Gut bacteria could be interesting as further developments emerge in the future.
Genetically we haven't significantly changed as a society in the last 30 or so years, and food was plentiful to the majority then. Evidence is emerging that the influence of genes is affected by nurture anyway. Can I claim to have "fat genes" because most of my relatives have been overweight by the age of 35? Or is it more likely that it's the effect of societal changes.
I find that an excess of sugar disrupts my digestive system. I became more cautious about alcohol in my 20s because of the effect on my system the following day. It makes me flatulent, and it stinks. I find it credible that if a person has in the long term consumed a diet involving a lot of sugars and refined, heavily processed foods that their gut flora is affected, disturbing their digestive processes and affecting their balance of weight management.
DS had food allergies and intolerance issues as an infant that took 6 months to go from triggering at weaning to going on an exclusion diet. Although he stopped having reactions, pain and eczema, he continued to have digestive issues with every single poo being a nappy blaster. At one of his dietician appointments he was recommended a strong probiotic powder which made improvements. When I've had digestive problems that have flushed me through, I've used it to help bring some balance back and it seems to help. It would be interesting to see if science can make useful developments in that area.
Something else that has changed in the last generation is availability of time and overworking. Lunchbreaks are rushed. People feel presentism is essential to keeping jobs in an insecure climate. Jobs have become cluttered with more admin and data tasks. It takes more time and energy to prepare nutritious food, and people struggle with that. Exhaustion is the enemy of a healthy weight, both through the metabolic effects of lack of sleep and seeking quick fix sugar rushes to keep going. I wish our society would slow down and evaluate what is genuinely useful and what is a box ticking distraction.
One of my weight gain phases was when I was walking over 6 miles a day. The 2 mile return trip walk to and from work was brisk and decent exercise. The rest of the milage hunting for medical records across a general hospital was wearing. By mid-afternoon, the urge for a can of coke to pep up the last couple of hours was very strong. On the way home, a "well earned" bar of chocolate quickly became a routine rather than a treat. The trouble is that it wasn't really walked off by all that walking. I thought I was just filling out as I was in my early 20s and didn't notice that those trats and rewards had be become routines. Fortunately for me, it was a short term phase and I went on to new habits when I went on to another course.
The hardest part isn't necessarily seeing that your weight is a problem, it's being able to make sustainable changes to the way you live when the barriers are becoming so ingrained into society.