Earlier quoted context omitted.
I read an interesting study on this. It showed that obesity rates were lower in places like Colorado, and theorized that the reason was that they were at the headwaters of their water source, so there was less opportunity for contaminants to enter their water.
That is interesting, but probably the reason is the highly active/outdoor culture of Colorado leads to lower obesity rates and people who are less obese move to Colorado.
The science on this altitude effect has been pretty thoroughly conducted, with potential correlating factors accounted for in multivariate regression models.
Denver and its suburbs are FILLED with people who are as unhealthy and inactive as the rest of the country. Even in these places, in the poorest neighborhoods, they are less obese.
I have an identical twin brother who lives in Portland, and since I've moved here I've stayed about 10-15 lbs lighter than him. It's a single, anecdotal data point, and completely unscientific, but he swears he eats healthier than me lol.