Half the mice were fed a vegetable-free diet and half the mice were fed a diet which included broccoli, green beans, corn, peas and carrots.

After 16 weeks, researchers measured cholesterol content in the blood vessels and estimated that plaques in the arteries of the mice were 38% smaller.

Although there was also a reduction in total cholesterol and body weight in mice fed the vegetable-rich diet, analysis showed that this could not explain the reduction in atherosclerosis.

Lead researcher Dr Michael Adams said: "While everyone knows that eating more vegetables is supposed to be good for you, no-one had shown before that it can actually inhibit the development of atherosclerosis."

Virtually everything American society has done for the past 100 years has made it easier for its people to be fatter, said James Sallis, a San Diego State University psychology professor, and others who gathered recently at the American College of Sports Medicine's annual meeting.

"We've built an unhealthy world in a lot of different ways," said Sallis, who was once dubbed an "obesity warrior" by Time magazine.

Sallis contends change will come only when the public demands walkable development, more federal money for parks and bike paths and even a tax on industries that promote sedentary lifestyles (he pointed to video game makers, movie theater chains and even electric Segway scooters).

Proof that people will accept an active lifestyle and walk to parks and shopping if they can is found in the "new urbanism" style of planned communities, the experts contend. They pointed to Denver's Stapleton neighborhood, an enclave of new homes built where the city's old airport used to be.

The neighborhood is a mix of shops, offices, parks, apartments and houses linked by wide sidewalks and meandering bike paths. Architecture varies from single-family homes to rows of brownstones. Tom Gleason, a spokesman for developer Forest City, said the design has been a hit.

For the new study, researchers measured how well 3,718 people did on simple tests, such as recalling details of a story. The participants, all Chicago residents 65 and older, took the tests three times over six years. They also filled out a questionnaire about what they ate that included 139 foods.

"We found that people who ate one fish meal a week had a 10 percent slower annual decline in thinking," said co-author Martha Clare Morris, an epidemiologist at Rush University Medical Center. "Those who ate two fish meals a week showed a 13 percent slower annual decline."