For a decade, public health experts assumed that living close to a supermarket was linked to a better diet and lower obesity. The closer you lived to one, the theory went, the more likely you were to eat more fruits and vegetables.

A new study led by the University of Washington School of Public Health finds that distance from a supermarket in Seattle doesn't matter when it comes to diet quality. What counts most is exactly where a person decides to shop. "Fruit and vegetable consumption was higher when people shopped at Whole Foods than when they shopped at a Safeway," said senior author Dr. Adam Drewnowski, professor of epidemiology and director of the School's Center for Public Health Nutrition. "Distance to the store had nothing to do with it. So where you shop matters."

The study, published March 13, 2014, in the American Journal of Public Health, found that shoppers at lower-cost supermarkets ate significantly fewer fruits and vegetables – nearly a serving less per day – than those who shopped at higher-cost supermarkets. Choices on where people shopped were likely based on individual factors ranging from economic necessity to lifestyle, culture and attitudes about healthy foods, said lead author Dr. Anju Aggarwal, research associate at the School's Center for Obesity Research.

"Clearly, people tend to bypass a multitude of supermarkets, grocery and ethnic stores near their homes to get to their primary supermarket of choice," Dr. Aggarwal said. "Contrary to general assumptions, it was shoppers at low-cost supermarkets who tended to travel the furthest. Shoppers at higher-cost supermarkets like Whole Foods also went out of their way." Researchers from the Center in collaboration with the UW's Urban Form Lab found that only one-third of shoppers regularly went to their nearest supermarket. Most traveled by car.

An earlier UW study, led by Dr. Drewnowski, found that driving distance to a supermarket did not predict obesity rates. This latest study is the first to look at where people actually shopped when they had access to a car, Dr. Aggarwal said. "The current paper provides insights into factors that make people decide where to shop for food," she said.

One of those factors is socioeconomic status. Dr. Drewnowski said the debate over "access to healthy foods" needs to be shifted from physical proximity to ensuring people in lower-income groups have greater purchasing power. One question, he said, is whether establishing a higher minimum wage is one possible way to achieve that.

The study was based on data from nearly 1,400 supermarket shoppers in Seattle and its suburbs, and came from the 2008-2009 Seattle Obesity Study. Using telephone surveys and King County government records, the Urban Form Lab geocoded locations of all food sources in King County, including respondents' nearest and primary supermarkets. Primary supermarkets of choice were categorized by price (low, medium and high) based on the cost of a basket of 100 goods.

Co-authors were Dr. Andrea J. Cook, Biostatistics Unit, Group Health Research Institute; Dr. Junfeng Jiao, Department of Urban Planning, Ball State University; Dr. Rebecca A. Seguin, Division of Nutritional Sciences, Cornell University; and Drs. Anne Vernez Moudon and Philip Hurvitz, Department of Urban Design and Planning, College of Built Environments, University of Washington.

The study was supported by the National Institute of Diabetes and Digestive and Kidney Diseases (grant # NIDDK R01DK076608).