Exercising 15 minutes a day extends lifespan, while watching television cuts lifespan, find studies.
Chi-Pang Wen of Taiwan's National Health Research Institutes and Jackson Pui Man Wai of the National Taiwan Sport University looked at more than 400,000 people who took part in a medical screening programme between 1996 and 2008. The volunteers were then followed for eight years on average.
Compared with individuals who said they were physically inactive, people who were in a group of "low-volume" exercise had a 14-percent reduced risk of premature death from all causes, and a 10-percent reduced risk of death from cancer.
The "low volume" category applied to people whose total amount of exercise of all kinds averaged 92 minutes per week, or about 15 minutes a day.
On average, their life expectancy was three years longer than inactive counterparts.
The benefits applied to all ages and to both sexes and are roughly equivalent in health impact to a successful campaign to discourage smoking, say the authors, who publish their work online in The Lancet.
Separately, researchers in Australia found that watching TV or videos for an average of six hours a day could shorten the viewer's life expectancy by almost five years.
The investigators used data from a cross-sectional survey of 11,000 Australian adults who were aged at least 25 at the turn of the millennium.
The data were then checked against Australia's national population and mortality figures for 2008.
The researchers estimated that in 2008 Australian adults aged 25 and older spent 9.8 billion hours in front of the small screen, and that this time was associated with 286,000 years of life that ended prematurely.
Every single hour of TV watched after the age of 25 shortened the viewer's life expectancy by just under 22 minutes, according to an extrapolation of these figures.
In actuarial terms, an hour in front of the box had roughly the same effect on life expectancy as smoking two cigarettes.
In an extreme case, someone who spends a lifetime average of six hours a day watching TV -- in the top one percent of the viewing population -- can expect to live 4.8 years less than someone who does not watch TV, according to their calculation.
The study, headed by Lennert Veerman of the School of Population Health at the University of Queensland, is published by the British Journal of Sports Medicine.
The investigators point the finger at TV watching not because of the programmes themselves but because of the dangers of physical inactivity that come from prolonged spells on the sofa.
A sedentary lifestyle is linked to cardiovascular disease, diabetes, excess weight and other health problems.