Are there any other biological factors for weight gain in mid life? I imagine it’s mostly down to lack of exercise and mental state.
Well, some conditions could probably cause it, but those are conditions are rare. Considering obesity is headed towards affecting 50% of the population in developed countries, those conditions are not a valid excuse for the vast majority of people. For this vast majority of people, it's lack of exercise and mental state, as you say.
I’m leaning more towards some chemical in our environment that is messing with our signaling hormones for hunger and satiety. There isn’t much evidence that people are getting less exercise than they were 50 years ago.