Episode 106 - Intermittent Fasting & Endurance Exercise. Should you do it?

Intermittent Fasting & Endurance Exercise. Should you do it?

Intermittent fasting has been around for most of human history but has seen a peak in interest in the last couple of years. 

  • What is intermittent fasting?
  • Physiologically, what happens to the body when you fast?
  • And should you dabble in intermittent fasting as an endurance athlete?

There is some excellent research in fasting and exercise performance so tune in for my recommendations on whether it is something you should or shouldn’t be doing as a triathlete.

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Aragon-Vargas LF. Effects of fasting on endurance exercise. Sports Med. 1993; 16:255–65

Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: a systematic review and meta-analysis. Scand. J. Med. Sci. Sports. 2018; 28:1476–93.


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Episode Transcription

Episode 106: Intermittent Fasting & Endurance Exercise. Should you do it?

Taryn Richardson 0:00

Welcome to the Triathlon Nutrition Academy podcast. The show designed to serve you up evidence-based sports nutrition advice from the experts. Hi, I'm your host Taryn, Accredited Practicing Dietitian, Advanced Sports Dietitian and founder of Dietitian Approved. Listen as I break down the latest evidence to give you practical, easy-to-digest strategies to train hard, recover faster and perform at your best. You have so much potential, and I want to help you unlock that with the power of nutrition. Let's get into it.

Taryn Richardson  00:43

Intermittent fasting is something that I have honestly had quite a lot of athletes come to me and ask me whether they should do it or not. So what I wanted to do is talk all about the research behind it in specifically endurance sport. Intermittent fasting has been around for honestly, most of human history. It's mentioned in ancient civilization so it's definitely not a new concept. But recently, it's become a bit of a thing. There's lots of celebrity endorsements and high profile athlete endorsements as well. In the literature, we've seen some benefits to some health markers in the general population. So things like weight loss, reducing blood pressure, and some improvements in markers in metabolic risk. So it makes sense why it's piqued people's interest in doing it as a strategy to improve our overall health outcomes. So what actually is intermittent fasting? Why would you want to do it? And should you do it as an endurance athlete?

Taryn Richardson  01:48

We actually have some really beautiful literature in intermittent fasting because Muslims all over the world fast every year for Ramadan. It's a month long period where there's no food or liquid consumed during daylight hours. And that has given us a really good understanding about intermittent fasting and sports because Olympics and other events often align with Ramadan. London 2012 coincided with Ramadan. So there was a lot of research that went into preparing athletes across the world from different countries. They were all doing their own research and assisting their own athletes to perform as best as they could in London.

Taryn Richardson  02:28

So there's many definitions of what intermittent fasting is. There is lots of different ways to do it. The 16:8 diet consists of fasting for 16 consecutive hours and then eating ad libitum, so as you want for 8 of those hours. There's the 5:2 diet which is an alternative day fasting protocol where one day, you have no or very minimal calories and then days consisting of food and liquid consumption, again, ad libitum. Now that can work for some people if they don't go crazy, the days that they are eating, but can also be crazy for people that have no structure and control. There's things like 20 hours of fasting and only a 4 hour window of eating. And then there's everything in between - 18:6, every other day fasting or whatever it is. People are making up their own rules. And there's no one clear method or way that seems to be better than the others. For people that do fast, it is really just about fitting it into your lifestyle in a way that is sustainable and maintainable.

Taryn Richardson  03:40

So physiologically, what happens when we fast? We actually know quite a bit, again about physiologically what happens in the beginning of the fasting period. The first phase in fasting consists of this post absorptive phase, which begins at around 3 to 8 hours into the fast depending on the makeup and quantity of the last meal that you had and that lasts about 12 to 18 hours. During this period, the blood glucose level is maintained by glycogenolysis, which is where we break down our storage form of carbohydrate, glycogen into glucose and we use our liver stores first. Then to keep up with the body's metabolic rate, which appears to be the same for the first period of fasting, we have also whole body lipolysis which is where fat stores are broken down into their individual components, so free fatty acids and our fat oxidation increases. So we have this increase in free fatty acids kicking around in our bloodstream, which becomes the source of fuel for our muscles.

Taryn Richardson  04:55

Interestingly, protein metabolism doesn't seem to be affected in the short term by fasting but we don't know what happens long term. The energy used in those first two to three days of fasting appears to come from glycogen, so carbohydrate and fat stores. And our protein catabolism or breakdown doesn't seem to increase until about 36 hours into fasting which is so interesting. Now it might take more than 10 days for the body to adapt to intermittent fasting. What we don't really know is what happens long term. So as an active person, we're not talking about general population stuff right now. We are talking about you as a triathlete. What do we know about our performance if we're trying to fast and do exercise?

Taryn Richardson  05:49

So there's a lot of literature around the different modalities of exercise. When it comes to high intensity exercise, so things like cycling to exhaustion or sprinting, both of those are very dependent on carbohydrate availability. So obviously, there's a concern if you're fasting with high intensity exercise. And the majority of research in this space is done in athletes fasting for Ramadan, which does show a decrease in sprint performance, so both impaired speed and power with fasting. And as an endurance athlete, there are some really good papers to draw on to decide whether this is something that would be useful or beneficial for you.

Taryn Richardson  06:31

There was a big review done by Aragon and Vargas in 1993. I'll link it in the show notes if you want to get nerdy and read it. But they looked at fasting anywhere from 24 hours to 4 days and the effects on endurance performance. Now, they saw with this big review, that there were negative effects in all but one study. Since then, Aird et al 2018 did another big systematic review and meta analysis. I'll link it in the show notes as well. But this type of study is the highest level of research quality, a meta analysis. They found 22 studies that fell into their categorisation of what they wanted to look at on the effects of fasting and performance. So they looked at fasting overnight in this review, so eight hours. And they actually excluded Ramadan because that helps us to avoid confounding factors, so things like lack of sleep, which happen during Ramadan. Because if you can only eat in the dark, then you're probably going to forego sleep to get enough calories in or get some food in so you're not starving.

Taryn Richardson  07:42

We also see a reduction in calorie intake during Ramadan, typically. So they removed Ramadan studies from their reviews so that we didn't have those confounding factors like not sleeping well and not eating enough that both of them are going to affect performance. Interestingly, what they found was that 54% of studies discovered that pre exercise feeding improved exercise performance during anything that was continuous anaerobic exercise, so longer than 60 minutes. And the remainder showed no significant difference. For aerobic sessions that were less than 60 minutes, 57% of studies found no difference in performance between fed and fasted conditions, which makes sense. And the rest, the other 43% found that feeding before exercise improved performance.

Taryn Richardson  08:33

So what that tells us is that fasting doesn't improve performance. It either has a neutral effect or it has a negative effect. So if I lost you there for a little bit with numbers, in summary, as a triathlete who is an endurance athlete and also has elements of high intensity, high performance sessions, if you want to perform in your sport, then I probably wouldn't recommend intermittent fasting. Studies don't show clear uniform results but one thing is clear that fasting doesn't improve any type of athletic performance. Research either has a negative or a neutral effect. We definitely do need to do research in a longer term sense. What does longer term fasting do to our bodies and performance? But if you're somebody that has come from a non triathlon background and has found intermittent fasting worked for you before the sport, it is unlikely to do any advantage when you then try and layer triathlon training into that.

Taryn Richardson  09:42

We really need to think about our nutrition so much more differently when we're setting ourselves up to train for three sports in a week. We need to think about our pre training nutrition, our recovery nutrition for all of our training sessions. And if you're fasting through a pre training session and then recovery, then you're losing all of those beautiful adaptations that we can get from our training if we intersect our nutrition in to support that. You're not priming the body ready to train hard either if you are fasting. That big meta analysis shows really clearly that you will do a better training session if you fuel it. And we know that the shorter duration, very aerobic stuff is either not going to have an effect, or there's no difference if you fast versus fuel.

Taryn Richardson  10:34

So you really need to look at your programming and go, what sessions am I fueling, what sessions am I fasting and be strategic around that, rather than just fasting all the time or just fueling all the time. You actually need to set those up with strategy to get the most out of them. And it all depends on your overall body composition goals as well. I know intermittent fasting is a way for people to not eat and so it gives them some rules and guidelines around when they can eat and when they can't. And that generally helps to restrict calories. But we can do that so easily just by understanding how to eat to support our training and eating the right building blocks for our body rather than filling it up with junk.

Taryn Richardson  11:21

The other thing to consider is I've heard of a lot of people say they're fasting, but they wake up in the morning and they have a milky coffee. You are no longer fasting, if you've put milk in your coffee. You are no longer fasting if you put milk in your coffee. You can have a black coffee, or a lot of people do what's called a bulletproof coffee where they're adding fats to their coffee. But as soon as you put carbohydrate in your mouth, you are turning on that switch to use carbohydrate as a fuel source and so you're no longer in that fasted state. So makes no sense to say you're fasting to lunchtime but you've broken your fast by having a milky coffee.

Taryn Richardson  11:58

So the bottom line is, I wouldn't recommend it if you're a triathlete. If you are the general population, maybe. But there are so many other ways that you can reach your goals, whether it's body composition, or overall health or metabolic markers, or whatever it is without having to restrict your window of eating. You just need to know how to build your plate real estate for what you need, not what you can eat. So any questions on fasting, send me a DM. I'm happy to chat about it. I will link these papers in the show notes if you do want to get down and dirty in the research. They're both really well done. But it's not something I would recommend for you. And if you are doing it, then maybe come and join us in the Academy and I can show you how to eat to support training without having to starve yourself all day and do a crap job of supporting your training with the right building blocks.

Taryn Richardson 12:53

Thanks for joining me for this episode of the Triathlon Nutrition Academy podcast. I would love to hear from you. If you have any questions or want to share with me what you've learned, email me at [email protected]. You can also spread the word by leaving me a review and taking a screenshot of you listening to the show. Don't forget to tag me on social media, @dietitian.approved, so I can give you a shout out, too. If you want to learn more about what we do, head to dietitianapproved.com. And if you want to learn more about the Triathlon Nutrition Academy program, head to dietitianapproved.com/academy. Thanks for joining me and I look forward to helping you smashed in the fourth leg - nutrition! 

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