Hey Ailish and all,
yes, obesity is generally a long-term adaptation and a strategy. It can be related to several mechanisms, which makes it worthy to discuss!
But weight gain without eating more can often be noticed quite shortly - which says it would be due to a trigger or a new mechanism.
Is there a plus function in brain stem or cerebellum-related organs that will result in weight gain? (Check out the teleseminar...)

And it's mostly the long-term energy-consuming (sympathetic) stress responses that will lead to depletion and function-minus in the long run.