[saiya-jin]

I think its a bit more complex than that. HIIT directly burns less calories, but it creates more muscles over time. Similar in weightlifting if you go for low reps high weight workouts which are basically HIIT.

In both cases, your metabolism adjusts - it feeds more muscles, its set into high-energy mode with 'fast' digestion.

I wouldn't recommend it to most though - if done properly its very taxing. I mean being in anaerobic state and pushing through is a hard sell to people who barely muster enough motivation to do occasional jog or spinning. One needs to be in top shape overall for it, no hidden heart issues etc.

[matwood]

I think people need both steady state and HIIT. Much like it's hard to go super heavy low reps all the time when lifting. It's good to cycle up and down.

As you mention, HIIT is also very hard for many people do even if they are healthy and want to do it. The body naturally fights going all out.

I've pitched training BJJ on HN (well to anyone really), and HIIT is yet another reason. People who train end up doing HIIT without even realizing. The body is in defense mode which breaks down some of the natural steady guardrails.