maximal heart rate,’ explains sports dietitian Renee McGregor. ‘The faster you go, the sooner your stores will deplete.’

McGregor recommends consuming 30-60g of carbohydrate per hour for the first three hours of your marathon, then increasing this to 60-90g of carbohydrate per hour after that.

Great sources of carbohydrate which you can consume on the go are sports nutrition products, such as carb drinks, energy gels or bars, or real food, such as bananas and Jelly Babies.

The best thing to do is to first check the packaging of your chosen product to find out how many grams of carbohydrate it contains. Then, you can work out how much of that product you’ll need to take per hour of your marathon to hit that 30-60g goal.

‘Start taking on nutrition in the first 30 minutes [of your run] and then every 30-40 minutes after that,’ advises McGregor. The atmosphere of big races like the London Marathon can be wonderfully distracting (in a good way), so if you need reminders to take your mid-run nutrition, set an alert to go off on your running watch, if you can, or make a note on your hand.

When it comes to gels, the quantity of carbohydrate they contain varies between brands, so once again check the packaging to find out how many you’ll need. If your gel of choice contains 23g of carbohydrate, for example, and you follow the consumption strategy outlined above, you would take six gels over a four-hour marathon to reach a carbohydrate quota of 34.5g of carbohydrate per hour (which gives you 136g of carbohydrate in total).

According to McGregor, it’s better to consume gels over the space of four or five minutes rather than all in one go, as this slower approach aids absorption.

It’s really important to practise your marathon nutrition strategy during your training, and to experiment with different products to find your best fit. And don’t try anything new on race day!

How much water should I drink?

It’s also really important to keep on top of your hydration during a marathon. ‘Along with maintaining hydration, fluid intake during endurance running helps to regulate body temperature (thermoregulation) and ensure adequate plasma (blood) volume,’ says McGregor. ‘When core body temperature rises owing to dehydration, plasma volume decreases, resulting in an increased heart rate, which accelerates fatigue.’

Alongside feelings of fatigue and exhaustion, other symptoms of dehydration to look out for include a dry mouth, dark yellow urine, decrease in urination, muscle cramping and headaches.

Your water requirements during a marathon will depend on your individual sweat rate, as well as your age, sex, weight, the intensity at which you run and the temperature outside. However, most runners will sweat between 400ml and 2400ml per hour, says McGregor.

‘Recommendations on how much water we should consume per hour when running vary from 300-800ml,’ explains nutritionist Kim Pearson. ‘Consider the outside temperature and how much you sweat, as well as your thirst,’ she adds.

It’s really important to hydrate before your run, too. ‘Research suggests about 530ml of water before and after a workout of any sort will support recovery and help prevent dehydration post-exercise,’ explains Pearson.

Start sipping on water around three hours before the race with an electrolyte tablet (which we’ll come onto next). Drink until your urine is clear – and if it isn’t, drink roughly another 400ml on top of this.

Do I need electrolytes for a marathon?

As well as lost fluids, you’ll also need to replace the sodium and other minerals which are lost in your sweat as you run.

Electrolytes help your body to retain fluid during heavy exercise when you are sweating, as they assist in drawing fluid into your working muscles, explains McGregor. ‘The sodium content of sweat varies, from 115mg per 1,000ml of sweat to more than 2,000mg. Most electrolyte tablets, salt capsules or sports drinks provide 250-300mg of sodium. If you are diluting your electrolytes into 750ml, this will mean having to consume in the region of 2,250ml of fluid per hour in longer races to meet your sodium requirements, which is hard from a consumption and transportation point of view.’

However, you can replace lost electrolytes in various other, more race-friendly ways. Electrolyte drinks and electrolyte tablets for dissolving in water are great options, as are salt capsules or sticks which can be taken directly.

‘I usually suggest runners take in 700-900mg of sodium per hour during longer training runs and races,’ says McGregor. She also suggests runners start drinking electrolytes in the 24 hours before race day.

How do I avoid stomach issues?

Runners often complain that they’re unable to stomach too many gels during a marathon, due to gastrointestinal distress or nausea. However, the idea that gels are to blame is a misconception, says McGregor.

‘The biggest cause of mid-run stomach issues is related to sodium imbalance, not sports nutrition gels or bars,’ explains McGregor. ‘If you are dehydrated and consuming glucose, it becomes highly concentrated in the gut. Blood is being directed away from the stomach to the working muscles, so the gut cannot absorb the glucose quickly enough, resulting in stomach upsets.’

What should I eat before a marathon?

In the three days leading up to your race, it’s important to up your intake on carbohydrate to fill up your glycogen stores – this is often referred to as ‘carb-loading’. The easiest way to do this – without overloading your stomach and causing you to feel bloated and sluggish – is by swapping elements of your usual diet for carbohydrate. So, for example, if you usually have a piece of fruit or yoghurt as your afternoon snack, swap it out for a couple of slices of toast or a hot cross bun.

For your pre-race dinner, opt for something which is high in carbohydrate but low in fibre and fat, which may upset your stomach. A good option is pasta with tomato sauce and garlic bread, or chicken, tofu or fish with potatoes, noodles or rice and a side salad. Don’t overload yourself, though, as this can make you feel uncomfortable the next morning due to the fact that your stomach hasn’t had time to digest all the food. One way around this is to have your main meal at lunchtime and then have a lighter meal in the evening, such as a sweet potato with soup and a piece of bread.

During training, make sure you experiment with different lunch and dinner options prior to your long runs. That way you’ll know what sits with you best, and come to your race with a tried and tested meal plan.

On the morning of your marathon, you’ll likely be a little nervous – but don’t worry too much if you struggle to eat all of your breakfast. If you’ve already adequately topped up your glycogen stores in the days leading up to your race, it won’t make much difference.

Again, for breakfast, opt for something which is high in carbohydrate but low in fat and fibre, such as porridge with a banana, nuts and honey or a white bagel with a banana and peanut butter.

Choose simple carbs – like white bread rather than brown bread – as these are lower in fibre and less likely to upset your stomach or leave you feeling gassy. Your body also digests them more quickly.

As with your other marathon fuel, experiment with different breakfast options during training to see what digests well and leaves you feeling full but not heavy or bloated – and stick with your preferred choice on marathon morning!

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