Think of it as changing the fuel rather than "boosting the metabolism". for example HIIT uses glycogen from the blood and muscles at a higher percentage than beta-oxidation. With that being said you'll still burn more fat (24-36 hours after exercising).

This just means your basal metabolic rate changes its source of fuel.

Epigenetics, and repsonses as a result of enviromental influences plays a huge role.
I'm a molecular kinesiologist.

I'll write something soon with the appropriate references- when I get the chance.
This is just referencing how personalized medicine is the way to go.
https://www.sciencedaily.com/releases/2015/11/151119143445.htm