Inactivity, fuelled by cars and a sedentary work life, has been dubbed the biggest public health problem of the 21st century, a global pandemic with dramatic impact on people's wellbeing. The latest reports suggest that around the world it was responsible for 5. 3 million deaths in 2008, which amounts to 1 in 10 people—even more than smoking. Not only does exercise make you fitter, it can also ward off numerous and often unexpected diseases, from heart attacks, to diabetes, some forms of cancer and dementia. There are tentative signs it might even make you cleverer, by boosting cognitive performance and brain function. "It's irrefutable that physical activity and exercise are beneficial for health," says sports scientist Chris Easton. The biggest benefits are seen when people go from being unfit to being moderately fit. At higher fitness levels, you don't see the same benefits in longevity and disease risk reduction. And although being vigorous is good, you don't need to go overboard.