The study, published in The Lancet and part of a series on inactivity published in the lead-up to the 2012 Summer Olympics, looked at quantifying the effect physical inactivity has on major diseases such as heart disease, Type 2 diabetes, breast and colon cancer and life expectancy.

Looking at data from 2008, the researchers found that, worldwide, physical inactivity causes 6% of heart disease cases (ranging from 3.2% in southeast Asia to 7.8% in the eastern Mediterranean), 7% of Type 2 diabetes (ranging from 3.9% to 9.6%), 10% of breast cancer (ranging from 5.6% to 14.1%) and 10% of colon cancer cases (ranging from 5.7% to 13.8%). Physical inactivity was also found to account for 9% of premature deaths (ranging from 5.1% to 12.5%), or more than 5.3 million of the 57 million deaths worldwide in 2008.

"How does physical inactivity compare with other risk factors for poor health?" the study asks.

"Although risk factors are classified on different scales (thus, the proportion at risk varies across risk factors), it is nonetheless informative to look at two established risk factors targeted for government action worldwide: smoking and obesity. Smoking was estimated to cause about 5 million deaths worldwide in 2000. The proportion of deaths attributable to smoking in China, one of the top five cigarette-consuming countries, has been estimated at 3.1% for women and 12.9% for men. By elimination of smoking, life expectancy at age 50 years was estimated to increase by 2.3-2.5 years in the U.S. population and 1.1-2.2 years in the populations of nine other high-income countries...As for obesity, if all obese people in the U.S. were to attain normal weight, life expectancy in the population was estimated to increase by 0.7-1.1 years at birth in one analysis and 0.5-0.7 years at age 50 years in another. Thus, physical inactivity seems to have an effect similar to that of smoking or obesity."