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  • Isaac J. Wedig MS, CSCS

Is Fasted Cardio Better for Fat Loss?

Fasted-cardio is a popular fat loss strategy that consists of performing cardiovascular exercise first thing in the morning, on an empty stomach, following an overnight fast.


Made popular over the years by bodybuilders, athletes, and fitness gurus alike, fasted-cardio has long been promoted as a superior means for achieving fat loss when compared to non-fasted cardio that is performed later on in the day.


In this article we'll discuss the rationale behind fasted cardio and assess whether or not it is a valid means of accelerating fat loss.




What if I told you there was a way to get your body to burn more fat during your workouts?


That would be fantastic, right? 


So fantastic, that it might even convince you to wake up at the crack of dawn, head to the gym, and jump straight on the treadmill, skipping breakfast all together. 


That sure doesn't sound fun, but could skipping your morning oatmeal and egg whites be the secret to accelerating your fat loss?


In a fasted state, such as when you wake up in the morning, your body's glycogen levels, a form of stored carbohydrate, are relatively low. With less carbohydrate available for fuel, the body should theoretically be forced to rely more heavily on stored fat for energy demands.


Additionally, in the fasted state, insulin levels are lower. Since insulin is a hormone that suppresses fat breakdown, then lower circulating levels should theoretically allow for greater rates of fat breakdown and utilization.


Not only does this theoretically make sense, it's been fairly well demonstrated that low-to-moderate intensity aerobic exercise, performed in the fasted state, does indeed result in greater fat oxidation (fat burning) compared to exercise performed in a fed state following a carbohydrate containing meal (Vieira et al, 2016). 


The type of fuel utilized during exercise is dependant on many variables, such as exercise intensity, duration, and an individuals level of training, however, details aside, its fair to say that fasted cardio typically results in greater fat use.


Therefore, it's fair to hypothesize that fasted cardio may facilitate superior fat loss when compared to cardio done in the fed state. 


Okay. Seems like a done deal then, right?


Let’s wrap this newsletter up! Fasted cardio leads to a greater amount of fat burning, so if you want to get shredded, then do fasted cardio.


Hold up. Not so fast there slick.


While the rationale behind fasted cardio is there, claiming that it's a superior means for fat loss suffers from one major flaw. There’s a big difference between ‘burning fat’ and ‘losing fat’. 


Fat loss, or the reduction of bodyfat, is a dynamic process that is determined by the overall net balance between fat storage and fat breakdown occurring over a 24-hour period.


Fat storage and fat breakdown are two opposing processes that are constantly fluctuating over the course of the day as you transition from meal-to-meal, from a fasted-to-fed state, and as you change your physical activity level and energy demands. 




For simplicity purposes, lets just say that fat breakdown and fat burning are the same thing. When the overall rates of fat breakdown exceed the overall rates of fat storage, this creates a negative fat balance, and the result is a loss in body fat. 


Since fat loss is determined by the overall 24-hour fat balance, and not the rates of fat burning that are occurring at any one time of day, then we simply can't assume that fasted cardio will lead to superior fat loss. Exercise comprises one, maybe two, hours of the day. That leaves twenty-some odd hours in which fat breakdown and storage are occurring. 


Research has demonstrated that although fat burning may be higher during the exercise period with fasted cardio, rates of fat burning during the 24-hours following exercise are much lower compared to non-fasted cardio (Paoli et al, 2011). Therefore, if you burn more fat during exercise, you’ll burn less fat the remainder of the day. 


This makes it difficult to tell which type of cardio will facilitate superior fat loss.

In order to really tell which form of cardio is better, we have to compare long-term changes in bodyfat between individuals who are habitually using either fasted cardio or non-fasted cardio and see if there is a differential effect. 


Shoenfeld et al (2014) was the first to do so, in which they compared two groups of women, both on a calorically restricted diet and both performing three hours of moderate intensity cardio per week. However, one group performed their cardio sessions in a fasted state and the other performed their cardio in a fed-state. After 4-weeks, both groups experienced a significant amount of fat loss and there were no differences observed between groups, suggesting that both forms of cardio were equally as effective.


Even further, a 2017 systematic review and meta-analysis examining the topic concluded that fasted cardio does not increase the amount of weight loss or fat loss compared to non-fasted cardio. Additionally, they suggested that weight loss and fat loss from exercise is more likely to be enhanced by creating a meaningful caloric deficit over a period of time, rather than exercising in a fasted or fed state (Hackett and Hagstrom, 2017).



Summary


There are certainly limitations to the current body of research, however, as the data stands, it’s sensible to say that fasted cardio most likely doesn’t provide any special fat loss benefit over non-fasted cardio. And if it does, the effect is probably very minimal. 


Recommendations

 

If you like to do fasted cardio because it fits your schedule, you don’t like to eat in the morning, or it helps to energize you for the day, then by all means, include it in your routine.  


If you find that you have low energy levels in the morning without eating and you aren't able to exercise for as long or as hard as you could in the fed state, then opt for fed cardio instead. 


The majority of research comparing fasted and non-fasted cardio controls for exercise type, duration, and intensity. However, in real life, with free-living individuals, these variables aren't controlled. No one is measuring your execs duration or intensity and requiring you to meet protocol. If fasted cardio negatively effects your exercise duration, intensity, or workout quality, then it may lead to less calorie burning and actually be inferior for fat loss.


Rates of protein breakdown also tend to higher during fasted cardio, suggesting that it may negatively effect muscle building and strength gain.


Taken collectively, its up you to decide whether or not to do fasted cardio. However, don't force yourself to do it under the false assumption that it leads to greater fat loss.



References


Hackett, D., & Hagstrom, A. (2017). Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis. Journal of Functional Morphology and Kinesiology, 2(4), 43.


Paoli, A., Marcolin, G., Zonin, F., Neri, M., Sivieri, A., & Pacelli, Q. F. (2011). Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. International journal of sport nutrition and exercise metabolism, 21(1), 48-54.


Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger J, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. JISSN. 2014;11:54


Vieira, A.F.; Costa, R.R.; Macedo, R.C.; Coconcelli, L.; Kruel, L.F. Effects of aerobic exercise performed in fasted v. Fed state on fat and carbohydrate metabolism in adults: A systematic review and meta-analysis. Br. J. Nutr. 2016, 116, 1153–1164.

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