Fasted Running. Yes or No?

We’ve all heard the stories of runners that have run 18+ milers on nothing but air, no breakfast, no in-run nutrition and no water; and it can definitely be a bit of an ego thing in the running community (I used to be one of those!). The question is, what purpose does it serve and should you be doing it?


I’m going put in a spoiler in right now – whilst results are often very individualised, I don’t recommend this for the runners I coach, and am not going to recommend you try it. Let me explain more…..

please note, I’m not a specialist in this field and the information below comes from recent and relevant articles and research, as well as my years of coaching runners. But each runner is individual and if you are serious about taking this further, please consult a well-qualified, experienced specialist. 

What is fasted running?

Scientific literature defines fasted training as not eating within 10-14 hours of a workout, so usually this would be an AM run. It’s an idea that has been around a long time and many runners swear by it.

Why has running fasted become popular? 

There are a few drivers behind this, but the main two seem to be weight loss, and increased utilisation of fat as an energy supply (for endurance running).

 1.  Weight loss:
It seems obvious – run without eating and you’ll have to burn your body fat – sounds wonderful! Sadly, it’s not that simple. Yes, you will increase the percentage of fat you burn when you run if you run fasted, but the primary source of fat burned during exercise isn’t the belly fat you’d like to use, but intramuscular fat. There are other negatives too… it increases the amount of protein you break down (in a non-fasted state, protein contributes approx 5% of your energy requirements for endurance training, but in a fasted state it’s doubled to around 10%). resulting in the breakdown of muscle tissue that can lead to a decrease in resting metabolic rate, a decrease in strength and an increase in injury risk.
Running fasted is also a major stressor on your body – it results in increased cortisol levels in the bloodstream, particularly in women, which promotes muscle breakdown, abdominal fat storage, higher fatigue and worse recovery. Not only that, but hard sessions are more effective when fuelled by glycogen, so you may burn more fat as a percentage, but your performance will suffer and you’ll burn less calories overall.

A number of studies have been done on fasted exercise for long term weight loss, and most found similar rates of weight loss in fasted and non-fasted participants, with little difference in body composition.  It just doesn’t seem to work. To meet weight loss goals, you’d definitely do better to fuel your runs. You’ll feel better, run better and nutritious fuel consumed during and immediately post workout is much more likely to be utilised for repair and recovery, rather than be stored where you don’t want it. Try and eat the exercise calories you have burned within a 1-2 hour window after working out. By doing this, it stops your body craving foods later in the day and you overeating and adding in more calories that you burned running.


2.  Increased fat utilisation as an energy source (and preserve glycogen in endurance runs e.g marathons / ultra marathons)
Again, this makes sense. Our glycogen stores are limited but we have much more energy in stored fat, even if you have a lower body weight. So relying on fat as a fuel for endurance events will preserve our glycogen stores and keep us running for longer. Research does indicate that training in a fasted state improves the ability to tap into fat stores sooner, and burns a higher percentage of fat during training sessions. But the caveats above still stand – it stresses the body, releasing cortisol, increasing protein breakdown and delaying recovery. As the brain is fuelled by glucose, it will also impact decision making as the race progresses. And the research is very clear on this – running on low carbohydrates will decrease your ability to hold a high intensity level if you plan on running fast.


3.  Other reasons: Convenience / it feels better / less digestion problems.
Running on a full stomach certainly doesn’t feel great. Most of us have also experienced runner’s tummy problems at some point or other. And not eating before a morning workout means 10 mins more in bed….
The best solution to stomach issues is to trial different foods to see what works (I have a client that can’t eat raisins for 3-4 hours before they run without getting an upset stomach, for example, but their friend always has them as a pre and during workout snack), and getting used to running with a little something in your stomach. It doesn’t have to be much. Stay near home whilst you try it, and preferably on an easy run! As for convenience, get out of bed ten minutes earlier and have something easier on your stomach. You’ll be fine!



Should I run fasted at all?
Well that’s all a bit doom and gloom for fasted running! But actually, short easy runs fasted won’t do any harm and you may feel good doing them. Generally, for easy effort runs of less than 60 minutes it is fine (but make sure you are well hydrated), but for harder or longer runs, a little sugar and protein goes a long way – it’ll boost energy, mental clarity and mood, allowing the body to better access energy sources and slow muscle breakdown. The general recommendation is around 150 cals, mostly simple carbs with some protein, but low fat and fibre. I use Mountain Fuel’s Morning Fuel for this (148 cals, some protein, mostly simple carbs) for convenience and easy digestion, but other ideas may include:

  • Half a protein shake with a banana or dates in for carbs (have the other half when you get back)
  • Instant porridge pots
  • Banana (maybe with a tablespoon of low fat Greek yoghurt)
  • Dates (a little peanut butter if you can stomach it)
  • Rice or potato balls
  • Nakd bar (or other similar dried fruit / nut bar)
  • Granola bar
  • A sports gel or chews
  • Half a white bagel or a slice of white toast with jam.



As with any new nutrition, try it out on an easier run close to home! And listen to your body. There’s lots of advice out here on the internet, but ultimately, we’re all individual and respond differently to different stimuli.

What about you….Have you discovered the secret to maintaining the ideal runners weight? Do you run fasted and love it?  Do you have the perfect pre run snack? Let us know….


Other useful resources:

MH Runners Club webinars on marathon training and nutrition

MH Runners Club Ask the Coach sessions


Join the Club at MH Runners Club

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Martin Hulbert

Running Coach & Personal Trainer Leicestershire

MH Health and Fitness Online Community