Running Fasted vs. Running Fed

By: Diana Davis, RDN, LDN, CDE
Without Limits Dietitian/Nutritionist
September 25th, 2019

This week I want to take a look at the research on running in a fasting state. I hope that this discussion  can help each athlete consider whether the potential benefits warrant a change in fueling before a morning workout or a later day second workout, and when you might want to practice this strategy versus when you truly need to take in food/fuel prior to training.

Research with endurance athletes provides evidence for the success of this strategy. However, recent interest is centered on a concept termed “train low, compete high” in which endurance athletes train with low glycogen stores to improve their response to training but compete with high glycogen stores to enhance performance. The purpose of the “train low, compete high” approach is to enhance the body’s ability to utilize fat in energy metabolism during exercise and, thus, spare limited glycogen stores to be used most efficiently during prolonged endurance exercise or intermittent high-intensity exercise that rapidly depletes carbohydrate reserves.

The benefits of “train low” techniques include the following:

  1. Enhanced muscle and metabolic shifts in response to training, such as enhanced transcription of several genes involved in training adaptation.
  2. Greater utilization of fat to fuel exercise.
  3. Less reliance on carbohydrate as a fuel during exercise.
  4. Decreased risk of gastrointestinal problems resulting from high carbohydrate intake during exercise.

However, despite evidence of metabolic adaptations to training and promotion of greater fat burning during exercise with “train low” approaches, no clear evidence of improved performance has been shown as a result of using “train low” strategies and it is not without risks.

Potential negative effects of repeated use of “train low” strategies include the following:

  1. Increased perception of effort during training (workouts feel harder).
  2. Reduced power output during training because of lower self-selected levels of power output (lower effort).
  3. Possible increased risk of illness.
  4. Possible increased risk of overtraining.

Another study found that body composition changes associated with aerobic exercise in conjunction with a reduced calorie diet are similar regardless of whether or not an individual is fasted prior to training. Hence, those seeking to lose body fat conceivably can choose to train either before or after eating based on personal preference.


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