Fasting and Training: Good or Bad?


by Alejandra "Alex" Ruani — Get free science updates here.

Have you ever worried that fasting and exercise don’t go together? Or perhaps you’ve heard the opposite: that training in a fasted state has its benefits?

We investigate the scientific studies that tried answering these questions in this PDF report.

Grab “Fasting and Training: Good or Bad?” below:

 

Download PDF NOW!

 

Conveniently download this 29-page science report. Contains links to extra reading materials and scientific references.

Topics covered in this report:

  • The health benefits of fasting
  • Fasted training: the dilemma
  • Does fasted exercise aid fat loss?
  • Can fasting give you an athletic edge?
  • Teaching muscles to prefer using fat for fuel
  • Is fat adaptation a mere theory?
  • Can fasting enhance endurance?
  • How are carbs stored in muscles?
  • What’s the main fuel used in bodybuilding-style workouts?
  • Does weight training cause glycogen depletion?
  • Fasting and muscle breakdown
  • BCAAs and the anabolic window
  • Fasting and high-intensity training
  • When fat fuel is too slow
  • Does ketosis increase or decrease power output?
  • Power decline at higher intensities
  • Your key takeaways
  • References and resources

 

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And let me know in the comments below what you think about fasting and exercise - I'd love to hear from you!

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Alex Ruani leads the research division at The Health Sciences Academy, where she and her team make sense of complex scientific literature and translate it into easy-to-understand practical concepts for students. She is a Harvard-trained scientific researcher who specialises in cravings and appetite neurobiology, nutrition biochemistry, and nutrigenomics. Besides investigating and teaching the latest advances in health and nutrition science, Alex makes it easier to be smarter with her free Science Catch-ups every other Thursday.
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21 Comments

  • Dan Frost

    Reply Reply January 22, 2015

    Firstly, Thank you very much for a very interesting article! With regards to training and fasting I have had a few bad experiences with it in the past. In particular a few months ago when I had to drop 8kg in a week to make weight for an MMA championships and it was possibly one of the worst weeks of my life…training 8 hours a day with a hugely restricted diet was not fun! By the end of it I dropped from 71 to 62.5kg but I felt weak, ill and even had open sores on my skin!(Now of course I am not saying this would happen to everyone, it is just my personal experience.) So personally I wouldn’t go with it if people are doing anything that requires any sort of power or high level of intensity/endurance…fighting 6 or 7 5min rounds is far too tough to do on no food! (Although the after fight dinner is pretty awesome :) )

    • Alex

      Reply Reply January 22, 2015

      Dan – Thanks for sharing your experience! Drops in power output where explosive strength is needed: not ideal if you’re in it to win it! I’m glad you realised that pretty quickly, most competitive athletes do :-)

  • Claudia

    Reply Reply January 22, 2015

    Very interesting article. I have been doing the intermittent fasting for about half a year now and love it. My main motivation were the health benefits that are described in an increasing number of peer-reviewed studies. I was very surprised when I started that I could still go for a run and that I didn’t suddenly feel too weak to keep going. I had wondered what the current research showed about fasting and exercising. Huge thank you for summarising. Getting up on a Thursday morning has become a lot easier since receiving these articles, because it is something to look forward to read over my morning coffee. It is really interesting every time. Well done!.

    • Emma

      Reply Reply January 22, 2015

      Wow I’ve learned so much with today’s report, it covers all my questions! I’ll pass it onto my brother who is obsessed about hypertrophy and bodybuilding. Brilliant!!! Like Claudia says, you give me a reason to look forward to my Thursday mornings!!

    • Alex

      Reply Reply January 22, 2015

      Claudia, Emma – music to my ears! Love having you in our community of (pretty smart) experimenters :-)

  • Melody Gold

    Reply Reply January 22, 2015

    Thank you for yet another content packed and researched piece. I have dabbled with intermittent fasting in the past, understanding it’s benefits for health, and had wondered how to fit it in around my workouts. I don’t lift weights, yet, but this piece answers my questions.

    • Alex

      Reply Reply January 22, 2015

      Medoly – love to hear that we’ve answered your questions!

  • Chris

    Reply Reply January 22, 2015

    A great morning read, but I would like to know, with the “majority of studies on the post exercise anabolic window of opportunity” having been conducted on participants who were in a fasted state, is the current advice of muscle fuelling 2 to 4 hours before exercise / competition backed up sound scientific study?…..what are the potential health and performance pitfalls of fueling with carbs before and then a protein / carb mix afterwards as well?

    My thinking here is despite knowing the calculation of body weight / gram of carb etc you would need to know how much glycogen you have used during the exercise to know how much carbohydrate to refuel with afterwards or risk converting the excess into body fat etc? or am I way off wit this thought? :)

    • Alex

      Reply Reply January 22, 2015

      Thanks, Chris! Pre-fuelling is one of the most heavily researched areas in sports nutrition and used to manipulate metabolic adaptations. Fuel mix, ratio, dose and timing (pre, during and post workout/event) are key for the elite athlete who is in it to win it, and the variables used to calculate them are training volume, frequency, length, type, athletic status, metabolism, gender, etc. which we teach in our advanced sports and exercise nutritional advisor course. In terms of hypertrophy, have a look at the second link on page 28 in the report, it’s a long read but I think you’ll enjoy it :-)

  • Sam Metson

    Reply Reply January 22, 2015

    As Palaeolithic creatures we were primarily adapted to burn fats – ketosis – either from our own body reserves or animal fats that we gained from hunting. The reason for this is that during winter and spring, through to early summer, there were so few carbs around to eat and no methods in use to preserve foods. The purpose of carbs and the appetite stimulation response from long summer days (circadian rhythm change) was to get us to lay down body fat, in an unconscious way, to prepare us for winter and the cold cycle. It was a protective mechanism developed by Nature. Leading thinkers like the neurosurgeon Dr Jack Kruse would say we are far stronger and have much deeper energy reserves in a state of ketosis, but it takes some considerable time to adapt to this state. I found a video link here that suggests 3 weeks as a minimum. https://www.youtube.com/watch?v=K7xwfOYI2bQ
    Today, we are able to draw on carbs all year round today without thinking, and almost completely disregard our circadian rhythm with the advent of artificial lights, TV’s and iPads. Because we are biologically so adaptive we can apparently manage in this changed state. However, chronic illness is also regarded as normal, especially as we age. It need not be like this.
    I believe when we are in a proper state of ketosis, then we are living in a way much more in line with our evolutionary roots and health, energy and strength are all at much higher levels. Modern living and fear of saturated fat makes this view quite alien.

  • Jacob Shepherd

    Reply Reply January 22, 2015

    Fantastic report, Alex, this has answered many questions for me and given me more to consider. I’ve recently started looking into nutrition so I really enjoyed reading this.

    My brother has been on a fruit/vegetable juices diet for the past two weeks so as to encourage his body to burn surplus fats rather than carbohydrate and I’ve personally seen the effects of “fasting and training” as he has lost over a stone. Have you personally done any studies into the current juicing fad?

    • Alex

      Reply Reply January 22, 2015

      Jacob – glad to hear that I could help, and thanks for sharing your brother’s experience! Some athletes drink beetroot juice an hour or 2 before the event to perform better than their rivals; studies show that it helps you run faster at the end of a 5km run (Murphy et al, 2012) or cycle longer (ca. 3%) in a 16km race (Lansley et al, 2011), or display higher power output in higher intensity activities (Cermak et al, 2012). This is because the nitrates in beetroot make the delivery of oxygen and nutrients into working muscles more efficient, improving fuel economy and stamina. Many athletes are now drinking tart cherry juice post-strenuous-training, studies show it lessens muscle damage, accelerates recovery and reduces inflammation (Howatson et al, 2010; Bell et al, 2014) and even improves muscle strength recovery after weight training (Bowtell et al, 2011).

  • Dima

    Reply Reply January 22, 2015

    I know regret all those years I ran after eating
    A banana for energy while I was trying to loose
    Weight .. Very informative .. Thank you for sharing Alex

    • Alex

      Reply Reply January 22, 2015

      Dima – nice light-bulb moment! :-)

  • RS

    Reply Reply January 22, 2015

    This is the article I have been waiting for, and it did not disappoint! Particularly impressed with the way it covers all types of training/competition. Time to set my goals and then choose my dietary needs accordingly. Thank you :)

    • Alex

      Reply Reply January 22, 2015

      RS – you’re so welcome, glad I could help!! Enjoy your planning session :-)

  • Deanna McCormack

    Reply Reply January 22, 2015

    I am wondering if consuming protein with no carbohydrate prior to working out has been studied? (Not in full ketosis, but rather just skipping breakfast). If the body has protein available prior to workout, will it choose consumed protein for muscle synthesis over muscle protein, and still burn fat at the desired rate?

    Or perhaps consuming a minuscule amount of carbohydrate, just to “light the fire” for ATP cycle?

    I am asked these kinds of questions constantly as a Health Fitness Specialist and Personal Trainer with the majority of my clients desiring fat loss, but performance and safety is a must!

    Thank you in advance for your response!

    Deanna McCormack, ACSM HFS

    • Alex

      Reply Reply January 22, 2015

      Hi Deanna! We dive into these topics in our advanced sports and exercise nutritional advisor course; in short, what you need to remember is that during exercise, in particular high-intensity, the rate of muscle-protein breakdown exceeds the rate of synthesis (i.e. building, growth, repair), even more so when muscle-glycogen stores are depleted, in which case more muscle-protein is converted into glucose for fuel, instead of preserving it for the post-workout synthesis. There’s hardly any uptake of dietary protein by working muscles, and if you take amino acids like glutamate and BCAAs, these are used directly from the bloodstream for energy to fuel the activity. Starting the training session with adequate hydration and sufficient muscle-glycogen stores has been shown to help prevent exercise fatigue and minimise the risk of fainting if these are your main concerns :-)

  • Jane Magan

    Reply Reply January 23, 2015

    I do not specialise in sports nutrition but the point about maximum fat loss (and the fact that it does not increase appetite later in the day!) is a great tip for those on a weight loss program. I shall certainly pass it on to my clients.
    I love the fact that you use scientific, peer reviewed research papers.
    Thanks for report.

    Jane Magan

  • Sam Metson

    Reply Reply April 9, 2015

    To adapt to ketogenic mode takes much longer than 4 weeks. More like 4 months. Neurosurgeon Jack Kruse has written extensively about ketogenesis and so has Ben Greenfield, who is a ironman competitor as well as other insane forms of extreme exercise. Mitochondrial output increases enormously once adapted successfully.

    • Daniel Wong

      Reply Reply April 19, 2015

      I followed Jack Kruse for a while until I realized that most of what he says is pseudo-scientific or anecdotal and even makes severe factual mistakes when trying to explain his stuff. There aren’t many blogs I trust these days. Ben Greenfield is okay but also comes up with dubious theories I find hard to believe. I prefer places where I am presented with the full pic that don’t underestimate my intelligence or try to get me all fired up on a single angle. That’s what I appreciate about the health sciences academy Alex and team

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