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Big retailers like Wallmart which provides service with food restaurants are the key responsible for obesity epidemic, study says.

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

"Due to intake of calories at a time in variables explain 37 percent of the rise in body mass index (BMI) rates and 43 percent of the rise in obesity," Courtemanche added.

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.

-Arun Kumar