How many calories do you actually eat and burn?

Electronic devices such as Fitbit and the Apple watch measure energy expenditure, are they accurate?

Image by Dariusz Sankowski from Pixabay

There are very many ways to approach weight control and loss. Tracking calories, our body’s energy coin, is front and center of many of them, as energy deficits will result in weight loss, and energy surplus will result in fat accumulation. Diets that don’t count calories also work on getting to a caloric deficit through other methods. Estimating caloric balance requires knowledge of input and output.

How many calories do you consume?

The traditional way of calculating caloric or energy intake relies on weighing and measuring your food, and knowing the caloric value of each specific food item. There are now apps aplenty – MyFitnessPal, Lose It, and Fitbit to name just a few – that help with assessing portion size and identifying food’s caloric value. It's still tedious, time consuming, and in real life we know that people’s assessments tend to be inaccurate and wishful, erring on the side of underestimating intake.

How many calories do you burn?

Assessing caloric output used to rely on charts taking into account age, gender, body size and activity level – sedentary people burn fewer calories than active ones.

Electronic activity devices are in wide use and are an advanced version of the pedometer, able to measure steps, distance, altitude, and some of them also heart rate. They come in all sorts of wearable and clippable forms. Many of them calculate caloric expenditure by an algorithm that uses our movement and our personal health information, such as gender, height and weight. Most smart phones have a built in health app that provides this info. The Fitbit calculates how many calories you burn, based on what you burn at rest – the calories that are used to maintain vital bodily functions such as breathing, heart and blood circulation, digestion etc. – and the extra calories burned during exercise. The Apple watch provides similar information, dividing calories to active and total calories. 

Is the caloric expenditure accurate? A study from Stanford University looked at six devices (Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2) used by 60 volunteers, and although heart rate measures were pretty accurate, energy expenditure measures were off. When compared to metabolic rate assessments measuring oxygen and carbon monoxide in breath – which are a more accurate way to assess energy expenditure – even the most accurate measures were almost 30 percent off, and the least accurate device was off by more than 90 percent. A recent 2020 study looking at four popular wearables (Apple Watch Series 4, Polar Vantage V, Garmin Fenix 5, and Fitbit Versa) also found that heart rate measures were pretty accurate, while energy expenditure measures were unreliable.

These devices are pretty good at counting steps and heart beats, but not as good at knowing how many calories went by. Mind you I still can’t explain the Samsung bonus: My husband's phone consistently registers an additional 15 percent mileage compared to what my iPhone calculates when we hike together. 

Can devices do the counting for you?

So, the search for an easy, objective way to measure caloric intake is still on.

A promising way to measure caloric intake relies on a simple math equation: Spare calories are stored as fat, so if you knew the energy storage, and you also knew how much energy you expend, you could calculate energy intake. 

As mentioned above, consumer devices that measure physical activity are ubiquitous. 

And there are now devices that claim to measure body composition. Electric scales that use bioelectrical impedance measure not just weight but also body composition (the scale sends an electrical signal through your body that measures the amount of fat vs. lean body mass) and can load this information to your personal account.

Combine the scale information, with the resting and activity energy measures, and you can potentially assess caloric intake without employing tedious food diaries and portion measurements.

How accurate are these measures?

A new study in the Journal of Nutrition compared the results of calculating caloric intake using the commercially available wearable devices for measuring caloric expenditure combined with a smart wi-fi weight scale (Fitbit Alta HR + Fitbit Aria), comparing them to the “gold standard”, the most accurate ways to measure energy expenditure and energy storage. For energy expenditure the most accurate method is doubly labeled water. For measuring storage of fat it’s Dual X-ray Absorptiometry for body composition. These accurate methods are difficult, expensive and require expertise, and are used only in specialized academic centers. This comparison was done for 2 weeks in 24 adult volunteers.

And the results: The Fitbit Aria’s ability to accurately measure fat storage was low. Energy expenditure measurements by the Fitbit device fared better in this recent study, and were quite accurate when compared to the doubly labeled water.

Alas, without an accurate measure of fat storage, the equation cannot be solved, and caloric intake cannot be reliably calculated by the existing electronic devices. 

So it’s back to scales, measuring cups and nutrition labels in order to know how many calories you consume, if you choose to go this route. You can’t base food decisions on these devices – at least not yet.

What’s all this counting good for?

Accurate and convenient automated calorie counting isn’t feasible yet – electronic devices have yet to deliver a reliable solution, even as they’re getting better at it. 

For people trying to eat better, tracking food may be a worthwhile exercise, at least for a while, because it helps you learn what’s in your food, and allows you to moderate caloric intake.

Until you actually count, you may not realize the caloric burden of frappuccinos, sugary drinks and fried foods, and the disproportionate portion sizes served in some establishments.

But once that exercise’s over, there are easier and more practical ways to eat well, maintain weight and enjoy life. 

A balanced plate of food (see MyPlate) can be assembled without scales, measuring cups and nutrition charts. And being glued to a digital screen even when you’re out and about outdoors, counting distance and steps rather than looking at the sky and registering birdsong diminishes wellbeing even if it can motivate you to exercise.

Much like with attaining financial literacy, some counting and accounting might be necessary in order to build good habits, but carrying a money or a calorie tally in your mind – or on your devices – takes away time and attention that can be better used.

Dr. Ayala