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Should You Trust Your Calorie and Step-Counting Apps?

Many Americans monitor health by using calorie and energy trackers.

“What is he doing?” I asked my friend whose husband had been walking from our kitchen to the living room and back for 20 minutes.

“His app told him that he hasn’t walked his 12,000 steps today,” she told me, “and he gets sort of obsessed until he does.”

If you are one of those people who depend on an app or device to ensure you have met your exercise requirements, you might ask how accurate these devices really are. You assume they are; otherwise why would you bother paying attention to their measurements? Your device told you that you took so many steps and covered so many miles. But, according to an article by Sara Harrison last spring, your device many not be measuring every step you take.

Devices that track your steps use an accelerometer to detect motion with electromagnetic sensors. Then your tracking device applies an algorithm that translates this motion into steps. We assume that regardless of how tiny, or gigantic our steps are, they are all counted as steps. But that is not what happens.

This algorithm matches your motion to a standard step (not yours) culled from studies of young men who walk at a normal pace and a normal speed. Your steps and those used as a standard may match more or less. But maybe they don’t. Maybe your gait is very short or long or you shuffle or walk unevenly because you are walking over a field strewn with rocks and tree roots… or you are walking on ice or sand, and your steps adjust to an icy surface or the lack of a firm surface caused by soft sand.

A friend’s husband has Parkinson’s Disease, and when she walks with him over the distance she might walk alone, she gets different readings because her husband tends to shuffle and walk slowly.

Unfortunately, fitness trackers are not able to detect all individual differences in gait and speed. Information might even be affected by where on the body the device is worn. In an article reviewing 67 studies using Fitbit to assess the accuracy of step recording and calorie expenditure, Feehan and her colleagues found that when people’s steps were measured in a laboratory setting, the device underestimated the number of steps taken; in a free-living setting (on your sidewalk), it overestimated. Moreover, they found that joggers should wear the device on their wrists for the most accurate measurement, but walkers should put it on their bodies (pinned to a shirt or belt) for the best results.

For those of us who want to know how our steps translate into calories used, the authors’ conclusions were dispiriting. All the studies confirmed that the devices were unlikely to provide accurate information about energy expenditure. Translation: Don’t eat that extra large ice cream cone because your device says you just walked off 1400 calories.

Also, the fitness equipment in a gym may not be reliable either in providing an accurate reading of calories used during a workout. Unless you are working out in an exercise physiology laboratory, don’t assume the calories that are displayed at the end of your workout are totally reliable. This is particularly true if the machine doesn’t include your weight in its calculation. According to several exercise physiologists, a stationary bicycle is the most reliable, and elliptical machines the least accurate in reporting calories use. If the arms are not used during the exercise on an elliptical machine, the reading will be even more inaccurate.

Common sense provides an alternative to the use of devices and calorie readouts from exercise machines. Elevated heart rate, sweating, fatigue, and increased rate of breathing are obvious signs that you are exercising strenuously. Look at people emerging from a vigorous aerobic or spin class and it is obvious that most, if not all, just had an active, calorie burning exercise session. Look at someone walking very slowly on a treadmill, talking on the phone, or chatting with the person on the adjacent treadmill, and again it is obvious that not too many calories are being used up.

Of course, knowing how many calories you are using up through physical activity may or may not influence how many calories you allow yourself to eat. “I just walked 14,000 steps, “you might say to yourself. “Maybe I’ll have dessert tonight.“ You might even check a food tracking app to see what the calorie content is in a cup of Belgian chocolate frozen yogurt, in case you decide to indulge yourself. Or you may record what you consumed yesterday to see how many calories you ate. You assume the information will be accurate. Will it be?

Researchers have studied the reliability of calorie counting apps compared to the gold standard of the USDA’s calorie and nutrient database.

In this project, the researchers tested seven popular (in 2019) food-tracking apps available on smartphones to test their accuracy. They entered three-day food diary records into the database of the individual apps they were testing, and also into the database of the USDA. All the apps reported similar figures for total calorie intake, and they all overestimated intake of carbohydrate intake compared to the USDA figures. But most of the apps significantly overestimated protein intake by more than 10%, and underestimated fat consumption by more than 6%, compared with the USDA standard. However, the researchers concluded that most of the apps did a good job in presenting sufficiently accurate information to be used by dieters and those who simply wanted to know what they were eating.

Ironically, using devices, apps, and calorie counters on exercise machines, regardless of their accuracy, are almost irrelevant. If you are interested in how many calories you are using up exercising, then you are exercising and that is what is important. If you are willing to enter into an app the foods you have eaten at home, at a restaurant, homemade or packaged, then you are focused on what you are eating. And that is important as well.

Ultimately, what you eat and how much you exercise will show up as improved health, better sleep, and, perhaps, better mood. And these do not require an app to track.