It adds more confusion to the muddy picture of what causes youngsters to gain weight.
Advocacy groups have suggested a strong link between obesity and the proximity of fast-food restaurants or the lack of supermarkets stocked with fresh food. But the new study by the Santa Monica-based Rand Corp. think tank found little support for that connection.
"You see lots of stories about the poor becoming obese because they're in neighborhoods with lots of restaurants and no access to healthy food," said Roland Sturm, a co-author of the Rand study. "We show that well, maybe those stories don't hold up."
The study examined the weight gain of 6,918 children of varying socio-economic backgrounds from 59 U.S. metropolitan areas as they advanced from kindergarten to third grade. Researchers compared the weight gain figures with the price of different types of foods and the number of food outlets in the areas.
They did not examine what the children ate, however.
The results showed that young children who live in communities where fruits and vegetables are expensive are more likely to gain excessive amounts of weight than kids who live in areas where produce costs less. That connection was stronger than the proximity to fast-food restaurants.
On average, children in the study gained 29 pounds. But for the region with the highest relative price for produce — Mobile, Ala. — children gained about 50 percent more excess weight as measured by body-mass index (a ratio of height to weight) than children nationally.
Among kids in the area with the lowest relative cost for fruits and vegetables — Visalia, Calif. — excess weight gain was about half the national average.
The study also found that many children who live in poverty have just as much access to grocery stores as kids in higher-income neighborhoods.
The finding, Sturm said, "flies in the face of a lot of theories," including some that the poor do not have enough access to grocery stores.
Susan Foerster, chief of the Cancer Prevention and Nutrition Section of the California Department of Health Services, said the findings support what she has heard from the low-income families she serves.
"Lower-income families are more price-sensitive," she said "They have to be careful with how much they spend in food, because housing and transportation is expensive in California."
Foerster said the study provides evidence that low-income people need help buying healthy food.
However, Elizabeth Frazao, a U.S. Department of Agriculture economist, noted that a separate USDA study showed low-income consumers, when given an additional dollar, didn't increase their purchase of fruits and vegetables.
"There's this perception that fruits and vegetables are expensive when maybe it's not so much the cost, but the pleasure and taste that people get," said Frazao, whose department funded the Rand study.
"If you have a dollar, would you rather buy apples, a candy bar or soda?" she asked.
Sturm suggested providing fruits and vegetables free to schoolchildren would improve their diets. The USDA launched such a program in about 100 schools, he noted, and it has been popular with parents and teachers, but it's not clear whether it's affecting children's weight.
He acknowledges more research is needed on the issue.
"There's no shortage of ideas when it comes to childhood obesity," he said, "but the data out there are a lot more limited" than the theories.