Fasted Running: How To, Benefits, + Drawbacks

Fasting in intervals is becoming a popular topic in the world of health and wellness. Why has this topic become a topic of conversation? Is it possible to engage in fasting without risking any harm, particularly for athletes? Research the advantages and disadvantages of employing an intermittent fasting diet while running.

Named the most popular diet of 2020 in a study by the International Food Information Council, intermittent fasting (IF) has become a favorite way for many to manage their weight and improve their health.

Contrary to most other diets, intermittent fasting puts more focus on

when
you eat rather than

what
you eat. Though it can be done several different ways, all types of intermittent fasting incorporate periods of eating and fasting throughout the day or week.

While depriving your body of essential nutrients and energy doesn’t seem healthy, preclinical studies and clinical trials show that intermittent fasting has broad benefits for many health conditions, including obesity, diabetes, heart disease, certain cancers, and neurologic disorders.

Given its potential to improve total body composition through weight loss, intermittent fasting has also gained in popularity among athletes seeking to get lighter, leaner, and take their athletic performance to the next level.

But can intermittent fasting be beneficial for runners, or does it do more harm than good?

Before we explore how intermittent fasting affects running performance, let’s take a closer look at what it is, what happens to your body when you fast, as well as some pros and cons of IF.

What is intermittent fasting?

Intermittent fasting is a dietary approach centered around eating and fasting for specific periods of time throughout the day or week. Many believe this dietary approach more closely mimics the eating patterns of our hunter-gatherer ancestors, who frequently had to go without food for extended amounts of time.

Unlike other popular diets that have strict food and nutrient requirements, time is the only real restriction with IF — which many people seem to like.

Types of intermittent fasting

All variations of intermittent fasting split either the day or week into eating and fasting periods. Here are the three most common types of intermittent fasting:

Time-restricted feeding (TRF): Commonly referred to as the 16:8 Method, time-restricted feeding typically involves limiting your daily eating window to 8 hours followed by a 16-hour overnight fast.
Whole-day fasting (aka the 5/2 Diet or Eat-Stop-Eat): With whole-day fasting, you typically fast for two, non-consecutive days each week, and eat normally the other five days. Some whole-day fasting programs like the 5/2 Diet allow you to eat 500-600 calories on fasting days, while others, like Eat-Stop-Eat, recommend zero energy intake for a full 24 hours.
Alternate day fasting: With alternate day fasting, you fast every other day, typically eating freely on the non-fasting days, and consuming about 25% of your daily calorie needs on fasting days.

What happens to your body when you fast

Whether you choose time-restricted feeding, whole day fasting, or alternate day fasting approach, your body moves through a fed-fast cycle resulting in changes to both your hormone levels and metabolism. Here’s what happens to your body when you fast:

Fed State (0-3 hours after your last meal)

In the fed state, your body digests and absorbs nutrients from food. As your body absorbs nutrients, blood sugar levels and insulin secretion increase. Insulin helps transport glucose (sugar) from the blood into cells to be used or stored for energy. Excess glucose that is not immediately used for energy is either stored as glycogen in the muscles and liver, or converted into triglycerides and stored as fat.

During and immediately after a meal, there is a natural shift in your body’s hunger (ghrelin) and satiety (leptin) hormones. Just before a meal, ghrelin levels increase to stimulate hunger. After a meal, ghrelin levels decrease while leptin, your primary satiety hormone, increases, helping you feel full and satisfied.

Early Fasting State (3-18 hours after your last meal)

The size and composition of your last meal affect how long you remain in the fed state, but typically your body will begin to enter the Early Fasting State 3-4 hours after your last meal.

As your body transitions from fed to early fasting, insulin levels decrease in the absence of carbohydrate (sugar) from food. Meanwhile, ghrelin increases as leptin decreases.

In the absence of food, your body begins to tap into its energy stores. Muscle and liver glycogen are used first, and converted back into glucose. As glycogen stores become depleted, the body ramps up lipolysis, converting fat into triglycerides as an alternate fuel source. As lipolysis increases, the body also breaks down protein (including muscle tissue) as a source of glucose for the brain and red blood cells which rely heavily, or exclusively, on glucose for energy.

Fasting State (18-48 hours after your last meal)

Around the 18 hour mark, your body will go into full-on fasting mode. At this point, your body’s glycogen stores are fully depleted, leaving fat and protein its main sources of energy. With insulin levels low, some fatty acids in the blood are converted into ketones and, at this point, your body starts to transition into ketosis. How fast this happens depends on the composition of your usual diet and your last meal. If you typically lean lower-carb, ketosis will kick in on the earlier side, around 18-24 hours.

Prolonged Fasting (48+ hours after your last meal)

Fasting for more than 2 days (48 hours) is considered prolonged, or long-term, fasting. During this stage, the body relies increasingly on ketones and muscle protein for sustained energy. Going without food for this amount of time is not recommended for most people, and should only be done under medical supervision.

Adaptive cellular responses to fasting

In addition to the hormone and metabolic changes, fasting also triggers some beneficial adaptive cellular responses that lead to reduced inflammation, oxidative stress, improved stress resistance, and the repair and/or removal of aging or damaged cells. These cellular changes can improve metabolic, cardiovascular, and cellular health, as well as counteract aging and disease processes

What Are The Benefits Of Fasted Running?

There are several potential benefits of fasted running. The benefits of running on an empty stomach include the following:

Fasted Running May Increase Fat Burning

It has been demonstrated that running while fasted can raise the share of fat being burned while running, implying that a larger portion of the energy that is being expended is from stored body fat rather than from reserved muscle glycogen.

In order to move your body through any kind of physical exertion, your muscles will require energy.

This energy (ATP) is generated by metabolizing the fuel that your body has obtained from the food you have ingested through your dietary intake.

Any carbohydrates consumed past what is needed by the body are converted into glycogen and deposited in the muscles and liver.

If your body has reached its maximal glycogen capacity, dietary fat and unneeded sugars are stored as triglycerides in fat cells. Protein then develops the structure of muscles fibers.

The amount of glycogen stored in the liver and muscles is restricted, and these supplies diminish during the stretch of time where one is not eating.

The Academy of Nutrition and Dietetics notes that athletes trained for a long period are able to retain up to 1,800-2,000 calories of energy, stored as glycogen in their muscles and liver. For smaller runners, the amount stored is usually around 1,500 calories.

Consequently, when you go for a jog in the absence of having had something to eat, your body tries to save the small amount of glycogen left by trying to use fat as a fuel source instead.

It is vital to understand that during fasted running, you are not expending more energy on average, just tapping into fat and carbohydrates at a different rate.

Fasted Running Can Reduce Digestive Distress

People who have delicate digestive systems usually find it beneficial to work out with no food in their stomachs in order to avoid cramps, pains in the side, flatulence, diarrhea, and feeling swollen.

Research has discovered that it is more probable to feel nauseous when doing intense physical activity if you have recently eaten before exercising.

When you work out, the flow of blood is diverted away from the digestive tract to provide additional oxygen to the muscles that are being used.

Due to this, the digestive process basically stops, meaning that anything in your stomach or intestines just stays there.

This can upset your digestive system, bringing about queasiness, abdominal swelling, and flatulence. This can prompt intestinal convulsions that prompt an abrupt craving to defecate amid your run.

Runners with delicate stomachs can find that even little bites can be disruptive to their digestive system. In such situations, running while having not eaten may be the preferable choice.

Fasted Running May Lead to Greater Weight Loss

Though running before eating won’t help people burn more calories, some research has revealed that individuals who choose to work out in this way will end up eating less over the course of the day.

It looks as though running on an empty stomach might make it easier to control one’s appetite by reducing the levels of ghrelin, the hormone associated with hunger, more than running following a meal.

Therefore, while running without having eaten beforehand will not result in more weight loss because it doesn’t burn more calories than running after eating, it’s possible that running on an empty stomach could help you lose weight faster by influencing your calorie intake.

The ability to shed weight is greatly linked to the balance between the amount of calories you take in and the amount of calories you expend every day, so if you practice fasting running, which results in you eating fewer calories than you would regularly have done, it is a possible way to facilitate a substantial weight loss.

It can be said that distinct joggers have different kinds of hunger responses when running on an empty stomach.

A few participants in running might notice that their hunger remains strong even afterwards, which may indicate that the calories burned off in the morning by running without having breakfast beforehand have caused a big deficit.

Fasted Running May Improve Blood Sugar Regulation

Runners typically worry about having a low blood sugar level before attempting fasted running, which is known as becoming hypoglycemic.

This can lead to exhaustion, dizziness, migraines, impatience, queasiness, and impaired ability.

Research has surprisingly found that fasted exercise does not lead to considerable drops in blood sugar, even for those with diabetes. Studies have suggested that working out without having eaten anything can actually augment insulin sensitivity and maintain blood sugar levels.

Despite these findings, if you have diabetes, you should consult your healthcare provider before trying fasted running.

The Drawbacks Of Fasted Running

Although there are some possible pros of fasted running, there are also potential drawbacks of running on an empty stomach, including the following:

Fasted Running Does Not Increase Fat Loss

A lot of people are inspired to participate in running on an empty stomach in the conviction that this action will result in greater reduction in fat or positive modifications in their body make-up.

Studies have demonstrated that regardless of whether cardio exercise is done when one is fed or not, the same amount of fat loss and modification in body composition are seen.

A review of five studies concluded that doing aerobic exercise in the morning after going without food for a twelve hour period did not have a major influence on the amount of weight lost or on changes to the body fat percentage.

Hence, it is essential to set realistic goals and be aware that running on an empty stomach is more of a mistaken belief than a fact in reality.

Fasted Running Can Increase Cortisol Levels

The hormone cortisol is a primary cause of stress in the human body. Studies have shown that prolonged exposure to high cortisol levels can cause fat cells to accumulate in the stomach.

Working out, going hungry, or skipping meals for an extended period of time can all put stress on the body.

Hence, research indicates that the association of running and not consuming anything before may lead to an important increase in cortisol levels.

Fasted Running May Cause Hormonal Imbalances

It appears that exercise when fasting can boost amounts of cortisol while also potentially contributing to the disruption of other hormones.

Your hormone levels can affect the chances of you getting injured, how quickly you are able to recover from a workout, and how much your body is able to benefit from it.

Hence, the possible effects of messing with your hormones due to exercising without eating should not be disregarded.

The Pros And Cons Of Fasted Running: Should You Run On An Empty Stomach?

When looking at the potential benefits and drawbacks of sprinting, most athletes discover that the minor time savings isn’t worth the exhaustion that comes with it.

Particularly if you are focused on your capabilities, bettering your times and getting stronger and swifter, this holds true.

Alternatively, if you endure a great deal of intestinal problems or don’t notice a decrease in strength or exhaustion while training on an empty stomach, this could be an option that is manageable and safe.