TOPIC REVIEW…Fasting and Performance

December 23, 2021

TOPIC REVIEW…Fasting and Performance

As the end of the year is nigh, we decided to have a look through our back catalogue of Podcast episodes and discovered we have released over 90 episodes and have had over 21,000 downloads since we started Podcasting back in April 2020, which is very exciting and all thanks to you, our loyal followers.

We went on to explore which of the episodes have been most popular, and the all-time most popular one is: Episode 18 Intermittent Fasting and the Female Runner.  We then decided to review it and give you any updates on the subject. So here we will:

  • Pull out key highlights from Ep18
  • Introduce some new information from recent studies
  • Give tips on including fasting in your daily/training life

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TOPIC REVIEW….Fasting and Performance


What are the various approaches to intermittent fasting?

There are many different approaches to intermittent fasting all of which are divided into three categories:

  1. Intermittent calorie restriction (ICR)
  2. Alternate Day Fasting ADF)
  3. Time Restricted Feeding (TRF)

ICR includes the 5:2 and the FAST 800 approaches:

  • 5:2 where you eat 500-600 calories on 2 days a week and eat to normal energy requirements on the other 5 days
  • FAST 800 – where you eat 800 calories every day for up to 12 weeks until ideal body composition is reached

ADF is when one day you eat to normal Energy requirements and the next day eat 25% of energy requirements only. For example 2000 calories vs 500 calories

TRF is when you fast during a set number of hours each day and eat during the remainder of daily hours. For example 14.10 16:8 or 18:6 or 20:4 – 16:8 possibly being the easiest to implement


What are the PROs and Drawbacks of Intermittent Fasting?

There are potentially many PROs and possible drawbacks to intermittent fasting. The choice is  very individual and this approach to eating may not be suitable for everyone, especially endurance runners.

Positive aspects of Intermittent Fasting:

  • Promotes weight loss and supports optimal body composition
  • Helps improve energy
  • More efficient recovery from training
  • Trains the body to switch from glucose use to utilising fat as fuel – supportive on long endurance runs

Potential Drawbacks of Intermittent Fasting:

  • May lead to STRESS and LOW BLOOD sugar levels, which could have a knock-on effect on cortisol levels
  • Cortisol manages energy by supporting blood sugar balance. If no food has been eaten for several hours and blood sugar drops too low the adrenal glands produce cortisol to signal production of glucose to bring blood sugar balance back into balance
  • Following an overnight fast blood sugar levels, in some people, may be TOO low. This could lead to cortisol being activated to bring glucose form the body’s store to help provide energy and balance blood sugar
  • Sometimes this can lead to overall cortisol levels being higher than they need to be (or should be) during the day. This can have a further knock-on effect on other hormones including sex hormones, neurotransmitters etc


FEMALE FACTORS – see below


What is the Current Research Saying About Fasting for Sports Performance?

There hasn’t been very much new data released since early 2020, however some factors worth mentioning include:

Exercising in a fasted-state (which is generally following an overnight fast and before eating the first meal of the day) allows higher levels of fat oxidation (so using fat as fuel) BUT this is only beneficial during exercise performed at low-to-moderate intensity. Overtime, exercising in the fasted-state could help increase the relative intensity at which maximal fat oxidation occurs.

5’AMP-activated protein kinase (AMPK), which is a compound that acts as an energy sensor at cellular level, regulates cellular and whole-body energy balance and is activated during exercise. Activation of AMPK can lead to a range of metabolic adaptations including:

  • Increases in glucose uptake
  • Fat oxidation
  • Mitochondrial biogenesis – so more mitochondria being synthesized – they are our energy powerhouses – so vital for runners

Some studies have shown that FASTED-state running results in greater increases in the activity of AMPK (following a 1h steady-state endurance exercise session at 65–70% VO2max). Studies have also noted that exercise performed with “normal” muscle glycogen levels plus additional CHO intake has a blunting effect on AMPK activity – in other words, AMPK is less effective. These effects appear to occur during low-moderate exercise levels only and performed for 90mins or less.


Tips on Including Fasting in YOUR Daily Life and Running Training

  • To get the best outcome make some compromises will need to be made. For instance; will you build your food timings around your run training or vice versa? Something to consider
  • Consider starting with TRF first – this may be the best approach to support a running training schedule
  • Possibly complete your run following an overnight fast then delay breakfast (following the 14:10 approach). This may encourage the use of fat as fuel and efficient AMPK activity both of which could help with optimal body composition, sparing glycogen and increasing mitochondria production
  • Get everyday nutrition optimal CONSISTENTLY before fasting – aim for healthy good quality food and plate balance (reduce carb portion if you are wanting WL) plus good hydration and limit stimulants like caffeine and limit alcohol/soft drinks
  • If required or if you think it may suit your lifestyle better, add Calorie Restrictions or Alternate Day Fasting once you have adjusted to this new way of eating
  • Start slowly and make adaptations/adjustments – you don’t have to do everything at once. As with everything you have to practice

Female Factors

TOPIC REVIEW…Fasting and Performance

  1. If fasting leads to a high requirement for Cortisol, that takes priority over making female sex hormones and the knock-on effect from this is disruption in sex hormonal balance possibly leading to: disrupted periods, fertility issues, low sex drive, peri and menopause symptoms and other hormonally driven health conditions
  2. Women appear to lose more weight and percentage body fat than men following fasting
  3. Metabolism of the substrates of Protein/fat/CHO may differ between males and females and this could be influenced by the menstrual cycle and oral contraception


TOPIC REVIEW….Fasting and Performance

  1. Fasting comes in various forms – it is about choosing then approach that fits best with your running training and lifestyle
  2. There are strengths and pitfalls to adopting a Fasting approach to everyday life and running training so be sure to weigh these up before embarking on any Fasting style
  3. Remember fasting may impact on sex hormone balance in some women leading to disrupted periods, fertility issues, low sex drive peri and menopause symptoms
  4. Ensure you have your everyday healthy eating plan embedded before embarking on Fasting to help limit its effects on BSB
  5. Finally, remember Fasting is not for everyone…it will depend on your training, your lifestyle, your current health and other factors

Related Episodes:


The suggestions we make during this episode are for guidance and advice only, and are not a substitute for medical advice or treatment. If you have any concerns regarding your health, please contact your healthcare professional for advice as soon as possible.

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