marathoners and fat


To run a half marathon (21 km or 13 miles) or a full marathon (42 km or 26.1 miles), our body needs much more energy compared to shorter races.

Although many runners believe that carbohydrates (pastas are usually the favorite ones) are the only source of energy, for a half or a full marathon runner, fat is also essential.

Therefore, we now show you the reason why fat is actually essential for half or full marathon runners.


Without a doubt, the bigger the distance to run, the more energy is needed in order to complete the run, (a marathon runner of 155 lbs will need approximately 2950 kcal to successfully run the full marathon).

The muscle contractions can be provided with energy from a wide variety of metabolic substrates, the most important being:

– Carbohydrates (stored in muscular and hepatic glycogen, and blood glucose)

– Fat

Even runners that have a very small amount of accumulated fat and seem to be very thin, have enough fat stored to provide the required energy to run a full marathon or more.

Therefore, stored fat in our body is an almost unlimited source of energy (you might be able to run a full marathon -26 miles- with it) and thus it is so important for those who run for a good amount of hours.

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On the contrary, given the fact that a runner can only store approximately 1,500 kcal of glycogen we can say that carbohydrates storages present in our body are really limited; i.e. 1,000 calories less than the necessary to complete a marathon.


So, if we were to rely exclusively of carbohydrates to run a marathon, we would be in serious problems of finishing the race, because it would be like pretending your car moves without fuel.

During a half marathon, your carbohydrates reserves may be enough, but you would probably get them close to being emptied.

Luckily the energy does not come from just one single source and there is always some contribution from one or another in different amounts (there is always one source that gives more than the other).

Meaning that, when we move, our body uses both sources of energy, but it will select the one that is going to dominate depending on different factors, which we are going to stand out:

The intensity of the run

When running at a relaxed and slow pace, our body tends to use a bigger amount of fat as fuel. Even though, when running at a faster pace, our body uses more carbohydrates (through stored glycogen in muscles, liver and bloodstream).

Your experience

However, training improves our fat burning capacity while we run, allowing us to use bigger amounts of fat while running at a faster speed.

Your diet

If you are a runner that bases his diet in big portions of carbohydrates, your body will get used to using glycogen as a main energy source.

Also, eating food rich in carbohydrates before running will promote insulin liberation and send a signal to our body to stop using fat as energy (or reduce its use) and instead use existing glycogen in the bloodstream.


The energetic requirements to run a half marathon and a full marathon (13 and 26 miles) are extreme and fat is essential because:

– Glycogen (carbohydrates) stored in our body is limited;

– Carbohydrates consumption (through gels, gum or energy drinks) has two risks: a) not being enough and b) creating stomachaches.

– Fat is a virtually unlimited source of energy.

flickr photo by ryanknapp shared under a Creative Commons (BY-SA) license