This is always promoted, however I read in a book I recently purchased by a Swedish nutrion and excercise guru that this should only be done when the intensity is very low (walking allowed, running forbidden). The reasoning behind this is that body-fat can only be metabolized in a certain pace, and if you exceed this muscles will be sacrificed as well. Is this bs or is the guy on to something?
Sounds like the Swedish guru knows a little something about exercise physiology.
This gets back to the low-intensity burns more fat propostion. Yes, during exercise, low-intensity exercise allows for greatest fat oxidation, but at the expense of total kcals burned and increased metabolic rate and excess post-exercise oxygen consumption. Those three factors, and more, are maximized when adhering to a high-intensity exercise program (i.e. HIIT).