BBC News have reported that 15 minutes of daily exercise is the bare minimum for optimal health.

Just 15 minutes of exercise can boost life expectancy by three years and cut death risk by 14%.  This research article was based on investigating the benefits of different levels of physical activity.

400,000 adults were consulted about exercise habits and monitored for eight years.  Compared to being inactive (low volume - less than 15 minutes per day) it was calculated to reduce risk of all cause mortality by 14%.  Each additional 15 minutes of exercise above this reduced mortality by a further 4%.

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