Can i control my metabolism?

I am currently eating around 500 - 700 calories a day and exercising regularly, I am 5 foot 9 and weigh 15 st. my concern is my metabolism slowing down as i enter starvation mode (even though i don't feel hungry and am eating 3 meals a day with a wide range of nutrients) I understand there are metabolism boosting foods out there so was wondering if i drank and ate things such as (green tea? celery?) would this aid to counteract my metabolic rate slowing down... thank you i appreciate any answers