Your guide to carb loading on marathon week

Get your pre-race fuelling correct to avoid hitting the wall.


Carbohydrate loading, commonly known as carb-loading is an important nutrition strategy used by endurance athletes to improve their performance by increasing the amount of fuel stored in the muscles. But many people don’t know when or how to increase carbs prior to a marathon meaning they arrive at the race with a huge handicap.

The reason carbohydrates are so vital to runners is because they serve as the body’s main source of energy. We store any carbs 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 as a source of energy.

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. Studies show carb loading improves athletes’ performance in endurance events lasting more than two hours, compared with a regular intake of carbohydrates. In this type of exercise high carbohydrate diets can improve performance by up to three per cent, according to a review in Sports Medicine. And the evidence points to carbs as being the best energy source for endurance exercise, rather than fats.

Why do runners carb-load?

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 coming up against the dreaded “wall”. If your body doesn’t have enough glycogen stored up, you can’t sustain performance over a longer distance because the body has effectively run out of fuel.

Runners carb-load to fill their bodies with the most accessible form of energy, says nutritionist Monique Ryan, author of Sports Nutrition for Endurance Athletes. ‘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,’ says Benjamin Rapoport, a 2:55 marathoner and Harvard researcher.

Which carbs should runners eat more of?

‘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 carbs but also in fibre, which can cause mid-race stomach trouble. Bananas are a low-fibre choice,’ says sports nutritionist Ilana Katz. Try avoiding too much fibre two days before a marathon to avoid cramps and diarrhea.

If you’re finding it hard to get enough carbs in, sports drinks are an easy alternative. However whilst carb loading works up to a point, the body has a limit on the amount of carbohydrate it can store as muscle glycogen. This means that athletes need to consume additional fuel during long events which is why runners keep eating gels during a marathon.

When should you carb-load?

It’s important to carb-load in training as well as during race week.

‘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, Sheffield Hallam University.

‘The more volume and intensity the programme has, the higher the carbohydrate and protein intake, and fat usually stays relatively consistent,’ he explains.

For example a 16 kilometre threshold run would need to be underpinned by greater carbohydrate intake. Meanwhile a long slow 32 kilometre run will cause a lot of muscle damage and require a greater amount of protein post-run to help aid 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 one day to maximize muscle glycogen storage, but give 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 as 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.’

And how much should you eat?

The precise amount you need to eat will depend on your weight but also how fast you intend to run. A runner aiming to simply complete a marathon who may take five hours, will need less fuel than a sub three hour runner who will be oxidizing 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 per cent of your calories should come from carbs, says Katz. You should aim to up your carbohydrate intake to 8-10g per kilogram of body weight (Anita Bean, The Complete Guide to Sports Nutrition, (A C & Black, 2003). For a 70kg runner, that works out to between 560g and 700g per day. Each gram of carbohydrate equates to around four calories, so that’s up to 2,800kcal carbohydrates per day during the carb-loading phase (for a 70kg runner).

Remember, you’re not eating many more calories per day than during the thick of your training, it’s just that more calories are coming from carbs.

If you step on the scales while carb-loading, expect to be above your usual weight. ‘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, you can try eating little and often rather than just super-sizing your usual meals. Eating five or six smaller meals is much more palatable than stuffing yourself only to feel queasy and lethargic. Another option is doubling up on carbs to make meals more dense rather than filling. For breakfast this could be three fistfuls of porridge with chopped banana, dried fruit and maple syrup with a glass of fruit juice. Lunch could be rice with potatoes and for dinner pasta. Also try to cut back on protein because it fills you up, meaning you could neglect your carbs.

How to time your carb load like a pro:

6-weeks before the marathon:

Practise loading two days prior to your longest run, and start eating more carbs and less fat and protein. ‘You’ll work out what foods agree and disagree with you,’ says Katz.

1-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.

2-3 days before the marathon:

Hit the switch. From now on, 85-95 per cent of your diet should be carbs. Eat after taper runs: ‘That’s when muscles best store glycogen,’ explains Rapoport.

The night before the marathon:

Don’t overeat. Dinner should be small but carb-heavy. Eat on the early side so you have lots of time to digest. ‘You should wake up on race day hungry,’ says Ryan.

The morning of the marathon:

Think big – three or two hours before the start, eat 150g of carbs, such as a bagel with peanut butter or a bowl of porridge and a banana, says Ryan. But don’t start trying anything new – stick to the same breakfast you’ve been having while training. Early race? ‘Get up at 3am, eat, and go back to bed,’ she says. Later start time? Plan to eat your breakfast on the train on the way to the start line, or pack a banana to have closer to the start.

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