I've been lurking here for a while and I've noticed that a common response to questions about belly pooches is "cortisol." From what I see here, on Mark's Daily Apple/Archevore/ other paleo-oriented websites, cortisol is elevated through stress that's both physical and psychological. The confusing part is that people are sometimes then told to reduce the HIIT and replace it with weight training, but I don't see how that works. I can see how the following scenarios work:
Lack of sleep - body is stressed due to lack of rest, cortisol rises
Crummy boss/workplace - cortisol rises as part of a fight-or-flight response
Enraged saber-toothed tiger ten inches from your butt - that's true fear.
HIIT - the heart rate rises, true, but a Crossfit metcon isn't necessarily a stressful situation. There's nothing life-or-death about it and I know that I can simply walk off mid-workout if I choose. There's no forced psychological component other than the self-imposed one. (Burpee penalties for quitting aren't true psychological stressors either.)
In weight training, the heart rate soars for brief(er) periods too, especially during sets. So theoretically that'd contribute to cortisol elevation too.
So, why does HIIT contribute to cortisol elevation but weight training doesn't? Is cortisol raised purely when the heart rate is elevated, regardless of the cause?
Cortisol is released when the brain senses a "fight or flight" response, both to emotional and physical stress.
Let's say you have a stressful desk job, your body may not release cortisol on a normal basis, but it will that day you suddenly realize you forgot to put together a report due in an hour…That panic, "oh crap," moment signals to your body that it's time to panic and cortisol is released.
In normal situations where the heart rate is elevated (maybe walking, weight lifting..depends on the person) if the body doesn't read it as "panic now!" then cortisol isn't released to come to the rescue. Often in HIIT, the body is pushed past what it can calmly endure. So the difference isn't the need for more oxygen or an elevated heart rate only…but the body's read that "it's time to panic."
Part of regulating cortisol is having a "relaxation" period after the stress, so I belive that is where the sleep comes in, but I suppose could also be controlled with mediation, yoga, taking the time to really come down and relax after a hard HIIT workout.
Just my thoughts on the matter.
Psychological stress is just as bad as physical stress, the physiological response is the same. The most important idea when discussing managing cortisol and stress is chronic vs. acute. Here's a clip from the master of the neurobiology of stress (and just an all around fantastic individual) Robert Sapolsky discussing this: sapolsky stress clip