Should I Run Without Eating Breakfast?

Ah, breakfast. It’s the most important meal of the day, right? Yet sometimes you oversleep and you’re in such a rush to get out the door and go on a run that you forget to eat breakfast. Your tummy is rumbling at the starting line, and you’re not sure what to do. Should you munch on something quick or can you forego breakfast when running?

Running without having eaten breakfast, which is known as fasted running, could improve your endurance, reduce your need for high energy intake, and boost your fat burning. You must be careful though, as the lack of energy from skipping breakfast can have adverse effects on your running performance as well.

Ahead, we’ll explain fasted running in full, including the benefits and downsides. Fasted running isn’t for everyone, and by the time you’re done reading, you’ll know whether you can safely exercise on an empty stomach.

Let’s begin! 

What Is Fasted Running? 

The concept of fasted running came up in our article on whether you should run right after waking up, which is certainly worth a read if you missed it. Fasted running is a specific type of fasted workout.

Although many of us associate fasts with periods of starvation, in actuality, you fast every day. When you go to sleep at night, that’s at least seven hours that you’re not eating. If you’re the type who goes about their morning routine before sitting down to breakfast, then maybe you fast for eight consecutive hours. If you skip breakfast entirely and then go for a run, by the time you’re done running, you get home, and you shower, perhaps you’ve fasted for 10 or 11 hours.

When you’re in a fasted state, your glycogen levels begin depleting. Glycogen is the converted form of glucose, a simple sugar that our bodies make from the foods we consume. Glucose is our energy, and since you haven’t eaten for so long when you’re doing fasted running, you haven’t supplied any of that energy to your body. 

Although this doesn’t sound good, it does have its perks, which we’ll discuss in the next section. Yet we want to stress what we mentioned in the intro as well, which is that fasted running is not right for everyone no matter how advantageous it can be.

The Benefits of Fasted Running 

Let’s talk about why running without eating breakfast can be good for your body. 

No Issues with Indigestion

If you remember our article on the foods to avoid before a run, we recommended skipping legumes, broccoli, artichokes, apples, pears, cheese, red meat, bacon, and spicy foods. The reason lots of these foods are inadvisable to eat before a run is that they take a long time to digest. 

Generally, the fattier and/or more protein-heavy food is, the longer it takes your body to process it. Oatmeal, for example, is a fast-digesting food while bacon is not. Although slow-digesting foods provide more satiety since they remain in the body longer, they’re also a runner’s worst nightmare. You’ll feel weighed down and sluggish. 

While sure, you could eat more oatmeal or chicken breast, yogurt, or even a banana before your run if you wanted food for quick digestion, even these foods can cause indigestion in some instances. The best way to ensure you’re at no risk of indigestion is to skip breakfast altogether. 

Could Increase Your Aerobic Endurance

What is your aerobic endurance as a runner? This term goes by other names as well, such as cardiovascular endurance or aerobic fitness. All refer to how long you can exercise without getting tired. Your level of aerobic endurance depends on how much oxygen your body uses while you’re engaging in physical fitness like running.

As you age, your aerobic endurance begins decreasing. If you’re in your 30s, this endurance is already going down. Fortunately, you might be able to give your sagging aerobic endurance a must-needed boost if you skip breakfast before a run every now and again. 

Remember, aerobic endurance is tied to your oxygen levels. VO2 max is a measurement that determines the levels of oxygen you use as you exercise. Studies such as the Journal of Science and Medicine in Sport in 2010 have suggested that your VO2 max levels can increase with less food in your belly.

Their study had 14 participants, six of them males and eight of them females. All participants were around 26 years old. The participants were split into two groups, one that ate before they exercised and the other group that didn’t. Then the participants engaged in endurance cycle ergometer training for five days a week over four weeks.

The group that fasted had elevated VO2 max rates regardless of their gender. While this is certainly a promising outcome, experts agree that more studies in this area must be done before we can draw conclusive evidence that foregoing food makes your aerobic endurance better. 

Might Lessen Your Energy Intake

Your energy intake is how much energy you make from what you eat, which is expressed as kilocalories. If you’re trying to lose weight, you’d want to control your energy intake, as the higher the rate of energy intake, the more food you’re eating. 

Two studies have shown that fasted running might help you do just that. The first is from The Journal of Nutrition that was published in 2019. The study involved 12 participants, all male, and each about 23 years old. The participants underwent three different trials with one week between them all in random order. 

First, they ate breakfast, only milk and oats. Then they’d rest for a while and engage in exercise. Later, they fasted overnight and then exercised without breakfast. When the male participants didn’t eat breakfast and then exercised, their energy intake rates were reduced for more than 24 hours. 

Another study on the topic comes from 2016 in the Journal of Nutrition and Metabolism. There were 12 participants in this study, all of them males around 21 years old. They too had less energy intake for 24+ hours after running without eating breakfast. 

The small sample sizes in both studies (and the lack of female participants) show that there’s still more work to be done in this area, but the potential is certainly there! 

May Burn More Fat

The reason that fasting is attractive to so many is that you can burn fat instead of glycogen. When you don’t supply glucose to the body, you need to get your energy elsewhere. The liver stores leftover glycogen, which is what your body will use first. When even that is gone, then you’ll start burning fat instead. 

It’s possible that foregoing breakfast might help you burn more fat when running than having a meal first. A 2017 edition of PLoS One examined this link. The researchers noted that, among their group of 10 participants, that the ones who exercised before eating had greater fat oxidation for more than 24 hours.

The Risks of Fasted Running

Now that you know what benefits you might be in for when fasted running, you should compare those with the following health risks. 

Won’t Help You Lose More Weight Than Eating and Then Running 

We mentioned before that you can lower your energy expenditure through fasted running, which might help you shed a few pounds. Yet if you think that you can see the weight melt off exclusively by running without breakfast, that won’t be the case. 

The Journal of the International Society of Sports Nutrition, in a 2014 report, reviewed the weight loss of a group of participants, 20 in all. Some of them exercised after a filling breakfast and others skipped breakfast and exercised first. The researchers report that how much weight the participants lost didn’t seem to vary much between the two groups. 

This doesn’t mean that fasted running won’t lead to weight loss, just that your weight loss potential isn’t more exceptional than if you ate breakfast. The most reliable way to lose weight is to burn more calories than you consume through running and other forms of exercise. 

Could Lead to Muscle Loss

Runners are already more prone to muscle loss compared to those who choose other means of exercise, and fasted running could accelerate that even more. That’s what this Journal of Physical Therapy Science study from 2015 states. 

The reason? It comes down to cortisol, a hormone from your adrenal glands that impacts your blood glucose levels and the health of your muscle cells. Not only does increasing cortisol levels in the body make you more stressed, but your muscle cell proteins begin depleting, leading to smaller and weaker muscles. 

Compared to other hours of the day, cortisol levels are more elevated upon waking. The study above says that running on an empty stomach (or doing any other kind of exercise without breakfast) can further boost cortisol. 

After one run or even a couple, the effects of the muscle loss shouldn’t be pronounced. Yet if you make fasted running a habit over many months, the possibility exists that your muscles might be even smaller for it. 

Increases Your Risk of Injuries

Food is energy. Without it, it can be difficult to awaken from the brain fog that a groggy morning leaves you in. This mental fogginess can lead to more mistakes. Perhaps you forget to stretch and so you strain a muscle. Maybe your running form is incorrect and you end up sore for days afterward. The injury risk is there, which is something to consider before lacing up your running shoes. 

Your body doesn’t only use glucose, but your brain does as well. The energy source keeps your brain capable of making quickfire decisions. Thus, without breakfast, your brain-body connection becomes muddied. 

Can Negatively Affect Your Performance

Even if you’re not doing a marathon or a race, you still want to run your best. Fasted running might not help you achieve your goals in this respect. 

In a 2018 publication, the Scandinavian Journal of Medicine & Science in Sports published the results of a collection of more than 45 studies. The general consensus was that eating and then exercising will help increase your aerobic abilities for longer, better fitness performance. 

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Should You Try Running Without Eating Breakfast? 

Now comes the question, is fasted running right for you? 

If you have certain health conditions, then the answer is no. Addison’s disease is one of these. This adrenal gland condition limits how much aldosterone and especially cortisol that your body can make. Fasted running can lead to plummeting blood sugar levels for those with Addison’s disease, which can be risky for your health.

Type 1 and type 2 diabetes patients will also have to skip fasted running. With both conditions, if you don’t eat for long enough, your blood sugar begins to drop. Should blood sugar levels get too low, you could have hypoglycemia. The problem though is that a lot of symptoms of hypoglycemia just look like side effects of running or exercising. These include hunger, elevated heart rate, sweating, and shakiness. 

If you don’t have the above health conditions, then yes, you can try fasted running. Only you can decide if the benefits outweigh the negatives. If you decide that fasted running doesn’t feel good to you, then you can at least say you tried it. 

Perhaps you like feeling lighter when running without breakfast, but you can’t stand how tired you are and how your running performance suffers. You don’t have to eat a whole meal before you run, even if it is first thing in the morning. Try noshing on a small snack pre-run and then having a bigger breakfast (or brunch) post-run. 

Final Thoughts 

Fasted running is a form of running without eating breakfast. There exist many purported benefits of fasted running, including fat burning for weight loss. If you’re eligible for fasted running, we hope the pros and cons in this article help you decide whether you want to give it a try! 

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Geoff Southworth

I am a California native and I enjoy all the outdoors has to offer. My latest adventures have been taking the family camping, hiking and surfing.

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