Updated: Mar 4, 2020
I know the gym bro’s lead you to believe…
“because you’ve not eaten and your stomach is empty, your body is only going to use fat for fuel”
There are lots of reason why for most cases this is just bro science. Firstly, alot of people who are doing fasted cardio usually do it in the morning and normally aren’t actually fasted. I know, the irony... This is because they would have eaten the night before, which will be the fuelling the morning workout. Hence the reason athletes are advised before any long endurance bout to eat a good bowl of pasta the night before.
But lets give the bro’s a chance here. Lets say indeed you were fully fasted; which by the way can be up to, or even longer than 24 hours to deplete glycogen stores. But lets say we were fully fasted and we have no glycogen present.
Then, this is where it gets tricky for the bro’s and where I think most people get it wrong. Fasted cardio should be at a very low intensity (50% of max heart rate), most people train way above that and when your heart rate goes nearer to 70% and above, your body is forced to use carbohydrates (glycogen) for fuel.
You see fat is highly oxidative, so assuming you’re reading this sedentary and not in the middle of doing HIIT intervals on the battle ropes, if you were I’d be very impressed. Then right now, your heart rate should be pretty low and you can breath super easy, meaning lots of oxygen is present. This is the perfect environment for fat burning.
This is the reason bodybuilders do an hour a day of cardio, around 50% of max heart rate
(referred to as a steady state cardio) before stepping out on stage. Because they’ve been cutting out carbs for the last few weeks, their glycogen levels are ultra low. Therefore, they must keep their heart rate low. If your body needs carbohydrates, and there are low glycogen stores, your body will make its own through a process call gluconeogenesis and turn protein or fats into carbs. Well the problem when glycogen stores are low and cardio is above 70% of max heart, your body next preferred choice isn’t fat but actually protein because it can’t mobilise the fat as there is little oxygen present. So it is forced to use protein, amino acids that are stored in the muscle, literally stripping your gains for fuel.
So, in summary, fasted cardio can be a method for fat loss. It can work for sure, but be mindful of your choice of exercise, that it’s not very intense (like a brisk walk), so your body favours the fat for fuel. I'd advise fasted cardio when trying to get very lean, on low calories or little to no carbohydrates in your diet. For example, when prepping for a bodybuilding competition. For most cases people who say they are fasted still have plenty of glycogen present and all that normally happens is they feel miserable and have a poor workout.
Sorry Bro’s this myths busted!!