Hi Vanilly I researched this the other day and the answer is yes it does lower your metabolism! All VLCDs lower your metabolism on average by about 10 - 15% and when you return to normal eating it wont go back up as you will weigh less so it doesnt need to be as high as it was before as there is less of you.
A lower metabolism is the reason for us feeling the cold more whist on this diet amoungst other things!
I did read that there are things you can do to increase it though. Some ideas are either being very hot so sitting in a sauna or steam room for a bit or the opporsite being very cold...not sure the best way to achieve this, prob just stick to the hot! Also drinking green tea is meant to naturally increase it.
My pharmacist told me it would speed it up but i didnt believe this when i heard it as it didnt make sense, if your not eating anything why would your metabolism speed up it hasnt got anything to do! Then when i read the articles it backed up my thoughts.
Nothing to worry about though its just standard with these types of diets.