Scientists using a new way to look at data concluded that the main reason that obesity has risen sharply in the United States in recent decades is predominantly because of over-eating rather than lack of physical activity.

The study was led by Professor Boyd Swinburn, chair of population health and director of the World Health Organization Collaborating Centre for Obesity Prevention at Deakin University in Australia, and was presented last Friday at the 17th European Congress on Obesity in Amsterdam, the Netherlands.

Scientists have long debated the extent to which consuming too many calories as opposed to not exercising enough is the main cause of the obesity epidemic and therefore it is not clear which should be the focus of public health campaigns to combat the problem.

Swinburn and colleagues are the first to look at the problem from the combined perspectives of metabolism, thermodynamics, epidemiology and agriculture.

Until now nobody has tried to put numbers to how much of the epidemic is driven by increased energy intake and how much by reduced physical activity, said Swinburn who said they concluded that:

“The weight gain in the American population seems to be virtually all explained by eating more calories.”

“It appears that changes in physical activity played a minimal role,” he added.

For the study, Swinburn and colleagues tested 1,399 adults and 963 children to find out how many calories they burned in their day to day lives.

This then helped them work out how many calories an adult needed to keep their weight normal, and how much children needed to grow at a normal rate.

Using national food supply data, they then calculated how much food Americans were actually consuming from 1970 to the early 2000s. The total consumed was calculated by taking away from the total produced the amount of food exported, wasted, or used for other reasons than human consumption.

Combining the food intake data with the calories needed to keep to a normal weight or growth rate, the researchers then predicted how much weight Americans would gain from the amount of food they were eating over the 30 years, assuming that food intake was the only factor influencing weight gain.

Swinburn and colleagues then compared their predictions with real data taken from the NHANES national surveys that recorded how much Americans actually weighed during the years 1970 to the 2000s.

The idea was simple, if food intake was the main reason for the rise in obesity, then their prediction would be in line with the real weight gain data, if it was only part of the reason, then you would expect the predictions to anticipate a lower rate of weight gain, leaving room for another reason, probably lack of exercise, to fill the gap.

What they found was that in children the predicted figures matched the actual weight gain figures exactly, suggesting that only increase in calorie intake was the reason for American childhood obesity rising over the 30 years since 1970.

However, the picture for adults was less obvious. Swinburn and colleagues had anticipated that they would be about 10.8 kg heavier, but the real figures showed the weight gain was only 8.6 kg, suggesting that while eating too many calories was the predominant reason for the gain, there must also have been an increase in physical activity over the 30 years or the weight gain would have been even bigger.

To reduce the obesity trend and return Americans to the average weights of the 1970s, Swinburn said that:

“We would need 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).”

Another way would be to increase physical activity by about “150 minutes a day of extra walking for children and 110 minutes for adults,” said Swinburn, adding that:

“Realistically, although a combination of both is needed, the focus would have to be on reducing calorie intake.”

Swinburn said this does not mean we should discount physical activity as a way to attack obesity. We should still promote it because it has many other health benefits. However, because of what we can realistically expect people to achieve, the emphasis in health promotion for addressing the obesity problem should be on encouraging people to eat less, he said.

“Increased food intake alone explains the increase in body weight in the United States.”
Boyd Swinburn et al.
Catalogue no: T1:RS3.3, oral presentation, Room: Elicium 1, 15.00 hrs CET Friday 8 May 2009.
17th European Congress on Obesity in Amsterdam, the Netherlands, 6 – 9 May 2009.

Source: European Association for the Study of Obesity.

Written by: Catharine Paddock, PhD