If using typical kitchen worktop of around 1mtr long you will need to brace/support it along the front edge and across the middle or two.
You are talking about placing 200+kgs on it, thats about 2-3 adults !
If you have ever carried a 3 mtr length of standard worktop you will know how readily it bows .
Also any cut edges must be well sealed as the chipboard core will blow out as soon as any moisture seeps on to it.
MDF must also be well sealed as it also absorbs and expands with moisture, though its horrible stuff to work with, dangerous dust when sawing etc and a total pain to paint and achive a decent finish, the end grain even worse.