Nano Girl Michelle Dickinson: Is your Fitbit telling fibs?

Wearables provide a swag of information on the users' heart rate, blood pressure and calories burned but with only 40-80 per cent accuracy. Photo / AP

You know a wearable wearer when you meet them. Conversations tend to gravitate towards how close they are to their 10,000 steps goal, or how they can prove they got less sleep than you last night.

Wearables - the relatively new term for technology that you strap to your body - are arriving on a wrist near you. Smart watches and health-tracking wristbands like the Fitbit are helping more of us to feel informed about the metrics output by our own bodies. These innocuous wristbands are actually smart devices providing thousands of Kiwis with information about vitals like heart rate, blood pressure and calories burned.

Although members of the wearables club may feel more informed with a plethora of personal data to download each day, the question of what they should actually be tracking for a healthy lifestyle still remains.

The Ministry of Health's physical activity guidelines recommend that all adults aged 18-65 complete at least 150 minutes of moderate intensity activity a week. A deeper look into the source of this miracle number reveals an interesting source which caught the attention of a University of Otago researcher.

Leon Mabire, a physiotherapy PhD student, found that the 150 minutes originate from research based on male British civil servants who walked to the train station as part of their commute. The 20-year-old study found that men who walked more than 15 minutes a day had a lower risk of heart disease than their non-walking colleagues. The healthiest men were those who walked for 30 minutes, five days a week, or 150 minutes a week.

They calculated that the average 75kg man would burn 6.4 calories per minute, or 1000 calories over a 150-minute-per-week exercise regime, and hence the guidelines were born.

The truth is we are not all 75kg, British or men so in collaboration with MoveMe Dunedin, Mabire asked 62 diverse men and women to walk briskly on a treadmill while hooked up to an oxygen mask. They found that the 75kg men burned around 6.4 calories per minute, agreeing with the original experiment.

However, the 50kg females only burned 3 calories per minute while the 160kg men burned a whopping 18 calories per minute. This clear relationship between increasing weight and burning more calories would suggest that obese adults should find it easier to lose weight from walking than skinny adults, but this is not actually the case.

In fact the heavier walkers burn energy more quickly so they end up burning carbohydrates stored in their muscles rather than using slow release energy from their fat stores.

Our brains need carbohydrates for fuel, so quick calorie burning causes the body to immediately want to replace them, making you feel more hungry and putting you at risk of eating more than you burned.

Monitoring calories burned using a health tracker may therefore lead to you overestimating the true value, or cause you to exercise too hard too quickly, both of which could lead to an increase rather than a decrease in your weight.

As research has found that wearable health devices are actually only 40-80 per cent accurate, Mabire's solution is a new algorithm based on age, sex, height and weight. His "calorie cost counter" calculation will provide a much more personalised calorie counter for your walking activities, with 94 per cent accuracy.

This New Zealand research has the potential to have a global impact on people's health, wellbeing and wearables happiness.

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