New York, Feb 3 (IANS): Big-box retailers like Walmart along with full-service and fast-food restaurants are key contributors to the obesity epidemic in the US, says a study.

"People are doing a lot of eating," said Charles Courtemanche, assistant professor at the Georgia State University in the US.

"Changes in variables related to calorie intake collectively explain 37 percent of the rise in body mass index (BMI) rates and 43 percent of the rise in obesity," Courtemanche added.

"And our data show that the pervasive presence of super centres, warehouse clubs and restaurants are responsible for most of these gains," Courtemanche said.

People in the US now live near an abundance of cheap and readily available food from a variety of sources.

The researchers built a comprehensive economic model comprising body weight and several state-level variables they categorised as general economic conditions such as unemployment and income.

They also factored in monetary or time costs associated with calorie intake (food prices, retail presence), physical activity (gasoline prices, fitness centres) and smoking.

Fitness centres and rising gas prices were found to work against the rise in BMI.

The research suggests these effects are not the same for everyone.

"The greatest rise in weight is concentrated among people already at risk for obesity," said Courtemanche.

The study was published was published by the US National Bureau of Economic Research.