A few thoughts:

-senior food won't necessarily result in weight loss. They do also make a lite formula.

- why not call you vet and ask?

- why not ask them? They have a link for "contact us" at the bottom of their web site, which is here:
http://www.chickensoupforthepetloverssoul.com/

And if you do ask them, and / or the vet, let us know what the answers are!!