If you are prepared to wear your aligners as instructed and keep your oral hygiene A1, there is no reason why you should loose weight.
One of the main reasons people loose weight while wearing invisalign is that they become lazy, and don't want to eat, as that means having to remove their aligners and clean their teeth. In these cases, fixed bracket braces are more suitable.
You need to take responsibility for eating properly and therefore making sure you don't loose weight.
Good luck!
