It might not seem like it, but when we are resting we are also consuming energy. This is necessary for our body to keep on working and sustain many of its functions, from breathing to the nervous system. However, the amount of energy that our body uses seems to have decreased, researchers found, which could then be a driver of obesity.

A team at the University of Aberdeen measured how much energy people spend while resting and found a declining trend over the past 30 years. While they don’t really know why this has happened, they suspect it might be related to dietary changes, with previous studies finding significant changes in diets in recent years.

“Studies of food supply suggest we are eating more food, but working out how much is difficult because people aren’t very good at reporting what they eat, and we can look at supply figures, but then it’s a problem quantifying food waste,” study author John Speakman said in a media statement. “People are also becoming more sedentary.”

A global problem

Obesity is a major public health and economic global problem. Prevalence rates are increasing in all parts of the world, both in rich and poorer nations. Men, women and children are affected. Overweight, obesity and health problems linked with them are now so common that they are replacing the more traditional public health concerns.

More than one billion people worldwide are obese, including 650 million adults, 340 million adolescents and 39 million children, according to estimates by the World Health Organization (WHO). Worldwide obesity has almost tripled since 1975, the WHO said, affecting the heart, livers, kidneys, joints and the reproductive system.

Speakman created a database of over 6,000 measurements using a method called the doubly-labeled water technique. “It’s a urine test in which a person drinks water in which the hydrogen and oxygen molecules were replaced with naturally occurring heavy forms, and then measuring how fast they are flushed out,” Speakman said.

Using this database, the researchers analyzed measurements of adults living in Europe and the US. This showed, once the data was adjusted for the effects of age and body composition, that total energy expenditure has declined since the early 1990s – 7.7% in males and 5.4% in females. But what was more surprising was the reason for this.

“By combining the database measurements of total energy demands with measures of resting energy demands it was possible to establish the contribution to this decline by changes in resting and activity expenditure,” Speakman said in a statement. “The decline is all down to a reduction in the energy we spend when we are at rest.”

While they don’t know why resting expenditure has fallen, they believe changes in our diets might be behind it. “We were able to see in mice that the make-up of the fat they ate affected their metabolism,” Speakman said. If the same effects are seen in humans, it might mean the decline can be tackled by changing our diets, he added.

The study was published in the journal Nature Metabolism.