A third of the world's adults are physically inactive, and the couch potato lifestyle kills about five million people every year, experts said in the medical journal The Lancet recently.

"Roughly three of every 10 individuals aged 15 years or older - about 1.5 billion people - do not reach present physical activity recommendations," they said in a report that described the problem as a "pandemic."

The picture for adolescents is even more worrying, with four out of five 13- to 15-year-olds not moving enough, it said.

Physical inactivity was described for the study as failing to do 30 minutes of moderate physical activity five times a week, 20 minutes of vigorous activity three times a week, or a combination of the two.

Inactivity increases with age, is higher in women than in men, and more prevalent in high-income countries, the researchers found.

A second study, comparing physical activity levels with population statistics on diseases like diabetes, heart problems and cancer, said lack of exercise claimed more than 5.3 million of the 57 million deaths worldwide in 2008.

It said inactivity was a risk factor comparable to smoking or obesity.

Lack of exercise causes an estimated six percent of coronary heart disease cases, seven percent of type 2 diabetes (the most common form) and 10 percent of breast and colon cancers, it said.

The human body needs exercise to help the bones, muscles, heart and other organs function optimally, but populations are walking, running and cycling less and less as they spend more time in cars and in front of computers, the investigators said.

The Lancet series called for global efforts to promote physical exercise by improving pedestrian and cyclist safety on city roads, for example, more physical education at school or promoting access to free public exercise spaces.