Washington D.C., being a small, densely populated region that is covered by a coherent public transit system, is very different from Colorado (which has large areas that are rural and heavily car-dependent) in ways that might well bias obesity rates independently of the (relatively small) effect from altitude.
A comparison between Colorado and an actual state (which will involve non-urban regions) might be better.
A comparison between Colorado and an actual state (which will involve non-urban regions) might be better.