Theories about why this is happening range from higher levels of obesity, higher density of calories in processed foods, and physical inactivity, to chemicals in food and water.

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"Density and street connectivity also matter, but mixed use is the most important factor relating to physical activity and obesity, " said lead author Lawrence Frank , now a professor at the University of British Columbia.

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That ideas should freely spread from one to another over the globe, for the moral and mutual instruction of man, and improvement of his condition, seems to have been peculiarly and benevolently designed by nature, when she made them, like fire, expansible over all space, without lessening their density in any point, and like the air in which we breathe, move, and have our physical being, incapable of confinement or exclusive appropriation.

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