In his new book on cities (in which he argues building restrictions are bad for a city's creativity), Glaeser observes that people in NY and LA are healthier and have lower cancer rates than in the US as a whole. Even though he'd like to think vigorous city life makes you healthier, he says there's also a selection effect as old people are likely to leave cities to retire.
But I do think cities make us healthier, i.e. through intense mating competition. As Glaeser explains, cities are love markets and hence young single people magnets. This creates an intense competition for mates where people try to be as attractive as possible. In the country side, you settle for less as you know your chances of meeting someone perfect are slim. This is why girls in cities pay 42% more on clothers and 25% more on shoes than those in the country.
So maybe this competition for mates also makes you eat less junk food and ice cream and exercise more to look fit and hence stay healthy. I often notice that people in cities are less fat than in the country side. That's true of NY and LA but also of London, Paris and Berlin. Pressure to stay fit and attractive may make you healthier.
Empirical evidence anyone?