The best way to fuel for an ultra marathon

Roughly speaking, you'll burn about 400 to 600 calories per hour during a run, but your body can only absorb 240 to 280 calories per hour. You'll never replace the deficit, but eating early and often will stop you from ‘hitting the wall’.

If you’re coming into a run carbo-loaded, you won’t need to begin fuelling until 45 minutes to one hour into the race. Those pre-race carbs (10 - 12 g per kg per day for 36 - 48 hours pre-race) will help promote the storage of glycogen (glucose), which will allow you to go longer before fatigue sets in.

After the first few miles of any race, you’ll want to start ingesting those 240 calories to 280 calories per hour, so set your watch for every 15 minutes to remind yourself to eat a little and drink a little at a time. Early on you can survive on gels and other carbs, but after four to six hours, add one part protein to four parts carbs (a mix found in most energy bars), but never exceeding 100 calories at a time.

The majority of the calories you’ll taking on board will be carbs and you should be aiming to consume 30 to 60g of carbohydrate per hour of exercise (the longer and harder you run, the more you need). The most common carb - and the most necessary - is glucose because it helps you burn fat. Glucose is also the only fuel your brain uses. It’s very easily digested, so it will give you lots of energy quickly without making you feel sick. You can get glucose from any number of products, including energy gels, bars and sports drinks.

Newer runners usually start with gels because they are the easiest product to ingest and they take the guesswork out of what you should be eating. Look for natural fruit sugars, brown rice syrup or maltodextrin in the ingredients. Some products also contain caffeine, which can help to give a mental and physical boost during the last hours of a race.

Few runners can stomach lots of gels or bars over a prolonged period, so finding agreeable real food is critical, especially over longer distances. Recommendations usually include turkey or chicken (with or without avocado) or peanut butter and jam sandwiches, chicken-noodle soup, pretzels and bananas. Try different options during your training runs and see what works best.

All sports nutrition advice can be thrown out the window if you get dehydrated. In the same way you fuel with gels and foods, you must also take in liquids. Keep in mind that those 240 to 280 calories you should be taking on board every hour include solids, gels and liquids, which means you can pair a gel or a sandwich with a sports drink to get in the necessary fuel. What’s more, liquids are absorbed more quickly than solids.

Your performance will suffer if you lose just three percent of your body weight in water and many ultra marathons require weigh-ins at aid stations to check hydration levels. Although hyponatremia, which occurs when a runner’s blood gets diluted, is of concern for a few, the most recent research suggests that drinking water and sports drinks as often as necessary is probably the way to go. Sports drinks are especially useful when you’re sweating a great deal because the electrolytes make you drink more and the sodium helps you retain more liquids and carbs.

If you do think hyponatremia is setting in - a watchband that suddenly becomes tight is often the first sign - stop and drink 1/4 cup of warm water with four stock cubes. Better still, consume some salty food - about 200 to 300 milligrams per hour. If it's a hot day, consider taking sodium tablets.

Dehydration in ultra runners tends to be the most prevalent the first eight hours of a race, so replacing the liquid lost through sweat is especially vital in the early stages. Since runners’ sweat rates vary between 0.5 to 3 litres per hour, the best way to determine how much you should be drinking is to figure out your individual sweat rate. To do this, weigh yourself in the nude, get dressed and then go out and run for an hour. When you get back, don’t eat or drink anything, just towel off and weigh yourself again. If you’ve lost 2 lbs, that equals about 1 litre of liquid per hour, which is your average sweat rate.