Should You Try Fasted Running? Here’s What the Science Says

If you’re an early morning runner, you’ve probably wondered if you should eat before your run or go out on an empty stomach. This ongoing debate sparks passionate discussions within the running community, with proponents highlighting benefits like improved fat adaptation and weight loss, while opponents raise concerns about performance, health, and weight gain.

Pro ultrarunner Kilian Jornet has publicly discussed occasional use of fasting before easy, low-intensity runs to understand how his body adapts to running with depleted glycogen (carb) stores. Meanwhile, other pro runners, like David Roche, attribute the “high carb revolution” as the primary catalyst for the increasing rate at which endurance running records are being broken.

So, what’s the real story behind fasted running, and is it a strategy worth considering for your training?

The Physiology of Fasted Training

Starting your run first thing in the morning without eating is the most well-known method of fasted training. Science indicates that exercise performed when glycogen stores are reduced (particularly in the liver) creates specific physiological effects on fuel utilization and metabolic adaptations. Specifically, the body relies more on fat as a fuel source.

Carbohydrates are stored as glycogen, predominantly in the muscles and liver.

Glycogen storage is limited to around 600 grams of carbohydrates (or 1,200 calories), which is enough for 90 to 120 minutes of continuous, intense activity. Unlike glycogen, the body can store up to 100,000 calories in the form of fat in muscle fibers and fat cells—enough for over 100 hours of running! Fat serves as the primary fuel source for the muscles during low- to moderate-intensity exercise. As exercise intensity increases to moderate-to-high, carbohydrates become the muscles’ predominant fuel.  The liver is unique in its ability to release glucose into the bloodstream, thanks to a specific enzyme that facilitates glucose transport across cell membranes. This is very important because the brain relies almost entirely on blood glucose to function. Skeletal muscles primarily use glycogen as fuel because they lack the enzyme needed to convert it into blood glucose. As a result, glycogen is quickly converted to glucose in the muscle cells, which rapidly fuels muscle contractions during exercise.

Given the body’s limited carbohydrate reserves in the muscles and liver, glycogen depletion during extended exercise is a significant contributor to physical and central nervous fatigue, respectively. Because the body has nearly limitless fat reserves in muscle and adipose tissue, fasted training is theorized as a way of teaching the body to better utilize fat for fuel, which preserves glycogen, boosts fat burning, and delays fatigue.

Is fat burning too good to be true? Let’s explore the potential benefits of fasted running.

Potential Benefits of Fasted Running

Improved fat adaptation

The primary supported benefit of fasted training is improved fat adaptation, meaning your body gets more efficient at burning fat as fuel. By training in a low-glycogen state, the body becomes more efficient at releasing fatty acids from adipose tissue, moving them to muscle cells and using them for energy. The process of fat burning has several components and regulatory mechanisms, and a critical component of fat oxidation is oxygen. Therefore, fasted training should only be used during low-intensity, aerobic training sessions. However, just because the body uses more fat when running in a fasted state, this doesn’t necessarily equate to weight loss.

Improved insulin sensitivity

Carbohydrate consumption leads to the breakdown of carbohydrates into glucose. Glucose then enters the bloodstream, resulting in elevated blood sugar levels. The pancreas detects this increase and releases insulin. Insulin facilitates the entry of glucose into cells for immediate energy utilization or storage for later use. Over time, in certain instances, cells may show less responsiveness to insulin (aka insulin resistance), limiting glucose uptake and leading to persistent hyperglycemia (elevated blood glucose). So fasted running may increase insulin sensitivity, which could help keep blood sugar levels in check.

Potential Disadvantages of Fasted Running

Performance impairment

Moderate-to-high-intensity exercise relies significantly on carbohydrates. Performing intense efforts in a fasted state can lead to early fatigue and impaired recovery. In long-duration activity, liver glycogen is critically important as its depletion leads to a decrease in blood glucose (sugar) levels. The sensation of “hitting the wall” or “bonking” during exercise shows the physiological impact of low blood sugar levels (hypoglycemia) on the brain and body.

Increase in cortisol

Cortisol, known as the “stress hormone,” is integral to the body’s stress response. Secreted by the adrenal glands, it modulates metabolism, blood glucose levels, and immune function. Cortisol is highest in the morning. When you exercise on an empty stomach, your body receives an added source of stress due to a lack of readily available energy, especially if you exercise at a high intensity.

Elevated cortisol levels present several risks to health and performance, like tissue breakdown as a source of energy. Studies indicate that women are more prone to utilizing lean muscle as an energy source, which can result in decreased muscle mass and increased risk for injury. Plus, cortisol exposure has been shown to affect the distribution of body fat, with elevated cortisol levels associated with increased fat deposition around the midsection. If your primary goal is weight loss through exercise, then fasted training could be counterproductive.

Immune system suppression

Strenuous exercise, while generally beneficial for the immune system, can cause a short-term period of immune suppression, increasing susceptibility to infections. In fact, research indicates that post-exercise immune function depression is the most pronounced following continuous, prolonged exercise sessions (90-plus minutes) of moderate-to-high intensity (55-75% of aerobic capacity) and performed in a fasted state.

Low energy availability (LEA)

Fasted training has the potential to create a domino effect of issues, especially in women. LEA is a mismatch between dietary energy intake and energy expended in exercise. This significant imbalance leaves the body’s total energy needs unmet, leaving inadequate energy to support the body functions and hormones required to maintain optimal health. For women, exercising in an energy deficit state significantly increases metabolic stress on the body. If this occurs for a prolonged time, ovulation can stop altogether (i.e., amenorrhea). The extended absence of ovulation, also known as functional hypothalamic amenorrhoea (FHA), leads to a consistent drop in the ovarian hormones oestrogen and progesterone. Low levels of these hormones can cause serious side effects, including loss of bone density and increased risk of stress fractures.

Fasted training can also induce changes to thyroid function, resulting in impairments to the heart, muscle, digestive function, and bone metabolism. Research has shown a significant drop in thyroid levels when restricting energy intake in menstruating women. Compared to men, research indicates that the female body faces a higher risk of energy imbalances when engaging in fasted training.

To Fast or Not To Fast?

Just because you can run fasted doesn’t mean you should. While some studies suggest benefits in fat utilization, exercising when the body is deprived of readily available carbohydrates has its drawbacks, like increased perceived exertion, diminished overall training quality, and compromised health.

When the body is adequately fueled, you can stay more consistent with training. And consistent training improves fat burning. Here’s why:

Mitochondrial Efficiency: Training increases the number and efficiency of mitochondria (the “powerhouses” of your cells) within your muscles. More efficient mitochondria mean your body becomes more skilled at breaking down fat for fuel.
Capillary Density: Increased capillary density in skeletal muscles improves the body’s ability to transport and utilize oxygen for energy production. A higher capillary density helps delay fatigue.
Glycogen Sparing: Endurance training naturally enhances your body’s reliance on fat as fuel, which has a glycogen-sparing effect (preserving your limited carbohydrate stores).
Increased Resting Metabolic Rate: Building muscle through exercise contributes to a higher resting metabolic rate, meaning your body burns more calories at rest.

Many runners don’t mean to run fasted, but they skip fueling because of time constraints, fear that pre-run food will cause stomach distress, or because they’re just not typically breakfast eaters. If you struggle to eat before a workout, it’s crucial to train your stomach to tolerate food before running. Without proper gut training, carb-loading on race day will likely lead to unwanted gastrointestinal issues. Translation: You may be spending more time in the porta-potty than on the race course.

So don’t fear that pre-run banana. You don’t have to run on empty to be a great fat burner. Consistent and well-fueled training fosters better performance (and a healthier body) than fasted training.