What you eat before a run is not a decision to be taken lightly. You need to tread a fine line, making sure you provide your body with the energy required without upsetting your stomach or eating so much that you feel sluggish on the run.
But what if the right thing to eat was, in fact, nothing at all? Fasted running is just that. You run after a long stretch without eating – before breakfast, for example. The result of this is that your body has little to no glycogen reserves (which you build up by eating carbohydrates) to use as energy, so it switches to using your fat reserves.
Why would you want this? A good question and one we put to sports scientist Barry Williams, co-founder of Group Evolution, which offers personal coaching in London and training camps for keen amateurs in its chateau in France.
What are the potential benefits of fasted running?
Advocates of fasted running claim a number of benefits, from improved competitive performance to positive changes in body composition to enhanced economy of movement. However, many of these claimed benefits are not supported by research, with much ambiguity over whether performance is actually improved and weight loss realised when compared with non-fasted runners. What is clear from studies, however, is that fasted running promotes numerous physiological changes that are health-positive.
Fasted running enhances mitochondrial biogenesis – an increase in the number of mitochondria in the body. These are the batteries energising our muscles, so a greater number of mitochondria is one of the main adaptations we seek through training to improve our physiological potential. All training has a positive impact but to varying degrees. Fasted running has been shown to stimulate mitochondrial biogenesis more effectively than non-fasted running.
Additionally, fasted running improves our fat-burning potential – the ability to mobilise and convert fat for fuel [known as fatmax training], offering the potential to change body composition. At the same time our regulation of insulin, the hormone which allows the mobilisation of carbohydrates for fuel, is improved, lowering our resting blood sugar levels.
These molecular changes, and some further adaptive changes, have major health consequences, lowering cholesterol, improving cognitive function and reducing the risk of cardiovascular disease and diabetes. And these benefits don’t come with obvious possible disadvantages, such as a noticeable increase in the perceived effort needed when running at a constant speed, or the need to catch up by eating more after fasted exercise.
Are there any downsides to fasted running?
Too much fasted running can hamper your carbohydrate metabolism, hindering performance at higher intensities and, in all likelihood, at race pace. It can prolong your recovery from exercise, hinder immune function, reduce muscle mass and disrupt the menstrual cycle.
So, fasted running is best used in moderation and should be carefully placed within programmes. It’s good for slower, longer efforts and you should not completely eliminate non-fasted days. These will help with recovery, keep the immune system in order and ensure your carbohydrate-powered metabolism stays tuned.
Does fasted running improve running performance?
There is not much scientific evidence showing that fasted running improves performance. The research that has been undertaken is inconclusive.
Numerous studies indicate that there is no significant advantage from fasted as opposed to non-fasted exercise, whereas a study of French triathletes showed that they improved their 10km time trial performance, economy and body composition.
It is not surprising that there is no clear-cut performance advantage. Even the most efficient fat burners can only use fat for fuel at 60-75% of their aerobic power threshold and the fat metabolism can only produce 850 calories an hour. Your aerobic power threshold is often called VO2 max and it’s the maximum oxygen uptake the body uses during exercise prior to the point of exhaustion.
Above 80% of aerobic power, we are reliant on mobilising carbohydrates for fuel and the carbohydrate metabolism can produce 1,500 calories an hour. So when we perform at race pace, we rely heavily on carbohydrate metabolism and the fat metabolism plays a supporting role – it helps us preserve our glycogen and carbohydrates for longer and therefore prospectively last for longer. This is important in endurance and ultra-endurance events but less so in shorter and higher-intensity races.
Unfortunately it is highly complex to test fasted and non-fasted training approaches in long distance endurance events like marathons in any form of controlled scientific study. What we do know is that some endurance athletes advocate fasted running intermittently.
Can fasted running help with weight loss?
Studies do not show that fasted running promotes weight loss any more than non-fasted running. It is believed that this is primarily because the carbohydrate metabolism generates more thermogenic activity, burning fuel for longer after exercise and negating the fat-burning benefits during exercise.
However, what is not in doubt is that a better-balanced metabolism has numerous health benefits as detailed above, and provides a better foundation for health and exercise.
- If this topic interests you, our running for weight loss guide goes into greater detail.
How long should a fasted run be?
There is no golden rule for the distance a fasted run should be. The length of the run is driven by the intensity at which exercise is conducted. Due to the lack of glycogen stores high-intensity efforts would need to be very short as they are glycogen-dependent, whereas long slow recovery runs could be undertaken in a glycogen-depleted state because they will be fuelled by fat burning. If looking for adaptation to fat burning for endurance events the longer, slower runs appear preferable. These would be one hour or more, potentially after a period of gradual adaptation.
What pace should a fasted run be done at?
Again the intensity is a factor. Assuming an endurance focus, the longer runs need to be conducted at 60-75% of aerobic power. Shorter, higher-intensity efforts can be conducted at race pace, but with the understanding that these will not last for long and need significant recovery afterwards. Replenishing carbohydrate stores almost immediately after such sessions would be essential to recovery and repair.
Will you feel awful during a fasted run?
There is no reason that you should feel awful as long as you run at the right intensity. Indeed, research shows that fasted running, conducted at lower intensities, does not lead to adverse reactions during the exercise, whether actual or perceived. If you do higher-intensity sessions then you will not be able to go as hard for as long. If you try to push through, you will feel bad, as you always would when pushing beyond your limits.
How much fasted running should you do?
Doing a third to half of your runs fasted would be a good place to start – nearer half if you are more endurance-focused or when it is early in your training cycle, and a third if you are more of a 5-10K specialist, especially close to or in race season.
If you just run for fun and your health, including for weight loss reasons, you could stretch this to two thirds.
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Nick Harris-Fry is a journalist who has been covering health and fitness since 2015. Nick is an avid runner, covering 70-110km a week, which gives him ample opportunity to test a wide range of running shoes and running gear. He is also the chief tester for fitness trackers and running watches, treadmills and exercise bikes, and workout headphones.