Can Fasting Before Exercise Improve Sports Performance?

April 10, 2020 ,

We often hear that it is important to have a snack or to eat carbohydrates before a workout in order to have enough energy. But could it also be beneficial for sports performance to train on an empty stomach? Let’s shed some light on this question! meal plans for endurance sports

Scientific evidence clearly shows that carbohydrates are the main source of energy for the body and muscles. However, this does not mean that it is absolutely necessary to always consume carbohydrates before and during exercise.

When exercising at a low intensity (e.g., walking or jogging slowly), the body mainly uses lipids (fat) for fuel. The more the intensity of the exercise increases, the more the body uses carbohydrates as a source of energy. This is because at high intensity, lipids are a less effective fuel than carbohydrates since they require more oxygen to perform the same effort. It is therefore important for optimal performance to consume carbohydrates before, during and after high-intensity workouts and events, for example interval training, marathons or a triathlons.

On the other hand, training with a low carbohydrate availability can help create adaptations such as allowing the body to become more efficient at using its fat stores for energy during exercise, which can in turn help improve athletic performance. However, it should be noted that training with low carbohydrate availability will affect the quality of the training and that a longer recovery time may be necessary. Thus, it is important to use this method for workouts of shorter duration (60 minutes or less) that do not require a high intensity. This is called carbohydrate periodization, i.e. determining when to consume carbohydrates and when not to consume them when planning an exercise schedule. Train Low refers to training with low carbohydrate availability. There are different ways to do this:

  • Training on an empty stomach in the morning before breakfast;
  • Performing two workouts in the same day, where you do not consume carbohydrates (or consume a very little amount) after the first workout, so that the second workout is performed with low glycogen stores;
  • Doing an extended workout (for several hours) without consuming carbohydrates;
  • Exercising before bed and not eating carbohydrates until after working out the next morning (Sleep Low).

In short, to improve athletic performance, it can be beneficial to fast before a short and low-intensity workout in order to get the body used to using fat more effectively during exercise. On the other hand, before, during and after more intense or prolonged exercises, it is important to consume carbohydrates in order to optimize performance and recovery. It should be noted that a lack of carbohydrates during high-intensity exercises can also contribute to weaken the immune system and increase the risk of infections.

Fasting before exercise in order to lose weight?

When it comes to weight loss, it doesn’t matter if you’re training on an empty stomach or not, it all depends on your total caloric intake. A meta-analysis showed that intermittent fasting was no more effective than a traditional caloric restriction for weight loss. In addition, consuming carbohydrates allows you to train at a higher intensity and thus burn more calories in total as compared to training on an empty stomach.

If you need help knowing what to eat before, during and after your workouts, don’t hesitate to consult with a dietitian who specializes in sports nutrition. SOSCuisine also offers a Sports meal plan specially balanced for athletes, active people and active families.


  • Headland et coll (2016). Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients; 8(6) : pii: E354.
  • Burke et coll. (2018). Toward a Common Understanding of Diet-Exercise Strategies to Manipulate Fuel Availability for Training and Competition Preparation in Endurance Sport. Int J Sport Nutr Exerc Metab; 28(5):451-463.
  • Jeukendrup (2015) 6 Ways to “train-low”.


Kathryn Adel
Kathryn holds a Bachelor Degree in Nutrition as well as a Bachelor and a Master Degree in Kinesiology, all from Laval University. She is a Registered Dietitian and active member of the Ordre professionnel des Diététistes Nutritionnistes du Québec (ODNQ) and of the American Academy of Nutrition and Dietetics. She holds the Monash University's certification for the FODMAP diet and IBS, and has considerable clinical experience in that area. She is also an accomplished athlete, having ran track and cross-country at a national level. Kathryn specializes in sports nutrition, weight loss, diabetes, as well as heart and gastrointestinal health.

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