There is the old joke in Houston about how you define a pedestrian: A person looking for their car. People don't do a lot of walking in the heat; perhaps that's why McAllen-Edinburg-Mission in Texas is the most obese region in America and Boulder, Colo. is the least.
But there may be a more important reason than the driving; it may be biological. A study by David Allison of the University of Birmingham found that air conditioning might make you fat.
One of the most intriguing factors listed in the study is the “reduction in variability of ambient temperature.” The widespread use of central heating and air conditioning means that most homes and offices are now kept at a relatively constant temperature year-round. Allison’s group found evidence that this causes the body to expend less energy, because it does not have to work to warm up or cool down, potentially leading to increased fat stores. In the South, where obesity rates are the highest in the nation, homes with central air increased from 37 to 70 percent between 1978 and 1990.
One doctor wasn't so sure, telling ABC: "Since people stay thin in all different climates, it is unlikely [air conditioning] plays much of a role." But that's not much of an answer; People in the southern United States are fat, and people in Italy or France generally are not. In italy, people often live in apartments with thick walls that resist the heat, and as seen in the photo I took in Milan last month, everybody has external shutters pulled down to keep the heat out. Not many people have air conditioning because they know how to keep cool. Few of the people I saw there were obese.
Somebody is going to say in comments that "correlation does not imply causation", they always do. But there IS a correlation.
There are other factors that might come into play; heat suppresses your appetite which reduces your food intake, while air conditioned restaurants are attractive cool places to be, which increases it.
Other effects of Air Conditioning:
A French study found that "people working in air-conditioned offices are almost two-and-a-half times more likely to suffer from respiratory infections than those in naturally ventilated buildings." From the Independent:
In the study, the French doctor examined the levels of air temperature, humidity, airborne bacteria and fungi in both air-conditioned and naturally- ventilated buildings. He found that seven out of eight symptoms were associated with exposure to air conditioning at work.
Dr Teculescu said: "Upper respiratory tract infections such as the common cold are one of the main reasons for absence from work. Air conditioning circulates the air and can carry airborne bacteria and fungi."