I started doing this 2 weeks ago that is 2 or 3 days with about 5 to 10 min. to warm up and then a couple of 20 sec. sprints. I seem to be getting more out of this than walking 30 min. 4 or 5 days a week. That is my mood has picked up some and I have cut down my eating just a little bit. As was commented it seems that the key to this is putting the mitrochondria under stress which reminds me of another study. In that study the subjects took about 1000mg of vitamin C and 400mg of vitamin E. and the control group didn't take the vitamins. When both groups did a few weeks of aerobic exercise the group that took the vitamins did not improve there insulin sensitivity. I do wonder if you would get much more out of doing these short HIIT sessions 5 or 6 days a week or if it's best to just do them twice a week.