Obesity researchers have discovered we burn the same calories on physical activities as we have done over the last 20 years.

They found that there has been no reduction in the energy we expend on day to day activities, suggesting increased food intake may be the more important factor in obesity.

Over the past 20 to 30 years there has been an enormous increase in the prevalence of obesity throughout the Western world.

Currently over 20% of the population in the USA and the UK are classed as obese (their Body Mass Index is over 30) with an additional 30 to 40% classed as overweight (BMI > 25).

The problem has been blamed on people eating too much, or exercising too little, or a combination of both.

Consensus over the past few years has been that an increased sedentary lifestyle has been a major contributory factor.

This view is consistent with perceived lifestyle trends such as increased car ownership; children being driven, rather than walking, to school, and the boom in television channels and computer gaming.

But now researchers from the Universities of Aberdeen and Maastricht in the Netherlands have discovered that there has been no reduction in the level of energy we are using in our daily lives over the past 20 years.

They have also found that comparisons of energy expended by people in both the USA and Europe differ little from those in pre-industrial societies in the Third World. Even more surprising, these levels were also the same as those anticipated from studies of wild animals.

Professor John Speakman, from the University of Aberdeen, co-authored a paper from the research, which is published online in the International Journal of Obesity. He said: "We found that people have not reduced their energy expenditure over the same period that obesity rates have increased enormously.

"It seems that we have been misled by the anecdotal information about levels of our physical activity. When actual measurements of energy expenditure are analysed there is no evidence for a reduction over time."

Professor Klaas Westerterp, from the University of Maastricht, was the co-author. He added: "Because we found no evidence for declines in energy expenditure over the last two decades, this work suggests that the obesity epidemic has been largely driven by increases in food intake."

The researchers made their findings based on data gathered in studies from Europe, North America and the developing world using a sophisticated isotope based technique called the doubly-labelled water method which permits direct measures of an individuals energy demands as they go about their daily business.

The study is the largest collection of data using this method and the results that are revealed provide a striking contrast to popular conceptions about what has caused the obesity epidemic.

Professor Michael Goran, Professor of Preventive Medicine and Director of the Childhood Obesity Research Center at the University of Southern California who is an international expert in the study of obesity, added: "This is an important study that puts into question a commonly held belief that the surge in obesity over the last few decades can be simply explained by a more sedentary lifestyle."