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What to do when you can't eat before running?

Feb 07, 2024

Do you often find yourself skipping pre-exercise meals due to lack of time, poor appetite or gut symptoms during running? Or do you frequently train fasted, hoping for potential benefits such as increased fat oxidation and improved insulin sensitivity. Is skipping fueling prior to running risky to your health and performance in the long-term? In this blog post, I will explore the best ways to fuel up before exercise and offer solutions to common challenges faced by runners.

What are the benefits of fueling up before running?

Carbohydrates (stored as glycogen in muscles and liver) are the primary fuel source during prolonged workouts or endurance events. Adequate glycogen stores are critical for maintaining blood glucose levels and enhancing physical and mental performance. This is particularly important for prolonged and high intensity exercise. Hydration is also essential for maintaining endurance performance, as even mild dehydration can impair exercise capacity and increase fatigue. Consuming a carbohydrate-rich meal or snack before exercise, along with proper hydration, can help maintain normal blood glucose levels, maintain blood volume and pressure, regulate body temperature and improve performance.

What are the risks of fasted training?

Fasted training has become a popular trend among runners, promising benefits such as increased fat oxidation, promoting fat loss, and improving insulin sensitivity. Training in a fasted state may indeed be beneficial for body composition and enhancing the body's ability to use fat for fuel, which can help prolong endurance performance by delaying the onset of fatigue. However, it's important to be aware of the potential risks that come with fasted training. Low blood sugar levels, decreased muscle glycogen stores, and impaired performance are just a few of the potential risks associated with fasted training, especially during extended or high intensity exercise. Furthermore, frequent fasted training can increase the risk of muscle breakdown and negatively impact immune function. While short, easy workouts may be suitable for fasted training, engaging in this practice too frequently can be a big risk to long-term health and performance.

What should be included in a pre-exercise meal or snack for endurance sports?

The ideal type and quantity of pre-exercise fueling and hydration depend on the amount of time before exercise and your individual responses. Sufficient time is needed for the meal to be emptied from the stomach, including extra time for delayed emptying that might accompany pre-event nerves. The right balance of gut comfort should be achieved, without being too full at the start of exercise or hungry late in the session. Choose a meal or snack that is easy to digest, rich in carbohydrates for fuel, low in fat, provides adequate fluids, and consists of familiar and enjoyable foods and fluids. Making sure you have appropriate pre-fueling options available will help ensure you fuel up well - keep your pantry and fridge stocked with foods and drinks that work well for you.

But my tummy doesn’t like food in it when I run! What can I do?

If you experience exercise-associated gut symptoms, you may find it difficult to eat solid food before exercise. Reduced blood flow to the gut, dehydration, and being nervous may cause stomach upset. Low-fibre carbohydrate-rich foods and drinks may be a better option, as well as experimenting with the timing of food intake. A reduced fibre intake can help prevent bloating, diarrhoea, and stomach discomfort. Eating solid meals earlier or replacing meals with liquid nutrition may help avoid problems. Commercially available liquid meals or homemade fruit smoothies are great if you have a poor appetite and can help you fuel up without feeling too full.

In addition to experimenting with the timing and composition of pre-event meals, gut training can also be used to improve tolerance to eating before exercise. This involves gradually increasing the volume and complexity of food consumed before exercise to train the gut to handle a larger load of nutrients without causing discomfort or gastrointestinal distress. This approach can take several weeks or even months and requires careful monitoring of individual responses to different foods and exercise intensities.

If you are really struggling to get fuel in prior to running, you could try carbohydrate mouth rinsing. This technique involves sipping and spitting a carbohydrate-rich drink regularly during exercise. Carbohydrate rinsing works through receptors under the tongue that recognize carbohydrates. These receptors activate regions in the brain associated with reward and pleasure, helping to maintain exercise intensity and enhance performance in a fasted state. Evidence is strongest for moderate-to-high intensity endurance exercise lasting 45-75 minutes. Consuming carbohydrate remains the optimal performance option, but in cases of an upset gut, carbohydrate mouth rinsing may provide some benefit (~2-3% better performance compared to no carbs in most studies).


Consuming carbohydrate-rich meals or snacks before sport and experimenting with the timing of food intake can help maintain normal blood glucose levels and improve performance. Ensuring adequate hydration prior to running and avoiding fasted training too often will help keep your runs enjoyable and support good performance. Athletes experiencing gut symptoms may find low-fibre carbohydrate-rich foods and drinks easier to digest. Gut training can improve tolerance to pre-exercise meals by gradually increasing the volume and complexity of food consumed. Carbohydrate mouth rinsing may help improve performance if you struggle to get any food in before exercise at all. Finding your perfect pre-run nutrition may take time but is worth the effort for achieving your next PB!

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  1. Beaven, C. M., Maulder, P. S., Pooley, A., Kilduff, L. P., & Cook, C. J. (2013). Effects of caffeine and carbohydrate mouth rinses on repeated sprint performance. Applied Physiology, Nutrition, and Metabolism, 38(6), 633-637. doi: 10.1139/apnm-2012-0363
  2. Chambers, E. S., Bridge, M. W., & Jones, D. A. (2009). Carbohydrate sensing in the human mouth: Effects on exercise performance and brain activity. The Journal of Physiology, 587(Pt 8), 1779-1794. doi: 10.1113/jphysiol.2008.164285
  3. McCall, A., & Davison, G. (2019). Carbohydrate mouth rinse and exercise performance: A systematic review and meta-analysis. Sports Medicine, 49(5), 813-832. doi: 10.1007/s40279-019-01086-3
  4. Ørtenblad N, Westerblad H, Nielsen J. Muscle glycogen stores and fatigue. J Physiol. 2013 Sep 15;591(18):4405-13. doi: 10.1113/jphysiol.2013.251629. Epub 2013 May 7. PMID: 23652590; PMCID: PMC3784189.

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