Hi, I read the major benefit of Hiit, especially for very morbidly obese people is that, it alters you genetically by activating parts of what is often a dormant survival system. Hiit essentially mimics situations that would occur in nature where short bursts of speed would get you away from a predators attacks then you would be moving out of its hunting ground dodging situations. Different genes activate as this becomes regular to tell your body not to store too much fat as 'movement is survival'. Your metabolism starts working again properly and in people who are largely sedentary this can often be the key to unlocking large weight loss. I think however the real life 'fight or flight' situations that can put massive stress on your body would effect your heart. But Hiit is controlled so should be system shock of a much lower magnitude. ( As an aside roller coaster rides and other thrill seeking activities as well as nature walks or even zoo visits can often help activate this system as well )
There are obviously other switched on/off or more like dimmered survival systems that interfere like weight gain to protect from cold or fat as a surplus, where there is a possibility of food shortage. Like crash gaining weight after diets. They did a twins study years ago that showed the cold one can be jammed on by things like emotional loss. Highlighted the difference in dna from a time when the obese twin had suffered, for example the death of a friend or gone through some emotional event that hadn't been dealt with. While the 'normal' twin living a similar lifestyle had not and had not started gaining weight. All the info is on the web for people looking to deal with large weight loss but I recommend looking at work by a guy called Jon Gabriel. He explains a lot of the science much better than I can and has the documentary evidence from research that I cannot be bothered writing out
Hope everyone has had an awesome Christmas. Good luck with whatever you are trying.