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Fasted cardio - yeah or nah?

Updated: Dec 16, 2021

You may have read the headlines: “Working out on an empty stomach burns more fat!” and wondered if this is the way to faster fat loss. Sounds like a pretty simple solution.


Is it actually true though? Read on to find out whether fasted cardio really is the way to faster fat burning and what impact (positive or negative) it might have on your performance.


Before we look at the effect of fasted cardio on fat loss, it’s important to have an idea of what is meant by ‘fat burning’.


Fat oxidation vs losing body fat

Often, people use the term ‘fat burning’ when referring to body fat loss. Or the term might be used when fats in the body are broken down and used for energy. However, these two ideas are not quite the same thing, so it can get super confusing.


When fats are broken down into their smaller components which are then used to create energy, this is called fat oxidation. While fat oxidation may be one step towards losing stored body fat, it’s only one part of the process.


Body fat is lost when the energy used up by our bodies each day is greater than the amount of energy coming in from food and drinks. When we use up more energy than we consume, we have to dip into our stored energy reserves such as our body fat, to fill that energy gap. And so, as those fat stores are used, we end up with less body fat over time.


But, if the energy we take in each day matches the energy we ‘burn’ doing activity etc, there is no need to use stored body fat for fuel. Any fats that are broken down for energy but not used will just be put back together and stored again as body fat.


Fasted cardio and ‘fat burning’

Fasting (going without food for several hours or more) creates changes in the body that reduce our body’s preference for using carbohydrates for fuel, switching to using more fat as a fuel source.


If we do aerobic exercise after an overnight fast – i.e. not having breakfast before you exercise – this can increase the rate of fat oxidation in the hours after exercise, compared to exercising after eating.


Great! So fasted cardio is better for fat loss?


Not so fast. Here, the devil is in the detail.


  • Fasted cardio doesn’t actually increase fat oxidation if you’re working out at higher intensities (around 70% VO2 max or higher, for those of you interested in the numbers). So really, we’re only talking low to moderate intensity cardio for a potential fasting benefit.


  • More importantly, the question to ask is whether this short-term increase in fat oxidation leads to an actual loss of body fat. As it turns out, fasted exercise has no effect on body weight or fat loss compared to exercising after eating, if people consume the same energy (calories) over the course of the day.


  • In saying that, it is possible that doing aerobic exercise after an overnight fast may lead people to eat less throughout the day, although the research on this is pretty limited. If training fasted does lead you eat fewer calories than you burn over the course of the day, this would contribute to fat loss over time.


Running fasted

"Train low, compete high"

Besides the question of fat burning, there is the potential that completing certain training sessions in a fasted state (or with low carbohydrate availability) may enhance various adaptations to training.


These adaptations include improving the aerobic capacity of muscles, up-regulating enzymes involved in energy processes or increasing the production of mitochondria - the energy 'powerhouse' of our cells.


This is where the principle of "train low, compete high"comes in. An athlete performs selected training sessions in with low carbohydrate stores to enhance adaptations to the training so that - theoretically at least - when they compete at a later date with adequate nutrition stores, they are able to perform to a higher level.


However, there appears to be a lack of consistent, favourable research showing that these adaptations do in fact lead to tangible performance improvements so the jury is still out on that.


Eating to fuel performance

While the idea of greater fat loss from fasted cardio and the potential for improving adaptations to exercise may be appealing, getting enough nutrition at the right time is key to fuelling exercise.


Making sure to have a good supply of carbohydrates in the body is important, especially at higher intensities and for longer duration events. Having plenty of carbohydrates to draw on helps us to train at our best and optimise performance.


Carbohydrate intake before, during and after exercise needs to be considered, to make sure we optimise the amount of fuel available and to replace the stored carbohydrate that we use up during exercise.


Eating protein regularly is a no-brainer for maximising muscle growth, helping our bodies to recover from and adapt to exercise.


Getting plenty of vitamins and minerals throughout the day keeps us healthy, enabling us to train and recover well and making sure we hydrate properly is important to exercise safely and perform at our best.


These nutrition basics are fundamental to being healthy, active and getting the most out of our training. So make sure you don’t ‘throw the baby out with the bath water’ by getting tied up with the theoretical fat-burning and performance benefits of fasting regimes.


The verdict

If fasted cardio is something that works with your lifestyle, you’re getting in enough nutrition throughout the day and you’re able to exercise safely and effectively on an empty stomach, you may find it works well for you.


However, don’t expect huge improvements in body fat loss or performance from exercising on an empty stomach. In fact, fasting is generally known to impair performance.


So, if it’s important that you get as much out of your training sessions as possible, if you have a game or event, or even if you feel a bit crummy working out on an empty stomach, make sure that you fuel properly beforehand.


As with any fasting regime, going without food for a long period is not suitable for everyone, for example if you are pregnant, on certain medications or have a history of an eating disorder. Speak to a health professional for advice tailored to your specific situation.


Key takeaways

  • Doing aerobic exercise at lower to moderate intensities before you eat breakfast may increase fat oxidation (breaking down fat) in the hours after exercise.

  • At higher exercise intensities (>70% VO2 max), training fasted provides no benefit in terms of fat oxidation vs training after eating.

  • Studies show that increased fat oxidation from fasted training doesn’t necessarily lead to actual body fat loss, if you are eating the same calories over the course of the day.

  • If fat loss is what you’re after, exercising after an overnight fast might be one strategy you could use to help you eat fewer calories overall. Having an overall calorie deficit (eating fewer calories than you burn) will contribute to fat loss over time.

  • Doing selected training sessions in a fasted state may improve metabolic adaptations to training. However, it's not clear that this will lead to tangible improvements in performance.

  • Getting adequate nutrition is key to fuelling performance and keeping you healthy.

  • Fasting is not for everyone. Speak to a health professional for individual advice.


Updated 21 October 2021


This blog is intended for information purposes only. Speak to a Registered Nutritionist or Dietitian for nutrition advice tailored for your specific needs.


References


Ferreira Vieira A, Rocha Costa R, Oliveira Macedo RC, Coconcelli L, Martins Kruel LF (2016) Effects of aerobic exercise performed in fasted vs fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition 116:1153-1164. doi:10.1017/S0007114516003160


Gejl KD, Nybo L (2021) Performance effects of periodized carbohydrate restriction in endurance trained athletes - a systematic review and meta-analysis. Journal of the International Society of Sports Nutrition 18:37. doi: 10.1186/s12970-021-00435-3


Kerksick CM, Arent S, Schoenfeld B et al. (2017) International Society of Sports Nutrition position stand: nutrient timing. Journal of the International Society of Sports Nutrition (2017) 14:33 doi 10.1186/s12970-017-0189-4


Kerksick CM, Wilborn CD, Roberts MD et al (2018). ISSN exercise & sports nutrition review update: research and recommendations. Journal of the International Society of Sports Nutrition. 15:38. Doi: 10.1186/s12970-018-0242-y


Thomas DT, Erdman KA, Burke LM (2016). American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Medicine and Science in Sports and Exercise 48(3):543-68. doi: 10.1249/MSS.0000000000000852. Erratum in: Med Sci Sports Exerc. 2017 Jan;49(1):222. PMID: 26891166.


Wallis GA, Gonzalez JT (2019) Is exercise best served on an empty stomach? Proceedings of the Nutrition Society 78:110-117. doi:10.1017/S0029665118002574


Zouhal M, Saeidi A, Salhi A et al. (2020) Exercise training and fasting: current insights. Journal of Sports Medicine 11; 1-28. http://doi.org/10.2147/OAJSM.S224919

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