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Fasted running: the pros and cons

should I try fasted running?
What are the pros and cons of fasted running? (Image credit: Getty)

There are many reasons why people run, including weight loss, cardiovascular health, for the competitions and for improved mental well-being. So, should we consider fasted running – hitting the trails on an empty stomach – as runners?

It is claimed by some that fasted running in the morning will bring benefits such as speeding up weight loss and reducing, in particular, fat stores. It’s a favorite habit of body builders, for example, who are keen to shred fat and build muscle. 

However, there are other reasons why fasted running may not be the best solution.

What is fasted running?

Fasted running, also called fasted cardio, is when you go for a run in a glycogen-depleted state. This usually means you are fasted running in the morning after a night’s sleep and without eating breakfast.

If you want to run fasted at other times in the day, you’ll need to abstain from eating for four to six hours beforehand. 

When your body has little to no glycogen reserves – which you build up by eating carbohydrates – to use as energy, it switches to using your fat reserves.

should I try fasted running?

Most fasted runners head out before breakfast (Image credit: Getty)

Fasted running – how the body works 

During exercise, the body uses two main sources of energy, fats (triglycerides) and carbohydrate (glycogen and glucose). 

When we run, the body naturally uses carbohydrate because this breaks down more easily for energy. However, when our bodies are depleted of carbohydrates, fat becomes the source of energy. 

Intensity also plays a part in how the body breaks down carbohydrate and fat. Higher intensity exercise – that is running at more than 65% VO2 max – is shown to use more carbohydrate (opens in new tab) while lower intensity (between 47-65% VO2 max) utilise more fat for energy (opens in new tab).

Fasted running benefits

Those that believe fasted running is good for us (opens in new tab), claim a range of benefits. These include:

  • Increasing the body’s ability to burn fat for fuel. Research, such as one study (opens in new tab) that looked at optimising fat oxidation to reduce the symptoms of metabolic diseases like obesity and type 2 diabetes, appear to point to benefits. 
  • Improvement of insulin sensitivity.
  • Reducing the risk of stomach and digestion issues for runners that can come with running too soon after eating.
  • Mitochondrial biogenesis, or an increase in the number of mitochondria in the body. These can be described as the batteries that supply energy to our muscles (opens in new tab). It is suggested that fasting can improve mitochondrial health.

Running fasted vs fed running

However, there is little scientific evidence showing that fasted running improves performance and most research that has been undertaken is inconclusive. There are also various documented disadvantages of running fasted. These include:

  • Muscle wastage caused by doing running sessions that are too long. 
  • Reduced carbohydrate metabolism, which can decrease performance at higher intensities.
  • Barriers to being able to work hard and fast, or manage speed work goals, due to a lack of quick fuel.
  • Reduction in the body’s ability to properly recover.
  • Disruption of the menstrual cycle.
  • Reduced immune system.

It is actually recommended by sports experts that if you plan to do fasted running, especially in the morning, you should aim for reduced intensity. Although intensity does depend on how fat-adapted you already are and your general fitness levels.

Nathalie Jones (opens in new tab), a dietician, said: “When exercising at lower intensities there is a lower reliance on carbohydrate as a fuel. This effect is greater with increased fitness. 

"Trying to work at higher intensities on less glycogen, runs the risk of not being able to train hard enough for the fitness benefit and can reduce the effectiveness of the session.”

should I try fasted running?

There are runners who advocate fasted running (Image credit: Getty)

How do I try fasted running?

Many people ask how far a fasted run should be for the best benefits but there is no one rule for all. It will depend on your fitness levels, whether you have built up to increased fated running distances and the sort of training you hope to do. 

High intensity efforts should be short because to be effective they are dependent on glycogen stores in the muscles. Longer and slower runs should be fine because they can be achieved in a glycogen-depleted state because they will be fuelled by fat burning. 

If you believe that adapting to fat burning for endurance events is a good idea, then it is a better idea to run at a slower pace and intensity and to build up the time on your feet.

Fasted runners will also require more recovery after a session where you have pushed yourself hard while in fat-burn mode. 

Nathalie said: "Fasted running is something that should be seen as highly personal. Benefits, or not, also depend on what you are hoping to achieve and how you train. The best advice is to seek the advice of a qualified running coach before starting."  

Fiona Russell is a widely published adventure journalist and blogger, who is better known as Fiona Outdoors. She is based in Scotland and is an all-round outdoors enthusiast with favourite activities including trail running, mountain walking, mountain biking, road cycling, triathlon and skiing, both downhill and back country. Her target for 2021 is to finish the final nine summits in her first round of all 282 Munros, the Scottish mountains of more than 3,000ft high. Aside from being outdoors, Fiona's biggest aim is to inspire others to enjoy the great outdoors, especially through her writing. She is also rarely seen without a running skort! Find out more at Fiona Outdoors (opens in new tab).