Fuel stores for running

Updated: Jun 2, 2018

‘What is the right fuel for an endurance athlete?’ is not an easy question to answer

Let us start with a comparison with a car. A car drives on 1 type of fuel. It’s either gasoline or diesel or … This principle unfortunately does not apply to our bodies.

Our body, has a more complex way of storing fuels for physical activity. It has the possibility to store energy in 2 ways. It’s able to store fats and carbohydrates, which can be used to generate energy. An important element however is that the storage capacity for fat and carbohydrates is not split equally. Fat stores in the body are far larger than carbohydrate storage capacity. On top of this, fat is a more efficient form of energy storage, releasing 9 kcal/g, compared with 4 kcal/g for carbohydrate. Each gram of carbohydrate stored also retains about 3g of water, further decreasing the efficiency of carbohydrate as an energy source.

The energy cost of running a marathon is about 2900 kcal, if this could be derived from the oxidation of fat alone, the total amount of body fat required would be about 320g, compared with 750g of carbohydrate and an additional 2,3 kg of associated water. Aside from the weight that would have to be carried, this amount of carbohydrate exceeds the total amount normally stored in the liver and muscles combined. The total storage capacity for fat is extremely large, and for most practical purposes, the amount of energy stored in the form of fat far exceeds what is required for any exercise task.

A runner with a body weight of 65 kg and a fat percentage of 20% can store approximately 13kg fat which equals approximately 91 000 kcal (1kg fat = 7000kcal)

The main problem associated with the use of fats as fuel for physical activity is the rate at which it can be taken up by muscle and transformed to provide energy. Fat oxidation can only supply energy at a rate sufficient to maintain exercise at an intensity of about 60% - 80 of the VO2 max. To generate energy to sustain higher exercise intensities, there is an increasing reliance on carbohydrate.

During most forms of exercise, a mixture of fat and carbohydrate is used to provide energy for your muscles to work. The intenser the activity the less fat an more carbohydrates are used.

Carbohydrates are stored in our muscles & liver as glycogen. For running a total of about 300g of glycogen can be stored in our leg muscles. About 100g of glycogen are stored in the liver which can be released into the circulation to maintain the blood glucose concentration at normal levels. This is sufficient for providing our body with approximately 1800 - 2300 kcal.

This means that our carbohydrate storage is limited and available for max 2h of training at low or medium intensity. During training that last more than 2 hours or intense trainings longer than 90 minutes or successive training (for example running in the morning, swimming in the afternoon and cycling in the evening) additional carbohydrates are needed and this mechanism can/must be trained. We often see athletes who train sober for hours & hours which causes an non-efficient combustion within the body.

Our training can be different from day to day. This means that our diets should be different from day to day to. On days with endurance training, the training effect may be better with less carbohydrates and more fats. On days with an intense training or a power training your body depends on carbohydrates.

More about how to eat right as an athlete & how many carbohydrates you need during training will come soon...

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