New research appears to settle the on-going debate over whether the culprit in the current obesity epidemic is a lack of exercise or too much food.
The research by Australian scientists used an innovative approach in order to examine for the first time, the relative contributions of food and exercise to the development of the obesity epidemic and they suggest that extra calories are to blame.
The research into the proportional contributions to the obesity epidemic combined metabolic relationships, the laws of thermodynamics, epidemiological data and agricultural data and found that the obesity epidemic in the United States since the 1970s was virtually all due to increased energy/calorie intake.
The research will settle to some extent the debate on where the public health focus should be even though experts agree that making it easier for people to eat less and exercise more are both important for combating obesity.
The study's leader, Professor Boyd Swinburn, chair of population health and director of the World Health Organization Collaborating Centre for Obesity Prevention at Deakin University in Victoria, says there have been many assumptions that both reduced physical activity and increased energy intake have been major drivers of the obesity epidemic, but until now their relative contributions to the rise in obesity since the 1970s has not been examined.
Professor Swinburn says the study demonstrates that the weight gain in the American population seems to be virtually all explained by eating more calories and changes in physical activity appear to have played a minimal role.
For the study the team of scientists began by testing 1,399 adults and 963 children to determine how many calories their bodies burn in total, under free-living conditions which is the most accurate measure of total calorie burning in real-life situations.
Once Swinburn and his colleagues had determined each person's calorie burning rate they were able to calculate how much adults needed to eat in order to maintain a stable weight and how much children needed to eat in order to maintain a normal growth curve.
The next step was then to work out how much Americans were actually eating, using national food supply data (the amount of food produced and imported, minus the amount exported, thrown away and used for animals or other non-human uses) from the 1970s and the early 2000s.
The findings were then used to predict how much weight Americans were expected to have gained over the 30-year period studied if food intake were the only influence - they used data from a nationally representative survey (NHANES) that recorded the weight of Americans in the 1970s and early 2000s to determine the actual weight gain over that period.
Professor Swinburn says if the actual weight increase was the same as predicted, that meant that food intake was virtually entirely responsible - if it wasn't, that meant changes in physical activity also played a role - if the actual weight gain was higher than predicted, that would suggest that a decrease in physical activity played a role.
The researchers found that in children, the predicted and actual weight increase matched exactly, indicating that the increases in energy intake alone over the 30 years studied could explain the weight increase.
Professor Swinburn says they predicted that adults they would be 10.8 kg heavier, but in fact they were 8.6 kg heavier which suggests that excess food intake still explains the weight gain, but that there may have been increases in physical activity over the 30 years that have blunted what would otherwise have been a higher weight gain.
The researchers say to return to the average weights of the 1970s, it is necessary to reverse the increased food intake of about 350 calories a day for children (about one can of fizzy drink and a small portion of French fries) and 500 calories a day for adults (about one large hamburger).
Professor Swinburn says alternatively, similar results could be achieved by increasing physical activity by about 150 minutes a day of extra walking for children and 110 minutes for adults, but realistically, although a combination of both is needed, the focus would have to be on reducing calorie intake.
Professor Swinburn emphasises that physical activity should not be ignored as a contributor to reducing obesity and should continue to be promoted because of its many other benefits, but that expectations regarding what can be achieved with exercise need to be lowered and public health policy shifted more toward encouraging people to eat less.
The researchers say that the findings would be reflected in other developed countries.
The World Health Organisation (WHO) estimates in 2005 were that approximately 1.6 billion adults (age 15+) were overweight; at least 400 million adults were obese and at least 20 million children under the age of 5 years were overweight globally.
The WHO says by 2015, approximately 2.3 billion adults will be overweight and more than 700 million will be obese.
A study was presented on May 8 at the European Congress on Obesity.