Eat Your Way to the Finish Line: Marathon Training

Physiotherapist Neil Smith’s monthly article to guide your Marathon training. This month he talks to dietician Sharmain Davis about fuelling your body during training.

So your training program is in full flow; the mileage is quickly ramping up; the new shoes are working their magic; you’ve got a big couple of months ahead of you! Are you ensuring that you’re properly fuelled for the high volume weeks ahead? 

Carb loading; energy gels; hydration; sports drinks; jelly babies….. what is the best way to avoid burning yourself out before you even reach the start line? Our expert sports dietitian, Sharmain Davis, has some excellent, practical advice for you as you prepare for the big day.

Proper training is crucial to succeeding in a marathon, however, if you don’t fuel your body with the right nutrition that distance to the finish line will seem a far greater stretch. Understanding your individual needs and meeting them is key to optimal performance and injury prevention.

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Carbs to Go 

Carbohydrates are your body’s main and preferred source of fuel especially during endurance exercise such as running. Carbohydrate food sources such as; bread, potato, pasta and rice are digested and stored in your muscle and liver as glycogen, ready to use as energy, as and when we need it. The problem is however, that we can only store approximately 2000 kilocalories from carbohydrate. Which sounds like a lot, but if you imagine a marathon requires on average 2500 calories to complete, the reality is it isn’t and you may hit the wall’ sooner than you think.

To put it simply, a higher muscle glycogen level will allow you to train harder for longer and a low muscle glycogen will result in early fatigue and a lower training threshold. Consuming carbs before and after training allows you to fuel that training session, it allows your gut to practice using carbs efficiently during exercise as well as supporting your immune system.

Take home message: A regular intake of carbs daily (specifically pre & post training) can help you to train harder, adapt and help to keep you free from illness and injury. 

But how much?

We all need some carbohydrates as part of a healthy balanced diet but active runners need more than inactive sedentary individuals. The amount of carbs you require very much depends on how active you are, specifically how many hours per week you spend training. Carb requirements are calculated on an individual basis using total body weight (in kgs) multiplied by carbs (in grams), which increases proportionately with time spent training.

Training duration/​situation #

Grams of carbs #

Time to consume #

3 – 5 hours per week/​or athletes with large body mass or need to reduce energy intake to reduce body fat

3 – 5g per kg of body weight

Each day

5 – 7 hours per week

5 – 6g per kg of body weight

Each day

1 – 3 hours per day

5 – 7g per kg of body weight

Each day

2 – 4 hours per day

7 – 8g per kg of body weight

Each day

4 hours+ per day

8 – 10g per kg of body weight

Each day

Very intense exercise – 6 – 8 hours/​day

10 – 12+g per kg of body weight

Each day

Before training/​event

1 – 4g per kg of body weight

Between 1 – 4 hours before training/​event

During training sessions/​events lasting longer than 1 hour

30 – 90g per hour

Start taking 45 minutes in

After a training session or between multiple events

1 – 1.5g per kg of body weight

Immediately after, aiming for 6 – 10g/​kg for rest of the day

Take Home Message: To estimate your carbohydrate needs, you can use the information in the table to establish the amount of hours you typically train each week and then calculate the suggested carbs in grams by your weight in kilograms.

Offset the Sweat

It is well known that exercise performance is impaired when dehydration occurs. We sweat during exercise to help regulate our core temperature; evaporation from the skin surface allows a cooling effect, aiding thermoregulation in the short term. Runners can lose a considerable amount of fluid as sweat especially during longer, high intensity training sessions and/​or exercising in warmer environments. The best way to avoid the detrimental effects of dehydration is to offset sweat losses by increasing your intake of fluid. 

Day to Day 

Dehydration is cumulative; therefore if you begin a training week dehydrated and subsequently fail to drink enough fluid, your performance during training sessions are likely to be reduced, hindering adaptation and any desirable gains.

Take home message: Runners can monitor and maintain hydration by aiming for regular urination, which should be pale in colour, by drinking frequently, ideally with meals and snacks that naturally contain sodium to enhance rehydration. Calculate your daily fluid needs based on; 35 mls x weight in kgs (+ ~500mls on a training day/​warmer climates).

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Race Day

Sweat rates and therefore fluid losses can vary considerably between individuals. Thus it’s important you are aware of your fluid requirements during your training sessions and race. With this knowledge you can then consider the practicalities of carrying and consuming sufficient fluid to maintain performance during the Marathon. The research indicates that a 2% drop in weight is on average the tipping point’ at which performance is affected. Therefore, the aim of your hydration plan should be to drink more than the minimum volume of fluid to avoid a 2% loss in body mass. The method below can be used to calculate your loss.

Sweat rate Method & Calculation


  • Measure pre-run body weight; nude or wearing minimal clothing.
  • Followed by a 30 – 60 minute run. Omit any food in this instance. 
  • Measure post-run body weight; nude or wearing minimal clothing.


  • Fluid losses = pre-run weight (kg) – post-run weight (kg) (+ fluid in litres)
  • Calculate your losses over a given period of time, considering that;1kg is equal to 1 litre of fluid, e.g. if you lose 0.8kg over 30 minutes (with nil fluid intake) then you can assume you lose about 1.6 litres of fluid per hour at that speed/​intensity, and in those particular temperate conditions. You can adjust either way if this changes.
  • The aim is to avoid a 2% drop in weight, so here’s an example;

Nutrition Consultation: For further advice regarding your Marathon nutrition strategy, you can arrange for a consultation with Sharmain Davis (Sports and Clinical Dietitian) at Pure Sports Medicine City Clinic, Threadneedle Street.