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Pros & Cons of Running on an Empty Stomach

Running is a great form of exercise, whether you take part in virtual races, hit the treadmill or enjoy cross-country sprints. Although fuelling your body is essential for intense training, some people choose to run without eating beforehand, also known as fasted exercise or training in a fasted state. Take a look at some of the pros and cons of running on an empty stomach and explore the science of fasted running with ACE Races.

What is Fasted Running?

Fasted running is essentially running on an empty stomach. People who take part in fasted running usually run in the morning after 6 to 8 hours of overnight fasting.

Pros of Fasted Running

Lower energy intake?

Fasted running may be a suitable training option for runners trying to lose weight as it can potentially help you control your energy intake. Recent research suggests running on an empty stomach will result in your body consuming less energy and intaking less energy. Once your blood and glycogen levels are empty, your body can access your liver glycogen stores and affect energy intake via the liver-brain neural network.

Increase aerobic endurance?

Some evidence also suggests that running on an empty stomach can increase aerobic endurance. Running in a fasted state has been associated with higher VO₂ max, which is your body’s maximum oxygen uptake during intense training and a measure of your overall fitness levels. However, further and more recent research is necessary.

Fewer digestion issues?

When long-distance running, it is common to experience stomach cramps, nausea, vomiting or diarrhoea. Running on an empty stomach could the solution to avoiding digestion issues. However, you could also try avoiding fatty, acidic or greasy foods before exercising.

Higher fat burn?

Carbohydrates are stored in your body in the form of glycogen and so when you haven’t eaten for a while, your glycogen energy levels are low. When fasted running your body may use more fat as energy instead, resulting in higher fat burn or oxidation. However, the results of this theory are varied and more research is necessary on the link between running on an empty stomach and increased fat oxidation.

Cons of Fasted Running

Hard to maintain a high intensity

Using your fat stores for fuel is not sustainable for a prolonged run, and will eventually cause fatigue. Fast running has been associated with lower endurance as it can be difficult to maintain a high intensity of fast pace with less energy.

An increased risk of injury

Fatigue can also increase your risk of injury when running. It can also prove difficult to pay attention and practise proper form when running on an empty stomach as your brain requires glucose to function effectively.

Potential muscle loss

Cortisol is a hormone, produced by your adrenal glands, that controls some of your basic functions such as blood glucose and stress response. It can also increase muscle loss when at high levels in your body by promoting protein breakdown in muscle cells. Most people take part in fasted running in the early morning, when cortisol levels are highest, resulting in potential muscle loss and reduced strength. Some studies also suggest that exercise in a fasted state could further increase cortisol levels.

A risk for some health conditions

Running on an empty stomach is not recommended for people with type 1 or type 2 diabetes as it could cause low blood sugar levels, also known as hypoglycaemia, especially if you take insulin. If you have diabetes, always check your blood sugar and eat a snack before running.

Fasted running can also cause an increased risk for people with other health conditions such as Addison’s disease. Speak to your GP to find out more.

Unsuitable for long-term weight loss

Although fasted running could potentially result in a higher fat burn, it isn’t very suitable for people with long-term weight loss goals. Recent studies with participants running before and after eating showed similar rates of weight loss. In fact, when you burn high levels of fat running on an empty stomach, your body will later compensate and reduce fat burn at another time by using more glucose stores.

Fasted running can also cause overeating as if you decide not to eat before a run, you may feel more hungry later in the day as your body continues to crave calories. This can result in you eating more to replenish the energy your body has used on your morning run.

Should I Run on an Empty Stomach?

Although there may be some cons to fasted running, it is generally recommended that you eat before training, and make sure you fuel your body correctly. If you do prefer running on an empty stomach, try to avoid any long-distance running or high-intensity training, and always take a break if you feel fatigued or light-headed. It’s also vital that you stay hydrated when running.

For more information on how to fuel your body and increase running performance, take a look at our ACE Races guide on the best things to eat before and after a run.

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