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Fuelling up for a marathon – What to consider

Updated: Apr 15

Photo of a group of people running a marathon

Running a marathon is one of the most challenging endurance events that an athlete can take on, whether the aim is to finish the race or to achieve a Personal Best. The reason for this is the sheer distance and therefore time it takes to complete it. There simply is not enough energy stored in the body’s muscles for high intensity running for longer than 2 hours, even if you top up with nutrition during the race. So pre-fuelling for a marathon is perhaps one of the most important nutritional considerations that an athlete needs to include in their race preparation plan.

What makes up this energy source?

Depending on the intensity that we run at, there is always a mix of fuels used in endurance exercise, whether that be carbohydrate or fat to fuel the working muscles. Most runners tend to rely more heavily on carbohydrate metabolism during an endurance event such as the marathon. For all athletes however, the higher the intensity, the higher the proportion of carbohydrate that is burnt to fuel the exercise. This means that more novice runners (as far as those capable of running 42 kilometres is concerned!) will most likely rely heavily on carbohydrates for fuel, and elite level marathon runners will be running so fast that carbohydrate metabolism predominates regardless.

Why does this matter?

Carbohydrate is stored in muscle (and the liver) as glycogen. The body always aims to keep our muscles ‘fuelled’ so that we can rapidly respond to a fight or flight situation. The energy is stored exactly where it is needed so muscles can contract without waiting for sugar to be supplied by the blood. There is however a finite amount of glycogen stored in muscle, which becomes significantly depleted after 2 hours of sustained high intensity effort.

By training with a mix of high intensity intervals and lower intensity longer efforts, athletes are regularly tapping into this energy source. This is constantly being recharged when we eat after training. That is partly why recovery nutrition is so important. For example, 60 minutes of high intensity intervals (training faster than race pace) will stress your muscle glycogen stores. This can take 48 hours to be restored by normal eating; have you ever noticed that monster appetite the day after a big speed session???

Because of this constant cycle of depletion and replacement of carbohydrate, the muscles of athletes are trained to

1) Burn muscle glycogen efficiently during exercise; and

2) Store more glycogen in the muscle than less fit people.

3) It is also the reason that athletes very rarely suffer from Type 2 diabetes – muscles are primed to take up sugar in the blood.

This is what we aim to maximise when we “carbo load” for a marathon (or any event around 60 minutes or more in duration). Many athletes tend to think this means “eat a big bowl of pasta the night before the race”. This is not serious carbo loading!

So what is carbo loading and what do you need to consider?

Firstly if we want to maximally fuel our muscles, we need to refrain from burning into our glycogen levels in the first place for the days leading up to the race. Most athletes will already be refraining from high intensity training during their “training taper” so that they don’t turn up at the start line fatigued. So already by doing this, every carb rich meal that you consume will contribute to fuelling up your muscles.

So eat more carbs, right? This is correct but proceed with caution! If we go from our regular healthy eating to downing massive carb portions, we are at risk of overloading our body’s digestive system. The last thing any athlete wants at the start line or during the event is to need to go to the toilet for “number twos”. Known as the “runner’s trots”, this can occur if we carb load too aggressively with the wrong types of food. So we need to consider reducing the amount of fibre that we eat for the days leading up to the event. Changing from wholegrain to refined white grains is a good starting point for many. Others may need to look at the types of grains that they eat.

Some runners regularly suffer from gut issues in training or competition. They may need to consider the types of foods that they consume for the days leading up to their event, including types of fibre (soluble vs insoluble fibre vs resistant starches) and other carbohydrate foods that can act like fibre in the gut. For these athletes, it is advised to see a Sports Dietitian that specialises in these issues. This will give them guidance on specifically what foods to consume in the leadup to the race, including appropriate amounts.

Another way to increase our carb portions is simply consuming a little less protein than usual to fit more starch on the plate as well as dropping the size of our vegetable serves somewhat. Omitting protein altogether is not advised as regular consumption of small protein serves is muscle preserving – very important at a time when we walk the line between resting up for the race and risk some ‘detraining’ which happens very quickly when we stop training. Similarly, omitting vegetables altogether is not advised as we want to keep our digestive system working efficiently (much like your muscles, the digestive system is also stressed during endurance exercise!) to avoid getting ‘bound up’.

The final consideration for carbo loading is how much carb to aim for and how long to do this for. Unfortunately there is no simple answer. It depends on level of training, expected average intensity for the event and what your training and “normal diet” consist of. A marathon runner that expects to run around or less than 3 hours will need to carb load very aggressively, which will need supplementation with nourishing fluids as food will never achieve the amount required for this level of performance. Those aiming to finish the race in 4+ hours will not need such an aggressive regimen.

On the morning of race day if you have done a good job with your race preparation, your muscles should be fully primed. A moderate to small meal should be just enough to top up liver supplies; unlike muscle, the liver breaks down glycogen to release glucose into the bloodstream. As a guide, a simple bowl of cornflakes or rice bubbles should be fine, or some toast with jam.

Seeing a Sports Dietitian is invaluable for forming a race preparation plan that maximises pre-fuelling for your race and therefore your chances of achieving your full potential. A plan will be made that takes all of the above into consideration and that fits with your current eating including the necessary tweaks. Such a plan is however best practiced at appropriate points in your training in the months leading up to the race.

Our Dietitian Chris Rauch works with endurance athletes and triathletes of all ages and levels from beginner up to elite. He is also involved in research and clinical sports practice at Monash University with a special interest in the effect of exercise stress on gut function, as well as substrate metabolism during ultra-endurance running. He is proud to be a part of a team leading the world in Sports Gastroenterology conducting novel exercise protocols for athletes suffering from gut distress in training and competition.

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