What Runners Need To Know About Fasted Running

Will running on an empty stomach make you a better runner? And is it a good idea?

What Runners Need To Know About Fasted Running

There has been much interest in fasted running (training in a fasted state) to promote fat loss and training adaptations over the last few years. Experts at Loughborough University are in agreement that there are benefits, with the common recommendation of morning runs to encourage the body to use more fat as fuel. While there has been debate amongst scientists regarding the effectiveness of this strategy to maximise fat loss, new research may have just provided another reason to stick with the occasional fasted run… it seems it could lead to faster performance gains!

Will it work for weight loss?

Years of sports science research has encouraged us to ‘carb up’ for our runs, making sure our fuel stores are full and ready to get us through a long or tough run. Common training theory suggests that the bigger the training load an individual is subjected too, the greater the adaptation, so if carbohydrate supports longer or higher intensity runs then this should bring the best performance adaptations and allow you to burn more calories along the way.

A new piece of research from Loughborough University has examined the difference in the use of fat as fuel during runs prior to breakfast (fasted) or post-breakfast (fed) and has concluded you do indeed burn more fat as fuel when running fasted, due to less carbohydrate availability. In short, feeding with carbohydrate blunts the uptake of fat as fuel. The authors do stress, however, that this does not necessarily mean fasted training will lead to weight loss, as this will depend on whether you remain in negative energy balance across the day. Interestingly, running after breakfast results in better appetite management across the day, which may be more likely to support long-term weight management. Running fasted may also have a negative impact on muscle tissue, increasing the amount of breakdown. One could argue that if muscle mass is not supported by this activity, then it may not be a good strategy for long-term fat loss, as muscle burns calories for us and any loss will reduce our daily calorie requirement. The good news is that running itself has been identified as a positive behaviour to encourage weight loss, not only helping to put us in negative calorie balance, but also blunting the appetite compared to sedentary individuals.

But aside from weight loss, the more interesting story regarding fasted cardio is that strategic scheduling of fasted runs may in fact help improve performance. So while you may be looking to drop a few pounds, you could get faster along the way!

Can fasting make you faster?

A new body of research is providing accumulating evidence to suggest that commencing fasted endurance exercise increases the transcription rate of several genes and proteins involved in the training response. It appears that exercise in a fasted state increases the activation of several signalling proteins with direct roles in promoting training adaptation.

The first study in this area to show impressive results was one by Hansen et al in 2005, which illustrated that ten weeks of training in a fasted state (with low muscular stores of carbohydrate) resulted in an 85 per cent increase in time to exhaustion compared with training in a glycogen-fed state. Although this study was impressive, it was limited by the use of one leg muscular contractions and extensions as the mode of testing, rather than real running. One of the first whole body studies in this area was conducted with 18 endurance athletes, who undertook three weeks of training and testing. They were split into a control group labelled as the ‘high’ group, training six days a week, alternating steady-state aerobic sessions at 70 per cent of their max and high-intensity training consisting of eight x five-minute maximal effort work bouts followed by one minute of recovery the next day. The experimental group conducted the same training sessions, but trained twice a day with alternate days off. On these days, the steady state session was performed in the morning followed by several hours of rest and then the high-intensity session. The steady state test was designed to deplete their stored carbohydrate considerably so they would be subsequently training in a ‘low’ carbohydrate state.

Interestingly, although the low-carbohydrate group didn’t perform as well during their high-intensity training sessions initially, by week three their performance was not different to the high-carbohydrate group, suggesting a performance adaptation to low carbohydrate stores. They also displayed increased rates of fat oxidation and carbohydrate restoration in the muscles between sessions, as well as displaying increased oxidative enzyme activity in muscle biopsies, which could translate to a greater aerobic adaptation after training in a fasted state. Essentially, the study suggested that training in a ‘low’ carbohydrate (or fasted) state could increase fat loss, potentially spare carbohydrate and lead to faster aerobic training gains, without a detriment to the intensity reached in training once the body is accustomed to training in this state. Further research studies have also backed this up. Sounds like the Holy Grail!

So, should you run fasted?

Choosing between fuelling or fasting for all of your runs is not really the question, as both conditions have advantages. It certainly appears that introducing some fasted training into your programme could bring about faster performance adaptations while promoting fat loss, increasing the percentage of fat used as fuel and potentially providing a carbohydrate sparing benefit, which can be utilised for important events. Balance is key though, as exercising on low carbohydrate stores has been linked with compromised immunity, increased susceptibility and poor quality training.

Conducting one to two fasted training sessions each week on days where you can either train twice to lower carbohydrate stores, or perform a high-intensity session in the morning prior to breakfast is a good compromise. To transform your weekend long run into an effective fasted session, you could simply run first thing in the morning after stopping food intake from 10pm the night before, ensuring you are well hydrated on water (for safety and to avoid complications with drinking too much water, it’s wise to add electrolytes to you water bottles).

In short, fasted training once or twice a week can help you increase fat loss as long as you stay in a negative calorie balance across the day, but it can also lead to faster training adaptations, so you can take time off your favourite events.


Chris Macdonald

Written by Chris Macdonald | 355 articles | View profile

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