You wouldn't jump in the car and set off on a long drive without fuel in the tank. However, there's a lot of noise about the benefits of fasted running, which is basically running on an empty stomach. This may sound like a bad idea – food is fuel, after all – but research suggests that there may be some tangible positives to a fasted running approach, as well as a few drawbacks too.
We've asked our running expert to look into what the science says to guide you on the pros and cons of fasted running. She delves into how the body responds and suggests the best way to try it out, if you're so inclined. So, lace up those trail running shoes, stay out of the fridge and give it a go.
What are the pros and cons of fasted running?
The pros of fasted running are that it increases the body's ability to burn fat as fuel and improves insulin sensitivity. It also does away with digestion issues while on the run and it's suggested that fasting improves mitochondrial health.
In terms of cons, muscle wastage can occur on long running sessions and the reduced carbohydrate metabolism decreases high intensity performance. The body is also less able to recover properly after a session and may suffer from a reduced immune system, while women may find that their menstrual cycle is disrupted.
For more details on the benefits and drawbacks of fasted running, see further down the guide.
| Row 0 - Cell 0 | Pros | Cons |
| Alternate fuel | It increases the body's ability to burn fat for fuel | Muscle wastage on long runs |
| Other factors | Improvement of insulin sensitivity | Recduced carbohydrate metabolism can decrease performance at high intensities |
| On the run | Reduces the risk of stomach and digestion issues on the run | Lack of quick fuel inhibits speed work |
| Wider issues | It's suggested that fasting improves mitochondrial health | Disruption of recovery, menstrual cycle and reduced immune system |
Meet the expert
Fiona loves running in all its forms. Based in the Scottish Central Belt, just a stone's throw from the Southern Highlands, she has a wealth of excellent road and trail running right on her doorstep.
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What is fasted running?
- Fasted running is when you go for a run in a glycogen-depleted state
- When your body has no glycogen reserves, it switches to using your fat reserves
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.
Fasted running – how the body works
- During exercise, the body uses fats and carbohydrates as energy sources
- Carbohydrates break down more easily for fast energy
- Higher intensity exercises use more carbohydrates
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 while lower intensity (between 47-65% VO2 max) utilise more fat for energy.
Fasted running benefits
Those that believe fasted running is good for us, claim a range of benefits. These include:
- Increasing the body’s ability to burn fat for fuel. Research, such as one study 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. It is suggested that fasting can improve mitochondrial health.
Fasted running drawbacks
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.
What do the experts say?
- For fasted running, aim for reduced intensity
- Intense workouts on less glycogen can be ineffective when it comes to improving fitness
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, 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.”
How do I try fasted running?
- How long your fasted runs should be depends on your fitness
- For higher intensity fasted workouts, go for a short amount of time
- For longer runs, go slow and build up the time on your feet
- Give yourself time to recover after a long run in fat-burn mode
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."
