Intermittent Fasting & Endurance Athletes

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. 

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.

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.

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.

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.

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.