Fuelling Versus Not Fuelling

How important is it to fuel those longer runs? Dr James Morton explains the benefits


by Dr James Morton

Dr James Morton is the Senior Sports Nutritionist for Science in Sport (SiS), the leaders in endurance nutrition. He has led and collaborated in over 40 research papers and publications and is also Senior Lecturer in Exercise Metabolism & Nutrition at Liverpool John Moores University (LJMU).


Fuelling strategies at marathons can differ greatly. Some runners will fine-tune their strategy in training, while others will see how far they can get on the day, not using nutrition until it is too late. Here, Dr James Morton from SiS looks at the advantages of having a fuelling strategy.

Why fuel during exercise?

The effects of muscle glycogen availability (the storage form of carbohydrate) on exercise capacity and performance is one of the foundations of sports nutrition. Indeed, a wealth of research over the last 40 years has consistently demonstrated that a high carbohydrate diet for several days increases muscle glycogen storage and that endurance exercise performance is subsequently improved. Similarly, depletion of muscle glycogen during exercise is closely correlated with the onset of fatigue, where fatigue is observed as an inability to maintain the same running speed. 

Why carbohydrate?

To avoid fatigue, it is now common practice to consume additional carbohydrate during exercise (usually in the form of drinks, gels and solids) so as to maintain carbohydrate availability and thereby maintain performance i.e. running speed. Indeed, numerous researchers have observed that feeding carbohydrate during exercise improves performance over and above that of simply feeding water, even when you commence the race with already high glycogen stores. Clearly, to truly maximise your marathon performance, 2 days of carbohydrate loading and a big breakfast are not enough!

What are the benefits?

It is currently thought that carbohydrate feeding during exercise may improve performance through multiple mechanisms that include a ‘sparing’ of muscle glycogen utilisation (i.e. you now have more glycogen available for later in the race), preventing hypoglycemia (i.e. a fall in blood glucose), maintaining high rates of carbohydrate oxidation (carbohydrate is the predominant fuel source for moderate to high-intensity exercise) as well as having direct effects on the central nervous system so that we perceive the exercise to be easier.

When should I fuel?

For these performance-enhancing effects to occur, most researchers agree that carbohydrate feeding should occur from the beginning of exercise as opposed to the latter stages when you have already ‘hit the wall’.  Indeed, a common mistake from runners is to only commence carbohydrate feeding during exercise after 90 minutes or so. Depending on your running speed, however, this may be too late as glycogen levels could already be depleted to those critically low levels that are associated with fatigue.

How much and how often?

The general rules of thumb for prolonged endurance events (e.g. > 3 hours) is to therefore consume 60g of carbohydrate per hour, usually consumed in 20g doses at 20-25 minute intervals. Research has also suggested that carbohydrate can be metabolised to the same extent when provided in fluid, gel or solid format, though it is important to note that many of us have personal preferences and may differ in our ability to tolerate these different forms of carbohydrate.

Why are gels usually the best option?

Regardless of your preferred source, you must always remember the golden rules of 60 g per hour. Many of the athletes I now work with tend to prefer the use of SiS gels given they are truly isotonic (i.e. don’t need water to be absorbed effectively) and also provide a simple serving of 20g of carbohydrate per gel – a far simpler way of consuming the required amount than the rather large handful of jelly babies it would take. 

How does this work alongside hydration?

If gels are your preferred approach then you should remember to also consume fluids to maintain adequate hydration, and current advice is to aim for around 500 ml of an electrolyte solution per hour and your thirst will likely dictate if you require more.

A few more things to consider

You should also remember to use a variety of flavours to avoid flavour fatigue and boredom throughout the race. Finally, ingestion of caffeine 30-45 minutes before you expect to hit the toughest part of the race can help give you a mental lift during those last 6 miles or so. Fortunately, many fluids and gels also contain caffeine and to achieve performance benefits should aim for at least 2 mg per kg of your body mass. 

Key Points:

  • Carbohydrate load for 2 days prior to the race by consuming 10 g per kg body mass per day.
  • Aim to consume easily digestible sources such as breakfast cereals, breads, rice, snacks and drinks etc.
  • Consume breakfast (e.g. cereals, toast, yoghurts etc) 3 hours prior to the race and aim for 2-3 g per kg body mass as well as 500-750 ml of an electrolyte drink.
  • Perhaps consume a carbohydrate energy bar 60-90 minutes prior to race.
  • Aim for 60 g of carbohydrate per hour in your preferred format or a mixture of fluids, gels and solids.

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