I can't BELIEVE I forgot to mention this one - which is one of my favorite trashy novels
The only catch is you MUST read the first of the two books...both written by Kathryn Harvey.
The one having to do with weightloss is titled
Stars which is the sequel to
Butterfly.
Both delightfully trashy, engrossing, and fun to read with absolutely no socially redeeming value
and a weightloss theme to boot (at least in
Stars.