Intermittent Fasting & Endurance Athletes

While approaches that restrict or remove certain macronutrients – like Paleo and Keto diets – have become more popular over the past few years, diets that focus on the timing of nutrition, like intermittent fasting, have popped up in the world of endurance sports. In this week’s post, we’ll look at what intermittent fasting is and the potential pros and cons from health and performance perspectives.

What is Intermittent Fasting?

Intermittent fasting (IF) is an eating pattern where a person cycles between periods of eating and periods of avoiding food <aka fast> for an extended time. Think of the typical diet as a dimmer switch, where we dial down the amount we are eating throughout the day. Intermittent fasting is an on-off switch for eating. 

What is a typical schedule for IF?

There are many different approaches to intermittent fasting, as the fasting schedules vary and so do the number of hours. Certain kinds of IF involve time-restricted eating, while others suggest fueling normally on certain days and restricting calories or not eating at all on others. The most popular methods include 16/8 (fasting for 16 hours, eating for 8 hours/day), the 5:2 diet (eat normally for 5 days and fast for 2 days out of the week) and alternate-day fasting.

What are the health benefits of IF?

A 2019 review released via The New England Journal of Medicine concluded that intermittent fasting may help with obesity, insulin resistance, hypertension, and dyslipidemia. The authors also noted that it could increase cellular stress resistance and benefit those suffering from neurological disorders. They suggested that IF might delay the aging process and potentially extend lifespan too, although this hasn’t been proven in human trials. 

A 2022 review in Nature Reviews Endocrinology stated that in previous studies IF has led to mild to moderate weight loss (3–8% loss from baseline) over eight to 12 weeks. However, this is comparable to weight loss achieved with traditional calorie restriction. The authors stated that the ability of IF protocols to help to manage weight long-term is still poorly understood, as the majority of studies to date have run for short durations. They also noted that while some trials have shown that IF improves cardiometabolic risk factors such as blood pressure, insulin resistance, and levels of LDL cholesterol, triglycerides, and HbA1c, other findings contradict this.

How does IF affect athletic performance?

Advocates for IF claim that it can improve training and race day performance by making your body “fat adapted” – i.e., able to utilize fat more effectively. Our body prefers glucose at moderate-high intensity for fast-acting energy because it breaks down quicker than fat. It’s important to fuel adequately before and after every training session. For longer runs and races, you’ll perform your best if you fuel during longer runs and races as well.

There’s evidence that failing to do so impairs athletic performance. A paper published in the European Journal of Sport Science found that participants had less power and became exhausted faster when they followed an IF protocol before a cycling test. A team of French researchers found that continual fasting compromised endurance during a 5K time trial, which they suggested was caused by decreased muscle contraction and reduced VO2 max.

Can IF be harmful?

As we explored in a previous post, taking in too few calories consistently can predispose you to Relative Energy Deficiency in Sport (RED-S), which can lead to a whole host of problems. These include disrupted hormonal balance, decreased immunity and bone health, cardiac abnormalities, and, ironically, slowed metabolic rate. This is why IF might be counterproductive, particularly if it leads to insufficient caloric intake. 

There is also research to support that even if total energy intake is adequate for the day if we have an intra-day energy deficit that is too great women can have metabolic disturbances. This research suggests that IF is the exact pattern to avoid to keep active women’s hormones more balanced.

When you’re training regularly, your body needs more fuel, not less. So if you pair the increased activity with fewer calories, there’s going to be a mismatch between the energy your body needs and what you’re providing it. As a result, your body could start limiting essential cognitive and physical processes to try and save energy, and you might down muscles to try and provide more fuel. This is one of the main reasons I don’t recommend IF for runners, cyclists, triathletes, and other athletes.

Who should avoid intermittent fasting?

In addition to athletes being wary, IF is not recommended for anyone who’s pregnant, has type 1 diabetes, takes insulin or experiences hypoglycemia, or has a history of or current eating disorder. As emotional volatility is associated with a caloric deficit, IF might also be risky for some people with mental health challenges.

If you’re unsure about whether IF is right for you, please don’t hesitate to reach out for a consultation.

1.Rafael de Cabo and Mark P Mattson, “Effects of Intermittent Fasting on Health, Aging, and Disease,” The New England Journal of Medicine, December 2019, available online at 2.Krista A Varady et al, “Clinical Application of Intermittent Fasting for Weight Loss: Progress and Future Directions,” Nature Reviews Endocrinology, February 2022, available online at Nashrudin Bin Naharudin and Ashril Yusof, “The Effect of 10 Days of Intermittent Fasting on Wingate Anaerobic Power and Prolonged High-Intensity Time-to-Exhaustion Cycling Performance,” European Journal of Sport Science, June 2018, available online at Brisswalter et al, “Effects of Ramadan Intermittent Fasting on Middle-Distance Running Performance in Well-Trained Runners,” Clinical Journal of Sport Medicine, September 2011, available online at 5. Fahrenholtz et al, ‘Within-day energy deficiency and reproductive function in female endurance athletes,’ Scand J Med Sci Sports, Mar 2018, available online

Disclaimer: The content in our blog articles provides generalized nutrition guidance. The information above may not apply to everyone. For personalized recommendations, please reach out to your sports dietitian. Individuals who may chose to implement nutrition changes agree that Featherstone Nutrition is not responsible for any injury, damage or loss related to those changes or participation.