HIIT can be disastrous for people who are not taking in adequate carbohydrate, as the main fuel for anaerobic exercise such as HIIT is glucose. I personally know several individuals who who overdid HIIT while eating ultra-low carb, and ended up losing so muscle they look like Auschwitz death camp survivors. Their metabolisms ground to halt, and they look and feel like shit, and walk around like emaciated zombies.
If you eat plenty of nutritious carbs, you can get away with HIIT and get great results from it. If you are eating an ultra low-carb diet, or perhaps in the final stages of contest prep, steady state cardio will tap into the last bit of fat reserves without catabolizing muscle, but absolutely do not over do it. If you have issues with ankle pain, do your cardio on an exercise bike or elliptical.
