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Why do marathon runners need carbohydrate?

This question is about Running
Sarah Achleithner
Marathon runners need carbohydrates to maintain output and blood sugar during a long distance run [1]. Working muscles prefer carbohydrates as fuel, and starting a run with adequate carbohydrate stores, and continuously replenishing carbohydrate while you’re running, has been shown to improve performance and delay fatigue. To that end, carb loading, or taking in more carbohydrates in the days leading up to a race (up to 10-12 g/kg of body weight), has also been shown to be beneficial for endurance athletes such as marathoners [2]. 
Inadequate carbohydrate intake before and during a long distance run can result in decrements in speed, duration, and energy levels, a phenomenon known as “hitting the wall” [1, 3]. Contrary to popular belief, low carbohydrates diets have not been shown to enhance training outcomes. Instead, low carbohydrate eating may even impair running performance if carried out for extended periods [2]. 
Carbohydrate food sources

References:

  1. Rothschild, J. A., Kilding, A. E., & Plews, D. J. (2020). What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions. Nutrients, 12(11), 3473.https://doi.org/10.3390/nu1211347
  2. Burke L. M. (2007). Nutrition strategies for the marathon : fuel for training and racing. Sports medicine (Auckland, N.Z.), 37(4-5), 344–347. https://doi.org/10.2165/00007256-200737040-00018
  3. Rapoport B. I. (2010). Metabolic factors limiting performance in marathon runners. PLoS computational biology, 6(10), e1000960. https://doi.org/10.1371/journal.pcbi.1000960