Your guide to carb loading on marathon week

Carbohydrate loading, more commonly known as carb loading, is an important nutrition strategy used by endurance athletes to improve their performance on race day. In essence, carb loading is a process by which you increase the amount of fuel stored in the muscles. But, many people preparing for a marathon aren't quite sure how or when to increase their carbohydrates before a 26.2-mile race – meaning they often arrive at the start line with a huge handicap.

Carbohydrates are vital to runners because they serve as the body's main source of energy. We store any carbohydrates we don't use as glucose in the liver and muscle tissue. During endurance or high-intensity exercise, the body can then access these stores for the energy needed to keep going.

The concept of carb loading in sport was introduced in the 1960s to help increase athletes' base levels of muscle glycogen prior to endurance events – and studies show that carb loading, compared with a regular intake of carbohydrates, does improve athletes' performance in endurance events lasting more than two hours. According to a review in Sports Medicine, diets that are high in carbohydrates can improve performance in this type of exercise by up to 3%. The evidence also points to carbohydrates, rather than fats, as being the best source of energy for endurance exercise.

Your body can only store enough glycogen (energy) to sustain 90 minutes of exercise. After this point, without sufficient extra fuelling, you’re in danger of running out of energy and facing the dreaded 'wall'. If your body doesn’t have enough glycogen stored up, you can’t sustain performance over a longer distance because your body has effectively run out of fuel.

Nutritionist Monique Ryan, author of Sports Nutrition for Endurance Athletes, says that runners carb load to fill their bodies with the most accessible form of energy. 'Filling your muscles with glycogen won’t make you faster, but it will allow you to run your best and help you avoid the wall,’ continues Benjamin Rapoport, a 2:55 marathoner and Harvard researcher.

'I eat a lot of rice,' says Rapoport. 'But bread, porridge, bagels, pancakes and yoghurt are also easy-to-digest options. Many fruits are high in carbohydrates but also in fibre, which can cause mid-race stomach trouble. Bananas are a low-fibre choice.' Try to avoid eating too many fibrous foods in the two days before a marathon to avoid cramps and diarrhoea.

If you’re finding it hard to consume enough carbohydrates, sports drinks are an easy alternative. However, while carb loading works up to a point, your body does have a limit as to how much carbohydrate it can store as muscle glycogen. This means that athletes need to consume additional fuel during long events, like a marathon – which is why marathon runners are encouraged to eat things like energy gels and chews during a race.

It's important to carb load in training as well as during race week to get your body used to the process.

'A lot of people get it wrong. They don't match their training programme to their food,' says Mayur Ranchordas, reader in nutrition and exercise metabolism at Sheffield Hallam University. 'The more volume and intensity the programme has, the higher the carbohydrate and protein intake – and fat usually stays relatively consistent.'

For example, a 10-mile threshold run would need to be underpinned by a greater intake of carbohydrates. Meanwhile, a long, slow 20-miler will cause a lot of muscle damage and require a greater amount of protein post-run to assist your recovery.

You can’t completely stock your muscles and liver with glycogen in just a single meal, ‘which is why you should start carb loading two or three days before your race,’ says Ryan. As you’re on low mileage at that point – and tapering – the glycogen will accumulate in your muscles.

Another tactic is to carb load in a single day to maximise muscle glycogen storage while giving your gut more time to digest – and this doesn't have to be the day before race day.

'The effects of greater storage of muscle glycogen don't disappear after one day,' says David Rogerson, principal lecturer in sport and exercise science at Sheffield Hallam University. 'The effects can actually last for up to three to five days. So, you could carb load two or three days beforehand – and so long as you're not doing lots of exercise on those days, you'll probably still maintain those elevated levels of muscle glycogen going into the race.'

The precise amount you need to eat will depend on both your weight and how fast you intend to run. For instance, a runner aiming to complete a marathon in five hours will need less fuel than a sub-three runner who will be oxidising a lot more carbohydrates. The faster runner will also create more muscle damage, so will need more protein post-race.

During this carb loading period, 85-95% of your calories should come from carbohydrates, says sports nutritionist Ilana Katz. And, according to Anita Bean, author of The Complete Guide to Sports Nutrition, you should aim to up your carbohydrate intake to 8-10g per kilogram of body weight. So, for a 70kg runner, that amounts to 560-700g of carbohydrate per day. Each gram of carbohydrate equates to around four calories, which – for a 70kg runner – equates to 2,800kcal of carbohydrates per day during the carb loading phase.

It's worth remembering, though, that you’re not eating many more calories per day than you are during the thick of your training – it’s just that, while carb loading, more of your calories are coming from carbohydrates.

If you step on the scales while carb loading, expect to be above your usual weight – and don't be alarmed or dismayed. 'With every gram of stored carbohydrate, you store an extra three grams of water,' says Katz. That means you will be well-fuelled at the start line.

To reach your carbohydrate target, try eating little and often rather than just super-sizing your usual meals. Consuming five or six smaller meals in a day is much more palatable than stuffing yourself only to feel queasy and lethargic. Another option is to double up on your carbohydrates to make meals denser rather than more filling. For breakfast, this could be three fistfuls of porridge with a chopped banana, dried fruit and maple syrup, accompanied by a glass of fruit juice. Lunch could be rice with potatoes and for dinner you could have pasta. Try to cut back on protein, too, because it fills you up and could mean that you could neglect your carbohydrates.

Six weeks before the marathon

During training, practise carb loading two days prior to your longest run by eating more carbohydrates and less fat and protein. 'That way, you’ll work out what foods agree and disagree with you,' says Katz.

One week before the marathon

Make a plan. 'A plan is especially important if you’re travelling to a race,' says Ryan. Pack plenty of snacks such as energy bars, sweets and crackers to keep you stocked up on carbohydrates.

Two or three days before the marathon

Hit the switch. From now on, 85-95% of your diet should be carbohydrates. Remember to eat after your taper runs, too – 'that’s when muscles best store glycogen,' explains Rapoport.

The night before the marathon

Don’t overeat. Your dinner should be small but carbohydrate-heavy and you should eat on the early side to give your body lots of time to digest. 'You should wake up on race day hungry,' says Ryan.

The morning of the marathon

Think big. For breakfast, two to three hours before you start your race, eat 150g of carbohydrates, such as a bagel with peanut butter or a bowl of porridge with a banana, says Ryan. But, whatever you choose, don’t try anything new – stick to the same breakfast you’ve had during training. Early race? 'Get up at 3am, eat, then go back to bed,' says Ryan. Later start time? Plan to eat your breakfast on the journey to the race, or pack a banana to snack on closer to the start.