Fasted cardio is one of those "love it or hate it" things. There are some people that swear by it. You'll see their IG stories of them hitting that morning run before eating. Or there are the people that say it's the dumbest thing ever, and that your body needs fuel to function.
So who's right? And what science is there to back up their opinion? Let's dive into the world of fasted cardio.
Immediately after you chow down on a burger, your body physiologically enters an absorptive state. For up to approximately three to four hours after eating, the digestive system takes up nutrients and breaks them down into basic sugars and amino acids. Sugars are simplified into glucose molecules, and insulin, a digestive hormone, is released to stimulate the storage of glucose in cells (Rice University 2013).
You may have heard endurance athletes refer to “quick carbs” as they pop a few gummy bears before heading out on a long bike ride- glucose can be utilized quickly after ingestion if energy is immediately exerted. The excess glucose (not immediately utilized) is stored as glycogen- that’s right, these are quite literally your “glycogen stores.”
Once the food has been entirely digested and stored, the body enters a fasted, post-absorptive state. (Does the word “break-fast” make more sense now?) In a fasted state the body relies on stored energy. In the absence of recently ingested glucose, glucagon is released. This hormone works as the opposite of insulin- it stimulates the conversion of glycogen back into glucose so it can be released from cells as an energy source (Rice University 2013). “Fasted cardio” refers to working out in the post-absorptive state, several hours after a snack or meal (most typically in the morning after 8-10 hours without food). The idea stems from a desire to burn fat. At first thought, this sounds like a foolproof plan. Before skipping the peanut butter toast, let’s break it down.
Working out on an empty stomach will certainly make the body likely to breakdown stored fuels like fat during exercise. This effect can benefit endurance athletes hoping to become more energy efficient in long exercise stints where food is not available. When it comes to “fat-burning”, however, this does not account for energy usage throughout the rest of the day. Studies like those by Edinburgh, et. al. (2019) and Hackett, et. al. (2017) suggest that the body simply balances its breakdown of energy sources after the fast is ended, leading to no significant difference in overall fat burn as a result of fasted or fueled cardio. In fact, these same studies noted that working out in a fueled state increased performance and allowed for higher intensity, suggesting better results in the long term (Hackett 2017).
For those looking to lose weight- fasted cardio could provide benefit by shortening the daily window of food intake, making caloric deficit easier to achieve in theory (but let’s not ignore that late-night tendency to overcompensate). However, those hoping to maintain muscle mass or improve their B.M.I. must consider that the body will not necessarily discriminate between breaking down fat versus muscle tissue.