Fasted Training

Fasted training is training without eating before hand, for example before breakfast. This type of training has become a topic of conversation in many fitness and health circles. But the benefits of this training are hotly debated.


It is known scientifically that as exercise intensity increases, i.e. from walking to running, so does the bodies usage of carbohydrate. But does this mean that we have to consume carbohydrate before training in order to train effectively?


Fat is one fuel source that everyone has plenty of stored in their bodies. But using fats to fuel high intensity activity is often inadequate as the metabolism of fat is a slow process. As a result, the body can’t keep up with the energy demands by using fats alone if you exercise at a high intensity for along period because the body needs lots of energy quickly. Protein is another source of energy but again its metabolism is a relatively slow process as the proteins have to be broken down before they are in a form which the muscles can use.


Fasted training usually means training first thing in a morning without having had any breakfast. This means you will become glycogen depleted very quickly as your blood sugar will be low and much of your liver glycogen will have been used in your overnight fast. As you start to exercise your muscle glycogen is also used up very quickly. Fat becomes the dominant fuel source, and training in this way does improve the metabolic pathways for burning fat.

There are also other benefits of such training:

1. Improved insulin sensitivity.

  • Insulin is released when we eat to help us absorb the nutrients from our food. The hormone initiates the removal of sugars from the bloodstream and directs them to the liver, muscles, and fat cells to be used as energy.
  • However, eating too much and too often increases the bodies resistance to insulin’s effects, and while poor insulin sensitivity increases the risk of heart disease and cancer, it also makes it harder to lose fat.
  • Eating less frequently (i.e. fasting more regularly) can help, because it results in the body releasing insulin less often, so we become more sensitive to it. This then helps individuals to lose fat and also get blood to their muscles.


2. Increase of growth hormone (GH).

  • GH is a hormone that helps the body make new muscle tissue, burn fat and improve bone quality, physical function, and longevity.
  • Along with regular weight training and sleep, fasting can help to increase the body’s GH. The effect ends when the fast does, which is a compelling reason to train fasted in order to keep muscle-friendly hormones at their highest levels.


However, there are two major drawbacks to fasted training where fat is the dominant fuel:

1. It is not possible to train as quickly as would be possible on carbohydrate as the body cannot produce energy from fat fast enough.

2. The immune system relies on carbohydrate to function. Prolonged endurance exercise causes a drop in immune function regardless, and a low carbohydrate availability only makes this worse. This can leave the body more prone to coughs and colds- not ideal if you are wanting to train hard.

So, based on the drawbacks of using fat for fuel, and the benefits that can come of such training for example by improving the metabolic pathway for fat metabolism, it has been suggested that the best way to use fasted training is at selected times, such as the early season, when the sessions are of a lower intensity and performance is not so important. Ideally fasted sessions should be no longer than 60 minutes so that the immune system is not put under too much stress.


Finally, to conclude, you should not train in a fasted state for all of your sessions, and based on current research during your racing season, you should train how you race.  Therefore the “train low, race high” approach should not be followed all the time as it does not improve performance. It must be remembered that carbohydrate is the dominant fuel for performing, and therefore carbohydrate should be available to the body in training in order to develop these metabolic pathways. Additionally, everyone is different. Fasting will not work for everyone, and the only way to see if it helps you to reach your goals is to try it out.


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For more information: Burke, L.M. (2010). Fueling strategies to optimize performance: training high or training low? Scandinavian Journal of Medicine & Science in Sports, 20(Suppl. 2), 48–58.