Intermittent Fasting For Runners: A Good or Bad Idea?

Intermittent fasting, or IF, is becoming one of the most popular weight-loss solutions. It sees men and women worldwide fast for predetermined amounts of time to reduce their calorie intake and lose weight. 

It is, however, also the subject of many debates over the safety of this weight-loss method, and more so for those who lead an active lifestyle. Is intermittent fasting safe for runners? What are the potential health risks associated with the calorie-reduction diet that you should know?

There are several things you need to consider if you wish to combine intermittent fasting with a physically demanding hobby. It’s just not a matter of whether you feel comfortable running on an empty stomach!

Let’s take a further look…

What is Intermittent Fasting?

Intermittent fasting for runners

Intermittent fasting is, in essence, a reduction of the overall calorie intake for those looking to lose weight and burn body fat fast. With celebs like Jennifer Aniston, Miranda Kerr, and model Heidi Klum raving about this weight-loss method, many are rushing out to try it. 

When choosing intermittent fasting as a weight-loss method, there are many protocols to consider. While it is a controversial topic that has health experts divided, the lack of scientific data surrounding the efficacy and potential health risks is enough to confuse anyone.  

Types of Intermittent Fasting

Intermittent fasting takes on many forms and includes the following most popular types:

Alternate Day Fasting

This type of intermittent fasting allows you to eat every alternate day, fasting on the others. 

Overnight Fasting

Overnight fasting is relatively easy and involves fasting for 12 hours overnight. 

The 16:8 Method

This method of intermittent fasting allows you to eat for 16 hours and fast for the remaining eight. 

The 5:2 Method

The 5:2 method sees those you use the method to eat for five days of the week and fast for the remaining two. You can choose which days are for fasting and which are for eating. 

Some reports indicate intermittent fasting decreases body mass and body fat, while others suggest that reducing calories causes fatigue, dizziness, and headaches. So who do we believe? 

The Pros & Cons of Intermittent Fasting for Runners

Any diet or weight-loss plan has several positive and negative aspects that should be considered before diving straight in. For runners, reducing calories could lead to a decrease in performance and have adverse effects on their body fat and mass. 


Let’s look at the pros of IF for runners:

Pro #1: An increase in muscle mass

Science shows that reducing calories will increase muscle mass for those who lead an active lifestyle. Building muscle is key for runners, so a fasting diet will benefit them greatly. 

Pro #2: Oxidation

Oxidation occurs when the body uses fat for energy rather than carbohydrates, resulting in a high-fat burn and weight loss. 

Pro #3: A Decrease in Body Fat

The goal of most eating plans or diets is weight loss. As IF focuses on calorie reduction, the body uses fat and stored glucose as energy, resulting in quick, efficient weight loss. 

Pro #4: A Reduction in Blood Glucose Levels

IF is a great option for those who want to regulate or lower their blood glucose levels. The diet plan has been proven to improve insulin resistance, resulting in better health for runners. 


Now let’s take a look at the cons of IF for runners:

Con #1: There is no improvement in a runner’s performance. Reducing calories and losing weight might be a driving factor for people who want to lose weight and look great. But, for runners, there is no scientific evidence to suggest that intermittent fasting impacts running performance or pace. 

Con #2: The diet can be tricky to follow. If you choose one of the intermittent fasting types mentioned above, be sure that you adapt your training program accordingly. A good running or training program should include a rest or recovery period after each session, usually when you have a small snack to regulate blood sugar levels, start recovery, and balance your metabolism. Ensure that the recovery period is not in the fasting portion of the diet. 

Con #3: A decrease in blood glucose levels. This con may seem odd as a decrease in blood glucose levels is also a pro of intermittent fasting. Those with high blood sugar levels will benefit from IF, while those within the “normal” limits could likely suffer from headaches, fatigue, and nausea from the calorie reduction. As mentioned above, adapt your training schedule to allow a snack while running and reduce the risk of injury or illness from fasting while running. 

Con #4: Interference with hunger cues. IF can interfere with your body’s hunger and satiety cues that tell you when to eat and when to stop. Unfortunately, intermittent fasting can result in overeating or disordered eating for many people.

Con #5: IF is not compatible with a conventional fuelling strategy for race days. Your competitors will likely be indulging in the many foods that can increase their stamina (and boost their performance). It will be much harder to compete with a similarly trained but better fuelled athlete.

The Health Risks Associated with Intermittent Fasting

Intermittent fasting schedule for runners

Intermittent fasting is often used as a weight loss tool. While many find it effective, a few health risks stem from this popular calorie-reduction eating plan. While the long-term effects are unknown, there are a few things that health experts and medical professionals need you to be aware of.

The Danger of Eating Disorders

Eating disorders are no joke and a very real risk for those who dive into intermittent fasting too quickly. Calorie reduction can cause hunger and fatigue. Ignoring our body’s cry for help for proper nutrition could result in overeating or binging, which often leads to bulimia. 

Losing weight is always a good sign that an eating plan or calorie-reduction diet is going well. But do you know when enough is enough? Intermittent fasting has quick results, and for those who have battled to lose weight in the past, the urge to continue losing weight well after you have hit your target could lead to anorexia. 

Constipation, Fatigue, and Dizziness

A healthy, balanced diet is necessary for sustained energy and keeps you focused and “regular.” Intermittent fasting has been known to cause constipation, probably due to a lack of carbohydrates and fiber. While this is not “dangerous” as such, it is uncomfortable. 

Fatigue and dizziness are to be expected when you eat a reduced-calorie diet. Fatigue is likely caused by reduced levels of glucose in the body, which is also known to cause dizziness. 

Increased Cortisol Levels

Cortisol, more commonly known as the stress hormone in the body, is increased when the body is put to the test on a low-calorie eating plan like intermittent fasting. This increase in cortisol can increase blood glucose, cholesterol, and blood pressure. These side effects can all increase your risk of heart attack, stroke, and weight gain.

The Risks for Runners

While we have discussed the negative aspects of intermittent fasting for runners, it is necessary to say that there is no definitive data or research that categorically states that IF should be avoided when running. 

IF can be successfully used to aid weight loss, but it needs to be carried out carefully. There are various methods of incorporating IF into your running and health regime, so choose the best for your needs as a runner.

Here are a few important Dos and Don’ts that will help you on your intermittent fasting journey:

DO Start Slowly

Rather than diving head-first into a strict diet, ease your way into it and choose a fasting method that is not a massive shock to your system. 

DO Stay Hydrated

Hydration is key to helping stave off hunger and dehydration while running. This is even more important if you are doing intermittent fasting, and it can help curb those cravings that crop up. 

DON’T Binge

Binge eating negates the principles of intermittent fasting and will have you piling on the pounds you have worked to get rid of. Try cutting down on the number of meals you eat gradually before you commit to a strict IF plan. 

DON’T Clock-Watch

Don’t stare at the clock as you wait to be able to eat again. Set a timer on your phone and try to keep yourself busy while waiting for your fasting period to be over. Focusing on the clock and watching the minutes tick by slowly can be frustrating, so try not to think about it too much. 

Our Verdict on Intermittent Fasting For Runners

Intermittent fasting for runners is a good idea if you want to lose weight fast. Fasting allows the body to use the glucose stored as fat for energy, reducing weight and fat considerably. 

You can indeed take part in an intermittent fasting diet as a runner, but note that it has no scientifically proven impact on your performance. 

Start slow and find an intermittent fasting method that fits well with your current training program, taking your recovery period or post-run snack into account. 

If you are unsure whether intermittent fasting is for you, speak to your doctor or trainer for tips on successfully incorporating intermittent fasting into your training schedule.

Author Profile

Thalia Oosthuizen

Photo of author
Thalia started running during the the pandemic as a way of getting out of the house. The running bug bit, and now much of her life revolves around everything to do with running - videos, podcasts, studies, books, articles, and interviews. She's also done several courses on running nutrition and mechanics to aid in her training and advising others.
Thalia Oosthuizen

Revel SPorts Contributor

Leave a Comment