The first thing I thought after reading this was how unfortunate it is for people on diets that they have to drink a sugary beverage in order to regain the willpower to avoid sugary beverages.
Yeap, that's one of the lessons to draw from this. Another is that it's twice as hard to change two habits at the same time: i.e. maybe people who quit smoking and find themselves getting fatter to this because of a self-regulation issue.
For what's worth, I remember the "magic number" for creating a habit is 60 days - and there's also good research on the stages one goes through when creating a habit and what's most likely to sabotage him/her at each stage. We really really need more "science in plain English" books about modern psychology.
Sounds like a negative feedback loop, it should stabilize at some point. The thing is, it might not stabilize where the person on diet would wish it to. :).
