By Adam Bisek

Amongst the vast annals of bodybuilding folklore lies the theory of fasted aerobic activity; “fasted cardio.” The notion implies doing a bout of aerobics in the morning on an empty stomach will result in greater fat loss. Much like many long-standing hypotheses, this one went years without being proven with hard evidence; a muscle-bound wives’ tale for all intents and purposes. This is what happens when anecdote marries seemingly sound logic because on the surface it makes sense. However, in the rhetoric to follow I will finally help distill this myth through the filter that is actual data.

Where did this idea come from?

It’s hard to pinpoint who or what timeframe brought this concept to the fitness space. However, we may be able to point to a study done in 1976 by Alborg and Felig. The study showed that when carbohydrates were consumed during a bout of exercise oxidation of glucose increased and lipolysis diminished. Essentially, when carbohydrates were consumed during activity the body used more carbohydrates as fuel and less fat. To further this a study done ~20 years later in 1997 by Horowitz and colleagues found that lipolysis was suppressed, and fat oxidation was reduced during activity when carbohydrates were ingested before exercise. So up until this point in fat-burning history, the fasted cardio hypothesis was only validated and perpetuated. Notwithstanding this surface-level logic, it may be easy for many to see at this point that the research was a bit short-sighted, that there’s probably a bit more to the story.

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Where the logic went wrong!

The aforementioned thought process is akin to “give ‘em an inch and they’ll take a mile.” The notion that increased fat loss during a relatively transient, short period of time during the day leads to greater fat loss over a 24-hour time frame is in fact, a stretch. In what seemed to initially confirm the long-standing hypothesis a study done by Paoli and colleagues in 2011 did show again that fat-burning was higher during fasted versus fed cardio. However, by measuring something called RER (Respiratory Exchange Ratio), which put simply is a measurement of the gases one exhales, they were able to measure what fuel source (carbohydrate or fat) subjects were using over 24 hours. What was found was that while the fasted group burned more fat during their exercise, they turned to burning more carbohydrate during the rest of the day. For the fed exercise group, the inverse was true, thus 24-hr fat loss would inevitably be the same. These findings were the first to truly refute the fasted cardio axiom, but again only revealed what occurs in a short time frame.

Finally, in 2014 a longer study was done that could very well have put the nail in fasted cardio’s coffin. Schoenfeld and colleagues found that when controlling for diet, that is the same amount of caloric deficit and macronutrient breakdown, over 4 weeks fasted and fed cardio resulted in the same amount of body fat loss. While some may point to the subject pool size and duration not being large or long enough respectively, this study did finally give solid grounds on which to say fat loss outcomes are the same whether you do aerobic activity fasted or in a fed state.

If the Schoenfeld study wasn’t enough a meta-analysis was done on this very topic in 2017; a study of studies. As you would imagine the Journal of Functional Morphology and Kinesiology (say that two times fast) concluded that their “review of a small number of studies does not support the use of fasted exercise for weight loss and positive changes in body composition.” They go on to say that fasted exercise hasn’t, however, shown any detrimental effects to body composition, and overall that there needs to be more research done that covers longer periods of time, volumes of exercise, and controls more for dietary habits.

To eat or not to eat…

By this point in the article, I would imagine you’ve surmised whether or not to do cardio fasted is a function of personal preference, and that maybe there’s a bit more context to explore. If that’s what you were thinking you were right (insert winky-face emoji). There’s has not been a ton of data on this exact topic to date, but from what we know (combined with quite possibly some common sense), simply getting in the activity is paramount when it comes to fat loss. Where things get a bit hairy are when we start talking about weight training or higher intensities of conditioning. These tasks, where carbohydrate availability may very well massively impact performance, may come with the prudent recommendation to be done in a fed state. This, however, is most certainly a topic for another discussion and involves a whole ‘nother examination of the current pool of research.

So, if you find it easiest for your schedule to wake up and zombie march on an incline, great, you do you. If an empty stomach coupled with the stairs to nowhere leaves you light-headed and uncomfortable, maybe “brekkie” prior to the gym is a better idea for you. At this point, if we are strictly talking about body fat loss, all else being equal, fasted or fed cardio are the same. There are plenty of bodybuilding myths out there but fear not as us in muscle-building academia will do our best to make things clear as to what will help you achieve your true 6-pack potential.

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Ahlborg, G., & Felig, P. (1976). Influence of glucose ingestion on fuel-hormone response during prolonged exercise. Journal of Applied Physiology41(5), 683–688.

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 Kinesiology2(4), 43.

Horowitz, J. F., Mora-Rodriguez, R., Byerley, L. O., & Coyle, E. F. (1996). Lipolytic Suppression Following Carbohydrate Ingestion Limits Fat Oxidation During Exercise 442. Medicine &Amp Science in Sports &Amp Exercise28(Supplement), 74.

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 Metabolism21(1), 48–54.

Schoenfeld, B. J., Aragon, A. A., Wilborn, C. D., Krieger, J. W., & Sonmez, G. T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition11(1).