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Fitness trackers designed to help people keep in shape offer no useful weight-loss benefit, a study has suggested.

Having every step counted does not improve the chances of losing weight and may even make some people fatter than going without.

The two-year study of 471 overweight people found devices that monitor and provide feedback on physical activity - wearable technology such as fitness trackers which are worn on the wrist like a watch - may not offer an advantage over standard behavioural weight loss approaches.

Participants were placed on a low-calorie diet, prescribed increases in physical activity, and had group counselling sessions as part of the study, one of the first into the effectiveness of fitness trackers.

Six months into the study, half of the volunteers were given a fitness tracker to monitor their progress.

But researchers with the University of Pittsburgh found those provided with wearable technology lost less weight than the control group by the end of the trial.

Specifically, those with the fitness trackers lost on average 3.5kg, while those left to their own monitoring lost 5.9kg on average.

Lead researcher Dr John Jakicic said it was possible those using fitness trackers may have felt they were able to reward themselves with "treats" more than other participants, prompting lower levels of weight loss.

The study was published in the Journal of the American Medical Association.

Sales of wearable devices including activity tackers and smartwatches have boomed in the UK with 10 million expected to be in use before the end of the year.