When you eat your body will burn the nutrients that are readily available first. Meaning that when you eat before cardio training the meal will be used as the energy source (and not fat).

Training FASTED means you have not consumed any food prior to training, your insulin levels are at baseline and your body is no longer digesting or absorbing food, typically 3-6 hours after consuming food. It is primarily performed in the morning as soon as you wake up, but DOES IT WORK? 

- Increases fat oxidation and the breakdown of fat cells. 
- Great option if you can't train on a full stomach
- Good for morning people who want to get it out of the way early in the morning

- Lack of food/energy in their system actually leads to decreased training intensity. 
- It doesn’t necessarily translate to a greater fat loss at the end of the day. Burning higher amounts of fat during one part of the day, can mean burning less fat later in the day and relying more on energy from foods eaten later on.
- If you train at a high intensity (hello HIIT & Crossfit), you may actually lose muscle as your body needs energy quick and glycogen stores (from food) are low.

Some research suggests that there is no change in body composition is comparison to fed training vs fasted cardio (Schoenfeld et al., 2014).

But your body can adapt to training in a fasted state by “learning” to use its glycogen stores more efficiently.

The final verdict: Fasted cardio is a great way to burn fat more efficiently, you just need to start slow and see if this way of exercising is for you. Also remember not to go overboard with the intensity to help preserve your precious muscle mass!

Body and Bone

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 Nutrition, 11(1), 54.
Van Proeyen K, Szlufcik K, Nielens H, Ramaekers M, Hespel P. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. J Appl Physiol (1985). 2011;110(1):236–245.