As my friend chowed down a huge burger and fries, he looked me squarely in the eyes and exclaimed, “Hey, according to my fitness tracker, I burned 1200 calories on this morning’s run.” That’s what happens when you’re a dietitian – people can’t help but verbally justify their food choices to me. At least this proclamation was original; I usually get, “I don’t usually eat like this.” Ya, ya, whatever…
Just to let you know, in case we get to dine together someday, I promise to keep my opinions to myself. That’s for several reasons. One, I have manners. And, besides, I know that one meal never offers the full-story. Because I previously struggled with eating disorders (and share this story at certain speaking events, many audience members have shared that they barely nibble in public and, yet, binge in private. But, most importantly, I won’t judge because, frankly, I’m no saint myself. I love to eat, too. And, while I probably wouldn’t opt for anything larger than a quarter pound burger and selectively eat fries only when they’re hot and crisp (hey, a girl has got to have standards, right?), I’m much more apt to indulge in chocolate, cheesecake, oh and warm crusty bread spread with butter (no oil dipping for me). We all have our favorites.
So, I did keep my mouth shut while he indulged. I let him enjoy the meal in peace. But, now I’m going public with what I really wanted to say, “NO, YOU REALLY DIDN’T! You really didn’t burn 1200 calories.” Now, mind you, I wasn’t with him on his run, but I DO know how a fitness tracker makes these calorie calls. My friend uses one of those popular weight loss apps to log his caloric intake (you too?). As he exercises using his tracker or simply records his exercise, that day’s permitted calorie intake goes up accordingly. That’s why he was indulging that day.
What You Need to Know about Your Fitness Tracker (and Calculated Formulas)
Here’s why I know my friend probably didn’t burn close to 1200 calories that day:
- The accurate way to assess calories burned is in a scientific lab. The resting metabolic rate (RMR), calories burned at rest, is assessed by measuring the amount of oxygen inhaled and the carbon dioxide exhaled. The same method is used for assessing calories burned during exercise.
- Fitness equipment, fitness trackers, apps and calculators on the internet are using a calculated formula. Four formulas are most commonly used. Some only take into consideration your gender and weight. Others add in age and height. None of the formulas take into consideration lean and fat distribution. And, of course, there’s a genetic component, too.
- Lean muscle mass is the largest determinant of how many calories we burn in a day. And, since this isn’t included, these formulas tend to be way off in obese individuals (weight greater than 20% above normal). Even the most reliable formula, the Mifflin-St Jeor equation, was found to be “accurate” in 82% non-obese subjects, but just 70% of obese subjects. Mind you, “accurate” was judged as being within 10% of the laboratory measured RMR. Overestimation was as much as 43% with another formula. Read more here: https://www.ncbi.nlm.nih.gov/pubmed/15883556
- Wrist-worn fitness trackers are not accurate. Just recently, a study found that while wrist-worn fitness trackers are accurate for checking your heart rate, none of the seven devices measured energy expenditure accurately. Even the most accurate device was off by an average of 27 percent. And the least accurate was off by 93 percent. Skin color was found to be another factor affecting accuracy.
- Exercise machines lie, too. A 2010 study at the University of California–San Francisco’s Human Performance Center, tested four different cardio machines’ calorie counters, compared to the scientific method. They found that all machines tend to err on the high side, and some more than others: Stationary bicycles overestimated calories burned by 7 percent; stair-climbers did by 12 percent; treadmills by 13 percent; and elliptical machines by a whopping 42 percent.
The Moral of the Story
Go ahead, if you want to, and check out the calculators and look at the calories burned on your electronics, including your fitness tracker. But, if you’re trying to match what you eat with what you burn, and you’re still overweight, don’t take the numbers seriously. This could explain the extra 50 pounds my friend carries around.