From this, we can easily deduce that it is strongly recommended to carry out vigorous exercises/activities while not in depletion of glycogen.
Therefore, fasted training should be carried out at low intensity at a steady state (aerobic activity, below the aerobic threshold)
In fact, during fasting, the levels of the hormone insulin are kept low, consequently there will be, in contrast, a high level of glucagon, an hormone that elevates the levels of blood sugar (glycemia), but also of the receptors at the level of the adipocytes, favoring the lipolysis (demolition of body fat for energy purposes).
But the glucagon, unlike the adrenaline generated by the HIIT, can not affect the intramuscular glycogen storage, so it's not catabolic.
Furthermore, a low-intensity workout do not increases the cortisol level as much as high-intensity workout. This indicates that low intensity workout are less catabolic.
