These studies are always interesting as they generate theories (like who lives where) that may or may not have merit.
At the end of the day it's no more complicated than calories in vs calories out. (Yes some calories are much better choices than others). Get into a 250 calorie deficit everyday by either excercise or diet or a combination and you will loose a half pound a week. Do that for 3 months and you drop 6 pounds. 500 calories and you double that. (Not unattainable for a healthy person). Extend it out and you double that every 3 months or so.
If only it were that easy right? The math is easy, but the application may not be which makes me wonder if people in the skinnier states are better at math...jk
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As has been mentioned it may be just modeling, if you're around fitter/active people that becomes more of a lifestyle choice perhaps?
Demographics come into play as well. Fast food loaded with the evil white stuff (flour, sugar and sodium) and the fact that you don't even have to get out of your car to get it and is less expensive (and easier to get) than other choices that may require more expendable income and time.
A sedentary life style is our doom. There was a time, not too long ago where it was a struggle to get enough calories in a given day as the amount of work required by our bodies just to put a meal on the table exceeded what the food provided.
Interestingly in NYC where people walk a lot they stay thinner without worrying too much about what they eat or how many sets they did at the gym today.
If you look at the Meditarian Diet you see lots of vegetables, meat and fish and not a lot of starchs, so again we are back to regions.
I guess the take away is that there is an abundance of information about what causes weight gain or loss. Some people (me included) have to be vigilant as we were evolutionary survivors, easily gaining weight in case of future famine. Still even then we are back to math....sigh.