Fitness trackers enjoy healthy sales despite lack of evidence they work

Soaring Christmas sales for Fitbit and Jawbone are, on the face of it, good for UK's obesity crisis. But there are concerns about privacy and effectiveness

One of this year's most popular Christmas presents looks like being a device that purports to help reverse the effects of festive overindulgence - the fitness tracker.

Sales of the distinctive wristbands - which monitor the number of steps taken, calories burnt, heart rate and quality of sleep among other things - have soared and the signs from retailers are that they will jostle for attention with, or perhaps displace, the traditional boxes of chocolates and bottles of whisky under the tree.