Apple, Samsung and Garmin smartwatches overestimate the number of calories you burn by at least 110, US scientists have discovered. Fitbit watches, meanwhile, are 'consistently glitchy' and produce both dramatic over and underestimations.Garmin watches are most likely to overshoot energy expenditure, while the Apple device is the most accurate. Those with a high level of body fat are more likely to get inaccurate results, according to the researchers, from Florida International University.Smartwatches are among the most popular types of wearable technology, with a third of Britons choosing to invest in one, according to YouGov polls.Around one in five Britons have the type that tracks fitness measurements, such as calorie counts, heart rate and sleep quality. But experts have warned users not to rely on the devices to monitor their weight loss plan.'You can't treat the calories burned number it gives you as accurate, because it's not,' said Jason Kostrna, the study's lead author and associate professor of kinesiology and exercise science at Florida International University. 'You can't treat the number of calories burned as accurate, because it's not, researchers say'You could be hundreds of calories off each week and easily end up in a calorie surplus when you think you're in a calorie deficit. 'Prof Kostrna added that 'improvements to the device or algorithm are needed'.In his study, published in the journal PLOS One, the researchers asked 58 adults to ride a special stationary bike whereby cyclists sit in a reclined position. Each participant did 60-minutes of cycling - a mixture of both moderate and vigorous exercise - while wearing one of four smartwatches; the Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5 and Garmin Forerunner 955. Volunteers were also fitted with a so-called metabolic analyser, which tracks calories burned by measuring how much oxygen is used while exercising.The kit, which is gold-standard equipment used in hospitals and laboratories, is a mask that's attached to a monitor which is worn across the back.The team then compared the output of the metabolic analyser to that of the smartwatches, and looked for discrepancies. All smartwatches produced errors in their readings for the number of calories burned during the cycling exercise. Garmin and Samsung overestimated by up to 25 per cent - and an average of around 110 calories - while Apple overshot by roughly 12 per cent. The Fitbit, which costs around £219, was found to produce the least accurate figure for the number of calories burned. Researchers reported repeated glitches - in some cases reporting implausibly small numbers, such as a single calorie for an entire workout. The devices are less likely to be accurate for those with a high body fat percentageIn other cases it failed to return a reading at all. On average, Fitbit's readings were around 128 calories off, in either direction.Meanwhile, having an above-average body fat percentage resulted in a further estimation of between 22 and 35 calories, across all devices. 'Higher body fat systematically increased the tendency for devices to deviate from the gold standard,' the authors stated. Last year, top fitness experts warned that the margin of error for smartwatches' caloric measurements can be between 30 per cent and 150 per cent. Dr Cailbhe Doherty, lecturer and assistant professor at University College Dublin's School of Public Health, Physiotherapy and Sport Science, said a watch may tell users that they burned 1,000 calories when they have only burned 300.The inaccuracy is due to the method smartwatches use to configure calorie expenditure, according to Dr Doherty. '[Smartwatches] measure heart rate and some variation of physical activity through movement,' he said.'Now that's very different to actually measuring someone's inspired and expired gas, which is what you would use to measure calories in the first place.'Essentially, the signal is so far removed from the original biometric rhythm that there's such a high degree of error that can be introduced that way.'The authors of the latest study say the readings aren't useless, but wearers should use the data to track trends over time, rather than focusing on daily calorie totals.'Track your own trends over time. And if tracking helps, enjoy it,' Prof Kostrna said. 'If it becomes too much, don't fixate on it. Focus on how you're feeling and use data to get a little better over time.'