Are Activity Trackers Accurate?
Interpreting your FitBit’s Calorie Burn
Short bonus post today, on a topic that has come up in my online coaching group this week.
The question is: should I log my calories burned through exercise?
Let’s say you’re tracking calories, using an app like MyFitnessPal. Apps like these make it very easy (and appealing) for you to add in your exercise. You can conveniently sync the app with gadgets like the Garmin, FitBit, or Apple Watch, allowing it to track your steps. Additionally, you can manually add in exercise, choosing from a variety of cardio and strength options.
I have a problem with this equation, from several points of view. Let’s start with how you see the information you’ve entered. When you add in steps or exercise, you are given a visual like this:
I ate 1,732 calories, but I ran a brisk 35 minutes, too. According to my MFP automatic calculator, I burned 486 calories, which means that I subtracted that from my eating, leaving me with 304 remaining calories to eat… even though I surpassed my calorie intake goal for the day.
Here are the three main problems I have with this over-simplified equation:
Math Problem #1: Your exercise is already accounted for in the calorie goal.
When you first create your calorie goal (whether you are using MyFitnessPal or some other calculator), you are asked about your activity level. When you answer that question, you are automatically multiplying your basal metabolic rate (the number of calories you would burn lying in bed all day) by an activity factor. You can’t see the math happening in these automatic calculators, but rest assured – your calorie goal is based on your stated amount of exercise. If you really did nothing but sit around all day, your calorie intake goal would be lower.
So if you allow the app to subtract your exercise and then eat more to “make up” for the exercise – you are overeating for your goal, likely putting you back into caloric maintenance or a surplus.
This is where I see so many people get stuck when they are trying to improve their body composition. Let’s play out the example from above. If I ate 304 more calories, I would actually be eating 2,036 calories. That is a whole 486 calories above my calorie goal, which already took my exercise into account. This is the math problem – thinking I’m staying in a deficit, I’m actually putting myself in a surplus. This is a very short road to frustration.
The only exception would be a major exercise event or otherwise very heavy level of activity. Let’s say you identified yourself as fairly sedentary (because of an office job, for example), but one day you have a day of several hours of intense yard work, snow shoveling, or something similar, that is very unusual for you. It is quite likely on these days that you have burned more than usual, and may need to eat a bit more to not feel completely burned out.
But even in this case, there is another math problem…
Math Problem #2: Activity trackers are notorious over-estimators.
I have seen several conflicting figures for how much activity trackers over-estimate calorie burn, but here’s the summary: no one is positing that trackers UNDER-estimate calorie burn.
Here’s why: the number of calories you burn in exercise is dependent on many factors, even beyond age, gender, and weight. Some of the factors are too complicated to determine, unless you are in a lab. For example, a very fit person tends to burn less calories during exercise than a less conditioned person, because highly active and conditioned people are more energy-efficient. You could see how, if you began regimented exercise program, your fitness would improve gradually, making your caloric burn a shifting factor over time.
Secondly, when we exercise at an extremely high level of intensity, or even over-exercise, we may slow down in other ways without realizing it. We may move around less, walk less, and generally expend less energy on days that we are worn out from tough exercise. The reality is that we burn hundreds of calories per day doing simple things like walking around and fidgeting – often more calories than we burn through exercise. So when that number decreases, we’re looking at an even bigger problem with the math. This is where the big picture is important, and you can end up missing the forest for the trees.
This makes calorie burn extremely difficult to calculate, because it’s not a static picture – it’s dynamic, with a complex orchestration of moving pieces, including mental states like motivation, self-assessment of activity, and level of energy. It’s not something that a FitBit can accurately capture.
So this leads me to my last problem with this equation, which transcends math and science:
Overall Mindset Problem: We take “calories in, calories out” too literally.
While a caloric deficit is truly the only way to lose weight, we sometimes don’t grasp this important fact of 21st century life: it is easy to eat poorly, and it is difficult to move well. Stated in the inverse: it is difficult to stay in a caloric deficit, and it is easy to be sedentary.
If we don’t realize this, we can easily under-estimate the calories we eat and over-estimate the intensity and volume of exercise we perform.
So how do we use activity trackers to our benefit? I recommend a Garmin Forerunner 35 on my recommended materials page, as well as using MyFitnessPal for my clients, so clearly I’m a fan! But how do we avoid these logical problems that I have laid out?
Practical Strategies for Getting the Most Out of Trackers
Strategy #1: Only use the tracker for steps and heart rate.
Your steps are incredibly important both to your overall health and to your metabolism. Keep it moving. Use your activity tracker to motivate yourself to boost your level of activity on a daily basis, and break up your sitting. Use it to monitor your heart rate during workouts, so that you can assess how intense your exercise actually is (remember to wear monitors nice and snug, a few inches above your wrist, so that it can accurate gauge your HR).
Strategy #2: Only track your caloric intake, not your calorie expenditure.
Calorie expenditure is simply too difficult to calculate. As I said, you would nearly need to be in a lab, under scientific study, to accurately determine it. However, foods have been scientifically tested for you, which means that you can generally trust the calorie counts you are taking in (assuming you are measuring and portioning correctly). Therefore, calorie intake is the only factor that you can measure with any degree of accuracy, and is the one to which you should give the most weight.
Strategy #3: If you track exercise, do it as an “exercise diary” to monitor long-term consistency.
This can be helpful for spotting patterns in your week over time. The most important aspect of exercise is its frequency and consistency, not its intensity. If you think you exercise four times a week, but you begin logging your exercise and realize you only consistently exercise twice a week, this is helpful information. You would know either to prioritize more exercise sessions, or decrease your stated level of activity in your calorie goal calculator.
When we start using exercise as a bargaining chip to allow us to eat more, or use exercise as a penance for overeating, we begin to erode what could be a positive relationship with exercise.
Don’t exercise to burn calories.
Instead, exercise to:
- Feel better overall
- Improve your heart health
- Enhance your mood
- Experience more energy
- Heal stiffness and pain
- Get stronger
- Feel and be independent
- Get outdoors
- Have fun with others
- Relax and de-stress
Exercise can be an incredibly valuable and positive part of your life for physical and mental health, social connection, and ease of living. Don’t make the mistake of creating a negative relationship with it, and instead focus on improving your interaction with both eating and exercise, so that you can enjoy a balanced lifestyle while you get real, lasting results.
I could go into the specifics of how many calories are in a donut versus how many calories you burn running for an hour. But, as you’ve seen above, this may be the missing the forest for the trees. In the end, you can ask yourself: “Am I hitting the treadmill for an extra hour because I don’t feel good about how I ate?” Or, “Am I going to exercise classes every week because I’m trying to lose weight quickly?” If the answer to either of these is “Yes,” it is worth it to re-assess. Not only are you not going to get the results you want, but you may also be setting up a dysfunctional, self-punishing cycle, in which “bad” food is a guilty pleasure and exercise is a punishment.
You can get out of that cycle anytime, and don’t be afraid to reach out for help.