The Science Behind Fasted Cardio

Fat…the one thing that everyone wants to get rid of. It slows us down, fluffs us up, and makes us think twice about bearing it all at the beach. Our culture’s focus on getting rid of fat has led to countless fad diets, pills, and motivational videos that oftentimes do not work. One of the most interesting methods of losing fat is known as the intermittent fast. It involves a person doing all of their eating in a short 6-10 hour window and “fasting” for the rest of the day and night. So what exactly happens when this takes place, and more importantly, does it work?

When we eat foods with high fat, or really just any type of food in excess, our body stores this extra energy in the form of triacylglycerides (TAG). This takes place during a type of reaction called an “esterification” where fatty acids are modified and then added to a glycerol backbone to be stored as TAG’s. This is how both saturated and unsaturated fatty acids are stored when the body has low current energy needs. These TAG’s make up our adipose (fat tissue) and can be used for energy at certain times. An excess of calories accompanied by little exercise can result in an increase of TAG synthesis, then resulting in an accumulation of adipose in the body. This is the fat that most don’t like as it is unnecessary for function and oftentimes is visually unappealing. So, how do we get rid of these TAG’s and adipose tissue?

Lipolysis is the inverse reaction of TAG synthesis as it breaks down fat stores in the body to be oxidized for energy. Lipolysis involves the breaking down TAG’s to liberate 3 fatty acids and one glycerol. This is carried out by a special group of enzymes called lipases, which cleave the fatty acid off of the glycerol backbone. The free fatty acids are then transported to the mitochondria of the liver and skeletal muscles where they are oxidized. The oxidation process involves breaking down the fatty acid chains, two carbons at a time, in order to convert these molecules into acetyl-CoA for ATP synthesis. In this process, fat is turned into intermediates used in the TCA cycle. This results in fat being “burned off” as a source of energy. Now, how do we make this happen?

Lipolysis is not always the predominant form of energy in the body. For the first 12 hours after a meal, Glycogenolysis (the breakdown of carbohydrates stored as glycogen) is the primary energy source. This changes around that 12 hour mark as the body goes deeper into a “fasted state”. It is at this time (ideally 14-16 hours after the last meal) that Lipolysis takes center stage and free fatty acids are liberated from TAG storage. Lipolysis also is very active during low to moderate exercise, think a brisk walk/jog that would keep your heart rate around zone 2-3. This is the scientific backing for accompanying an intermittent fast with some sort of cardio. Say your last meal was at 6 p.m. on tuesday, and at 9 a.m you went for a 15 minute jog while keeping your heart rate around 60-70% of its max. This is a peak condition for fat burning as it combines fasting conditions with a low to moderate exercise.

Overall, fasted cardio and fasting in general definitely makes scientific sense if you want to burn some “stubborn” fat that you have been holding onto. Just be sure that you hydrate properly and make sure that you consume enough fuel in your eating window. Trying to cut calories significantly while doing fasted cardio could be dangerous as you may become faint.