Why you should NEVER trust a fitness watch to count how many calories you're burning; HARRY WALLOP puts the leading brands to the test, with very surprising results....

Have any of your friends started wearing a chunky ring? Or a strap around their wrist with no watchface? And can’t stop talking about their resting heart rate?

Then they have almost certainly become part of a growing army of consumers who have embraced ‘wearables’ – anything from a smartwatch that can measure your step count to increasingly sophisticated, and expensive, rings and arm straps that analyse your sleep patterns, cardio-respiratory fitness and stress levels.

Once considered the preserve of Lycra-clad cycle bores, these wearables are increasingly being seen as a possible saviour for the creaking NHS.

Last week, Health Secretary Wes Streeting laid out his ten-year vision, explaining how technology – notably the NHS app – would become more useful once it has access to data collected from wearables.

This is potentially exciting – freeing up GPs and alerting people to a possible decline in their health. But can these wearables really monitor your health as accurately as a doctor?

They use a variety of different sensors, such as an accelerometer for measuring steps. Most shine small lights through your skin to detect the blood flow in an artery, which is a way of measuring your pulse.

However, though wearables can be accurate for step counts and heart rates, many experts warn that calorie counting – vital given our obesity crisis – is a different matter.

To find out how good wearables really are at this, I tried seven of the leading devices, wearing them for a week. I then took them to My Vital Metrics in London, a lab run by sports entrepreneur Owen Hutchins. It helps elite athletes, including those at England Rugby and Liverpool Football Club, as well as amateurs who want to track their fitness and health.

First, I needed to have my basal metabolic rate (BMR) measured. This is the number of calories you burn even when you are lying down because breathing and pumping blood requires lots of energy.

Hutchins performed various tests on me, including a Dexa scan to get my body fat percentage, which is needed to get an accurate BMR. He also measured how much oxygen I burn just by breathing.

Wearables do not have access to your BMR. All they have is the data you supply to them when you first register and set up your device: your age, height and weight. This allows them to calculate your body mass index (BMI) – which is a loose substitute for body fat percentage.

The gadget companies then use various equations – many of which were developed more than a century ago and are not very accurate – to estimate your BMR with the information they have.

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After these initial tests came the main part: a VO2 max test to measure the amount of oxygen your body can absorb and use when exercising. This involved me being strapped to an exercise bike wearing an accurate heart monitor strap and a mask that measured how much oxygen I breathed in and out.

‘The difference is what your body absorbs. For every 207 milliliters of oxygen your body has consumed, you’ve burned one calorie,’ Hutchins explained. ‘For calorie counting, this test is the most accurate. It is literally measuring how much energy you are burning, based on the amount of oxygen you are absorbing.’

While being measured by Hutchins’ machines, I was also using my wearables. So, how did they do when it came to assessing my calorie burning?

We have displayed the accuracy of each device as a percentage – how much they were above or below the accurate score.

A score of 90 per cent means the device underestimated how many calories I was using, potentially leading me to cut out too much food if I was on a diet. A score of, say, 110 per cent means they overestimated how much I was burning, potentially leading me to eat too much. A score of 100 per cent means perfect accuracy.

£34.99, buy it here.

Decathlon is a French retailer, with 42 stores in the UK and a cult following for its astonishing range of sports equipment, from water polo goals to spearfishing gloves. It also has a small range of own-brand wearables.

This one is very cheap and looks just like a small Apple Watch. It can receive WhatsApp messages and measure heart rate, step count and stress levels. Its calorie accuracy, however, is dire. At this price, it’s perhaps no surprise.

Calorie accuracy: 37 per cent

£349 (down from £599), buy it here.

This is a seriously chunky watch aimed at serious sports people.

If you are a cyclist, it can measure your wattage, it has a triathlon mode (it is waterproof down to 100m) and it can tell you if you are ahead or behind your personal best when running.

Its calorie accuracy, however, is poorer than the steep price would suggest.

Calorie accuracy: 78 per cent

£399, buy it here.

Apple was the first mobile brand to get into wearables and this watch, now a decade old, does an awful lot. It answers calls, tracks menstrual cycles and is waterproof down to 50m.

One downside: a poor battery life.

Its heart rate accuracy – compared with my official heart monitor – was excellent.

Its calorie accuracy is not perfect, but – compared with the others – isn’t too bad at all. In fact, it is the winner.

Calorie accuracy: 92 per cent

£139.99, buy it here.

One of the first wearable fitness devices to hit the market, Fitbit has been in the UK since 2009. Now owned by Google, it has a range of trackers as well as larger, more souped-up watches.

The Charge 6 is a tracker with a small screen, so you might struggle to read the data. But you can sync it with an app on any phone and read it more easily there. When I wore this with other devices, it always estimated a far higher number of steps than rivals.

But, although it overestimated steps, it underestimated calories burned – by quite a lot.

Calorie accuracy: 72 per cent

Price: £169 to £349 a year (a subscription model), buy it here.

The Prince of Wales has been spotted wearing a Whoop, along with golfing great Rory McIlroy and football champion Cristiano Ronaldo, underlining its status as the wearable of choice among the elite.

A Whoop is a strap. No screen. That’s it. All the stats are sent to your phone, providing you with ‘scores’ rather than raw data. Each day you get a sleep score, a recovery score and a ‘strain’ score (what it calls exercise).

However you can see steps, calories, heart rates and plenty more besides in the app. But, despite the welter of data, graphs and ‘insights’, its calorie accuracy is distinctly substandard.

Calorie accuracy: 66 per cent

£349, buy it here.

Is this jewellery or a wearable? Oura’s popularity with Los Angeles’ finest including Prince Harry, Jennifer Aniston and Gwyneth Paltrow is partly thanks to the fact it is quite elegant.

It does mean, of course, you can’t see any real-time data – Oura collects your steps, heart rate and stress levels throughout the day, sends the data to an app on your phone and presents it as scores at the end of the day.

You get scores for ‘readiness’ (how well recovered you are), sleep, activity, heart rate and stress, as well as more standard medical data if you want.

It can be quite clever, correctly working out that I had been dancing at a party, for instance.

But it also struggled – because I was stationary – to recognise that I had been busting a gut on an exercise bike. Odd. Its calorie calculation isn’t too terrible.

Calorie accuracy: 86 per cent

£279.99, buy it here.

Lightweight, easy to set up and with a great battery life, there’s a reason why Garmin devices have remained reliably popular for many years.

This pricier model has

some fancy features such as ‘Sleep Need’, which calculates how much shut-eye you should be getting each night based on your activity, stress and recovery levels.

For serious sports people, the connected app on your phone offers lots of clear data from the length of your strides to estimated sweat loss.

This was the only device to overestimate my calorie

burn. And did so by a noticeable amount.

Calorie accuracy: 112 per cent

Conclusions...

Some of the wearables are great smartwatches, some are brilliant at analysing sleep patterns and running routes. But they are uniformly pretty rubbish at measuring calories. Only one, the Apple Watch, got within ten per cent of the accurate score.

Hutchins, then, is clear that people need to treat the calorie data wearables provide with scepticism.

‘Wearables are popular because people think it’s personalised data,’ he tells me. ‘Unfortunately, the reality is that they’re using a whole bunch of algorithms to predict you – not to measure you.’

Does it matter that they can’t accurately measure calories? Hutchins warns: ‘If you are using these things to inform your day-to-day decisions about nutrition, for instance, you could come a cropper.’