Fasting before a run may seem like a strategy to enhance fat burning and improve performance, but recent medical research suggests that fasting may also have detrimental effects on metabolism and performance. Fasting before a run can significantly decrease resting metabolic rate (RMR) by up to 10%. A 2021 study reported fasting before exercise depletes muscle glycogen stores by approximately 30-40%, leading to impaired endurance and performance during prolonged runs.
Drawing upon findings from recent medical journals, let's explore other impacts fasting can have on metabolism and performance.
Fasting has been shown to reduce resting metabolic rate (RMR), the number of calories your body burns at rest. A slowed metabolic rate can reduce overall energy expenditure across the day and potentially hinder weight management efforts.
Fasting shifts the body's fuel source from carbohydrates to fat, a process known as metabolic inflexibility. While this may seem beneficial for fat burning, it can impair the ability to utilise carbohydrates, which is essential for high-intensity exercise performance efficiently.
Fasting can disrupt hormone levels involved in metabolism, appetite regulation, and energy balance. For example, prolonged fasting may lead to increased production of cortisol, a stress hormone that can promote muscle breakdown and increase fat storage.
Fasting before exercise can deplete muscle glycogen stores, the primary fuel source for high-intensity exercise. Reduced glycogen supply can impair endurance, power, and overall performance during a run.
Fasting can delay recovery by limiting the availability of nutrients needed for muscle repair and glycogen replenishment. Lack of critical nutrients can prolong muscle soreness and fatigue and impair subsequent training sessions. This further impacts your motivation to train consistently and burn calories.
A slow metabolism is not good news for runners keen on losing weight. Our blog, Weight Loss for Athletes, may be helpful.
In conclusion, fasting before a run can slow metabolism, impair substrate utilisation, alter hormone levels, deplete muscle glycogen stores, and delay recovery, ultimately compromising athletic performance.
To optimise metabolism and performance, it's essential to fuel your body with a balanced meal or snack before exercise. In a previous post, we discussed some of our go-to recommendations for pre-run snacks.
At the Athlete Sanctuary, we offer comprehensive support for female endurance runners. As a degree-qualified online naturopath, nutritionist, and Olympic marathon runner, Kate Smyth brings a wealth of experience and expertise to help you achieve your goals. For personalised support and tailored meal plans, visit www.athletesanctuary.com.au.
Kate Smyth is a sports naturopath, nutritionist and female-centric running coach. She is the founder of the Athlete Sanctuary - a holistic healthcare clinic for athletes of all levels and sporting codes.
Kate has a thirst for knowledge, with two bachelor's and a master’s degree under her belt. She has been involved in sports for many decades and competed for Australia in the Commonwealth Games and Olympic Games marathons with a personal best time of 2 hours 28 minutes.
Kate’s path into high‑performance sport didn’t follow the traditional script. A late bloomer and recreational runner, she found her spark during the Sydney 2000 Olympics, watching her idols surge into the stadium. That moment ignited a commitment that would quietly and profoundly reshape the course of her life.
Eight years later, she realised her own Olympic dream, representing Australia in the women’s marathon at the 2008 Beijing Olympic Games.
Her running career spanned the Commonwealth Games, multiple Australian representative teams, and national‑level competition across cross‑country, track, and road racing. With a marathon personal best of 2:28, Kate was one of Australia’s all time fastest female marathoners.
But her journey was far from linear. Significant health challenges forced her to question conventional medicine, sports nutrition and traditional training models. What felt like setbacks at the time became turning points, pushing her to explore deeper, listen more closely to her body, and ultimately develop a more sustainable, female‑centred approach to performance.
These experiences now form the foundation of the work she shares with other women: how to train smarter, nourish deeply, honour physiology, and build resilience from the inside out.
She holds three degrees including a Masters and Bachelor of Health Science (Naturopathy). Kate is an accredited athletics coach with Athletics Australia and a member of NHAA.
Kate’s expertise is widely recognised, leading to regular invitations to speak on podcasts, at seminars, within industry education forums, and across corporate and women’s health initiatives.
