Generally sedentary lifestyle and unhealthy food are considered root cause of obesity epidemic. However recent study revealed that intake of unhealthy food is the main culprit behind such a drastic increase in number of obese persons.

Led by Professor Boyd Swinburn, researchers at the World Health Organisation Collaborating Centre for Obesity Prevention, at Deakin University in Victoria, Australia, said that the findings would be reflected in other industrialised countries such as Britain.

Researchers calculated what should be the weight of Americans based on their current, higher food intake, and compared it with their actual weight. Increase in present weight as compared to estimated weight would suggest reduced physical activity. Comparison of data showed that adults weighed less than could be expected from their diet, suggesting that, if anything, they had increased their physical activity over time.

But in case of children, estimated weight was equal to real weight which showed that changes in physical activity had no impact on children in the US growing fatter.

Professor Boyd Swinburn, said that the findings would be similar for other developed countries in showing that changes in exercise played a minimal role.

He said: "This is a call to focus public health attention more on the energy intake side. There is no evidence that a marked reduction in physical activity has been a contributor to this epidemic."