Your smartwatch is lying to you.
There’s no denying smartwatches are cool and these days it seems like everyone has them. They brought health monitoring and personal data to the masses and made it fun to talk about. Who wouldn’t want to know about their heart rate or how balanced their steps are? As such, smartwatches have been at the top of fitness trend charts for the past few years. But, when it comes to calories, how effective are they at actually capturing your calories burned? Let’s take a look.
How has caloric burn historically been estimated?
When it comes to calories, there are a few big equations that have dominated the field. Some of these equations have been around since the early 1900s (like the Harris Benedict equation), but others are slightly newer coming from the 1990s (like Mifflin-St. Jeor). The name of the equation matters little as they all effectively calculate Basal Metabolic Rate (BMR) based on your age, height, weight, and sex. Based on these four inputs, the equations estimate lean body mass (also known as fat-free mass), which is key for accurately predicting BMR. But why is BMR important? BMR is based on the number of cells your body needs to supply energy to. Each person has about 37 trillion cells in their body, and even when you’re not being active, they still require energy to function and keep you going. So, BMR is significant when thinking about total energy expenditure. Once they estimate your BMR, they then make a total guess as to how active you are (see our blog post on Physical Activity Level (PAL) for why making assumptions here can lead to poor training outcomes). By multiplying BMR by PAL they then estimate your total energy expenditure (aka your caloric burn). As you can see, this is an okay estimate at best, as it’s based on quite a few assumptions along the way.
How does my watch calculate caloric burn?
Your watch, with a couple of sensors, can make the estimate from the equations a little bit better. Most watches use an accelerometer (which tracks motion) and a heart rate monitor. Together they can help your watch make a better guess at how active you are (your PAL). While this is a little better, it still guesses your lean body mass based on your age, height, weight, and sex. It also is worth noting here that watches have repeatedly failed accuracy stress tests. At best, some watches were 30% off, and at worst, others were up to 90% off when it came to estimating energy expenditure! Errors are this large because the human population is so diverse (and this abundant diversity is something Calorify can capture).
A watch’s estimate might be close for some, but for others it can estimate caloric burn to be significantly higher or lower than the real burn– enough to impact training. For example, one pound of fat stores approximately 3,500 calories. Therefore, if you were to eat in a caloric deficit of 500 calories each day for a week, you should expect to lose about a pound. But, depending on how much you’re burning and how quickly you’re looking to lose weight, a deficit of 500 calories each day for a week might not be right for you (and based on your PAL, it might not even be sustainable!).
In one customer’s case, their watch estimated their burn to be 2,851 but Calorify measured their actual burn to be 3,444. Their watch was undercounting by 600 calories, about a 20% error. This is one of the best case scenarios for errors that we have seen.
If this customer wanted to lose weight, they could burn fat and build muscle eating 3,152 calories a day, and could eat 2,626 for max safe fat loss. However, if they used their watch data (2,851 calories burned/day) and followed the fat loss math from above (suggesting a 500 cal/day deficit), they would calculate that they should be eating 2,351 a day to lose a pound of fat in a week. But that is well below their actual max safe fat loss, making it an unsustainable cut, putting them at risk of injury and more.
In the literature, smartwatches are consistently >750 calories off, and are sometimes up to 4000 calories off. This is where, if your watch data is hundreds or thousands of calories off, it can have a real impact on your training and goals.
Fun fact!
Ever wonder where the 2,000 calories a day number comes from? The short answer is nowhere–it was made up. In the 1990’s, the FDA pulled together public survey data and decided on 2,000 calories as a nice, easy to work with number for food labels. Unfortunately, based on the literature, we know that people chronically under-report the amount of calories they intake.
Thanks to Pontzer et al., 2021 we can begin to visualize this discrepancy. In the graph below, you can see that very few people actually need 2,000 calories, with the majority needing significantly more. Also note how your smartwatch doesn’t even begin to capture the variability between everyone’s individual burn.
Calorify’s Caloric Burn Measurement
So, now that we know a watch isn’t great at estimating calories burned, what’s the next best thing? In the past indirect calorimetry has been used to measure total energy expenditure, but in order to get your measurement you have to live in a sealed room in a lab for up to a week! Calorify’s doubly labeled water (DLW) method is the gold standard as it is just as accurate and you get to live your life as normal while testing! By directly measuring how quickly H2O and CO2 is expelled from your body, we essentially make your body its own indirect calorimetry chamber. But, instead of the system being a room or a mask, it’s just your body!
Unlike your smartwatch, Calorify cuts out the middle-men, the proxies and combination of different estimates, and gets down to what really matters: making a measurement. Calorify simplifies metabolism testing to work with your schedule. We would love to help you figure out what your actual burn is, and get you on track with whatever fitness or life goals you may have. We’re here and we’re excited to help–just reach out!
