To exercise fasted or fed? It’s a controversial topic with strong views coming from both camps. For those promoting weight loss, exercising fasted is often an essential criterion to ensure the most ‘fat burning’ possible. Then there is the opposing argument that you want to be well-fuelled before activity to help power the length and duration of your exercise. As with most things in nutrition, the simple logic of both camps falls down when you take into account the complexity of physiology and psychology of us as human beings. In this podcast, I’ll look at the fasted versus fed exercise debate.
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To exercise fasted or fed? It’s a controversial topic with strong views coming from both camps. For those promoting weight loss, exercising fasted is often an essential criterion to ensure the most ‘fat burning’ possible. Then there is the opposing argument that you want to be well-fuelled before activity to help power the length and duration of your exercise. As with most things in nutrition, the simple logic of both camps falls down when you take into account the complexity of physiology and psychology of us as human beings. In this podcast, I’ll look at the fasted versus fed exercise debate.
Links referred to in the podcast
Episode transcript
To access the full episode transcript, go to the following link and select the individual podcast episode and then click on the ‘Transcript’ tab https://thinkingnutrition.buzzsprout.com
Connect with me
Instagram: doctimcrowe
Facebook: Thinking Nutrition
Twitter: CroweTim
Hi, Tim here. Every now and then I take the opportunity to have a mini-break from putting out new episodes. But it also presents a great opportunity to dip into my back catalogue of episodes to pull out some that got a lot of listening engagement, and which are also topics that I still get asked a lot about. And for this week, it is on the perennial questions asked by exercisers the world over if they’re trying to also lose a few kilos: is it better to exercise fasted or fed? Enjoy
To exercise fasted or fed? It’s a controversial topic with strong views coming from both camps. For those promoting weight loss, exercising fasted is often an essential criterion to ensure the most ‘fat burning’ possible. Then there is the opposing argument that you want to be well-fuelled before activity to help power the length and duration of your exercise. As with most things in nutrition, the simple logic of both camps falls down when you take into account the complexity of physiology and psychology of us as human beings. In this podcast, I’ll look at the fasted versus fed exercise debate.
People do exercise for all sorts of reasons. General health and fitness, training for competitive sport or to help shed a few stubborn kilos are all common reasons. So, I’ll take that last point first: if your aim is to lose weight, are you better off exercising on an empty stomach? It is a seductive idea: you want to ensure all that sweat is using up your body’s fat stores to fuel its endeavours rather than grinding your gears and go nowhere because you’re metabolising the meal you just ate before exercise. But what actually happens?
Well, I’ll take a step back first and talk about ‘fat burning’ in general when it comes to exercise. Well-meaning advice claims that you need to exercise at a low-to-moderate level of intensity to burn fat. Now yes, it is true that the body burns the greatest percentage of fat at lower intensities of aerobic exercise, but at higher intensities, you burn way more total kilojoules – and more fat kilojoules overall. If you were are only concerned about burning the greatest percentage of kilojoules from fat, then sleeping is where it is at, but the total energy cost of this is tiny. Low-intensity workouts do promote weight and fat loss; you just have to do them for a longer time. When time is limited to exercise, then it makes sense to work as hard as you can safely do so to get the most health, performance and weight loss gains from it.
So with that out of the way that it is the intensity and duration that matters most for long-term weight balance, then what about the follow on question of whether you’re fasted or fed during exercise - will it get you to a fairly similar place? Doing cardio exercise does increase fat oxidation and it seems that people who do this in a fasted state oxidise more fat than glucose, but this doesn’t translate to increased body fat loss all things being equal when comparing it to a similar situation when doing the exercise fed. Why? Short-term fat oxidation burning is pretty meaningless as it’s the long term balance–over days and weeks–that will dictate whether you’re losing or gaining body fat. So, if you exercise fasted and enjoy a small spike in fat burning, it is offset by the ‘delayed gratification’ that you’re just shifting what you would have eaten to later on so would be in a larger state of energy excess at that time. Now, for the person who exercises fed, sure, they may have had less fat oxidation during exercise, but the energy deficit will be delayed as they won’t be eating as much later. Again, remember I said here that all things being equal in that our fed and fasted exercise scenarios were both eating the same about of food over the day.
But, let's delve into the science. A 2017 meta-analysis looked at the effect of overnight fasting before exercise compared to eating before exercise and how this affected weight loss and body composition. And I’ll link to the study in the show notes. And while only 5 studies could be included in the review, the conclusion was that performing exercise in a fasted state did not influence weight loss or changes in lean and fat mass. These findings support the notion that weight loss and fat loss from exercise is more likely to be enhanced through creating a meaningful caloric deficit over time, rather than exercising in fasted or fed states. https://www.mdpi.com/2411-5142/2/4/43
What about exercise performance when fasted though, is it impaired? A 2018 meta-analysis on fed versus fasted cardio found no difference in exercise performance when workouts lasted less than an hour. But the situation was different when workouts lasted more than an hour and eating before the workout improved performance. And I’ll link to this study in the show notes https://pubmed.ncbi.nlm.nih.gov/29315892
I want to stress that the findings from this review represent the average of many individuals so there are always lots of inter-individual differences, but as far as blanket population advice goes, there is no strong reason to recommend someone exercise fasted or fed when it comes to performance or weight loss goals when we’re only talking about an hour or less exercise. Go with what you body tells you. I’m a morning runner and really struggle to run with food in my stomach – I just feel sluggish for it so whether it’s psychological or physiological, I just feel better running on an empty stomach. Others need to eat before exercise as they feel their blood sugar falling and weakness happening from it.
So, whether you train fasted or not comes down to personal preference and scheduling. Do you prefer or enjoy training fasted? Then train fasted. Do you find you’re not hungry in the morning and eating before a workout makes you nauseous? Then, yep you guessed it, train fasted. If you feel you need to eat before exercise, then eat.
Intermittent fasting
Another extension on the question of exercising in a fasted state is how it applies to intermittent fasting. On the surface, some of the principles of intermittent fasting do seem to go against the core sports nutrition concepts of refuelling and spreading food intake (especially protein) over the day. But in fact, there could be some benefits from ‘training low’ at times where carbohydrate especially is limited to promote favourable fat adaptation. Periods of ‘training low’ are then offset by times of ‘training high’ for high-quality sessions and race day where peak performance is needed and where you would revert back to standard feeding regimens. So, intermittent fasting could fit within into this type of protocol if it was periodised.
A 2019 review on this very topic of intermittent fasting and athletic performance, which I’ll link to in the show notes, concluded that there is probably no great benefit on athletic performance while fasting. But everyone’s mileage is different here and your own performance would be the best guide. This is separate though to using intermittent fasting for weight loss to help improve the power-to-weight ratio in a sport where a performance benefit would be likely, but intermittent fasting is just one in a long line of ways to achieve weight loss. https://pubmed.ncbi.nlm.nih.gov/31283627
If you’ve consistently trained in a fasted state, the metabolic and muscle adaptations you’ll acquire will boost performance when you compete in a fed state. And that’s everything, isn’t it? A fasted workout trains you to perform under difficult physiological conditions of low fuel availability and that comes in handy. You probably wouldn’t enter a race or powerlifting match in a fasted state, but the fasted workouts you did in the months leading up to competition make you more likely to win.
Summary
Fasted training isn’t superior to fed training and if your goal is fat loss. If you enjoy training fasted or your schedule only permits training early in the morning, then train fasted. There’s very little difference between cardio in the fed or fasted state with regard to fat loss, muscle preservation, daily caloric intake, or metabolic rate. What really matters, then, is you. Some people feel lighter and energised when they do cardio on an empty stomach, while others feel light-headed and sluggish. Fed or fasted state: pick whichever makes you feel better.
So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.
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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.