[kazinator]

If we are talking about control over body fat, it's not so much about metabolic rate, as about mobility of the stored fat. If your adipose fat does not respond to reasonable caloric restrictions, you're simply going to have a hard time.

It is not actually hard to burn mobilized fat. Each one of us burns a whole lot of dietary fat in a given year: a lot more than what we have stored in our bodies. Why is it that we can burn through all that dietary fat, far in excess of how much body fat we can burn in the same period? Because that fat it is mobile: it is circulating in the body in a form that is ready for use.

Thermodynamics provides us with a summary of what is going on, which doesn't reliably translate to a method. If we measure the total energy output of the body while and monitor the energy input, then the body mass and composition changes will be reliably related to those variables.

Not all body composition changes are favorable, though.

If you could simply cut 500 calories a day, and reliably have the deficit made up by burning body fat, it would be laughably easy for anyone to achieve a vanishingly low body fat level.