Poor town planning which limits opportunities for children to take exercise has been blamed for fuelling an increase in obesity.
Leading US paediatrician Professor Richard Jackson called for a rethink in the way towns and cities are developed.
He said living in a walkable neighbourhood helped people keep off an average of seven pounds (3.17kg).
Professor Jackson made his comments at a lecture at London's Institute of Child Health.
"We have "designed" a lot of incidental exercise out of our lives"
He said humans were so adaptable that they quickly adjusted to the environment in which they found themselves.
However, while this was an advantage in evolutionary terms, it spelled bad news when that environment provided little opportunity for exercise.
Humans were designed to keep active, he said, and they were not designed for the modern, sedentary lifestyle that had become the norm.
He said the environment should support people to make healthy choices, but increasingly children were not given the option of walking.
"Prescribing a minimum of physical activity is useless if there is nowhere to exercise," he said.
"How a neighbourhood is designed dictates how people get around, for example walking or bicycling versus automobile use."
Professor Jackson, who is professor in both public health and urban design at the University of California at Berkeley, said technology had brought both "good" and "bad" news.
Labour saving technology
He said: "Technology has eliminated a lot of the really backbreaking labour from our lives.
"But we have also "designed" a lot of incidental exercise out of our lives, such as walking.
"In 1969, 48% of American students (90% of those who lived within a mile) walked or bicycled to school.
"In 1999, only 19% of children walked to or from school and 6% rode bicycles to school."
Dr Ian Campbell, medical director of the charity Weight Concern, said Professor Jackson was "absolutely right".
He said: "The development of obesity in the past 30 years is a direct result of environmental change.
"The fact that environment sustainability and health are inextricably linked needs to be recognised by politicians and public health officials and definitive action taken.
"Then, and only then, will we see decreases in levels of childhood obesity in this country."
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