Making sugary caffeinated beverages supported by billions in ads sure seems like a method to try to affect peoples choice though.

Perhaps although I really think a lot of food people are eating today will be considered poisonous at some point.

The phenomenon of people from skinnier (but developed) countries spending some time in the US and complaining that they gain a lot of weight suggests a pretty damn strong environmental component. Strong to the point of overwhelming most other factors, to my eye, at least.

Is there some study about this “phenomenon” this or is it all anecdotes?

My very first, naive attempt at DDG’ing this revealed a bunch, yeah. Quantified weight gain in immigrants, studies of gain then loss again once moving back to the original country, all kinds of angles of approaching it.

A second search with a slightly more specific topic of “weight gain in us immigrants” on DDG was mostly studies (or pages collecting multiple studies) on page 1 of the results.

Many people are overweight by choice in the same way alcoholics or crack addicts are addicted to substances by choice. There are very powerful chemical and subconscious factors at play that make it incredibly difficult to exercise one's will. That semiglutides are proving to be remarkably effective at helping people both make better dietary decisions as well as reduce substance use, should tell you that the problem isn't as simplistic as you've made it out to be.

This is kind of missing the point. People are in groups that do all kinds of things normatively that add up to obesity, like dualistic thinking leading to depression while be embedded in a culture of dissociation. Yes, it's a choice, and as the cultures that gatekeep people from meeting their needs grows and spreads, making and sustaining that choice becomes harder. Maintaining cultures that harm is also a choice. No one is an island. We're all programming each other and ourselves in every moment while not taking care about how we do it.