This question comes up all the time doesn’t it?
Well I have a couple ideas that are rooted in science and not “what a friend or a friend said” or “I read it somewhere” haha
1) Stress – Stress releases hormones such as epinephrine which raise RHR and respiration therefore burning more calories (I wouldn’t advise adding stress to your life!)
2) Lean Body Mass – more muscle- more calories expended
3) Total Body Weight – People who weight more have to work harder to move their body its that simple
4) Hormones – lower than normal thyroid levels can result in an up to 15% lower RMR
5) Supplements – caffeine and many other supplements also raise RHR and can in turn expend more calories.
6) Exercise – HIIT….but WHYYY?! I can’t believe many trainers can’t explain this but its because of something called EPOC excess post exercise oxygen consumption. Basically when you push your body to its limits it needs to work harder to recover and that leaves your body working harder (increased metabolism) for a sustained period of time even after the workout is done.
Hopefully this brought some fresh insight in to your question.