Why chasing “calories burned” is a fool’s errand

This article is going to take a deep dive into the science of calories in/calories out related to exercise and weight loss, and will provide a detailed analysis of how technology may be impeding your health and fitness goals.  Lastly, I will make some suggestions of how to use the tech that we currently have available as a way to actually enhance the training experience and push you closer to your goals.  

If you’re interested the science and are looking to gain an understanding of why and how certain things work (and why certain things do not) for body composition and weight management, I encourage you to read the entire article.  If you’re reading after your workout this while waving your arms back and forth trying to close an “exercise ring,” or just want a simple answer, you can look no further than the back of the napkin summary. You can use the following links to jump around by topic if you prefer.

Contents:

1. Back of the napkin summary
2. The technology just isn’t there yet
3. The problem with using heart rate
4. Metabolic realism & why your body doesn’t care how your jeans fit
5. Your flexible metabolism: your body is pretty freaking cool
6. A better way to use technology to advance your health & fitness levels

Why is it a waste of time, effort and stress to track calories burned during physical activity?

Simple answer: You’re not measuring what you think you’re measuring.  

When you constantly check your watch or activity tracking app to see how many calories you’ve burned during a particular workout, what you’re interested in is not actually the total number of calories burned.  You’re interested in the quality of the workout.  Did this workout help me reach my goals?  Did I work at a high enough intensity to (insert goal)?  Did this workout “balance out” my (insert some less than “ideal” eating habit from the previous day)?  Am I “allowed” to eat more today because I “burned” more at the gym?  When you see that you “burned 800 calories in a 40-minute bootcamp,” you think you’ve accomplished more towards your goals than you did “only burning 200 calories,” during a strength training session.  

If these questions sound similar to what’s going through your mind as you deliberate over choosing “Functional Strength Training” or “Cross Training,” let’s reframe the entire thought process.  The number of calories your watch tells you burned is in no way indicative of the quality of the session.  The reasons for these logical fallacies are outlined later in this article, but the bottom line here is:

When you get down to the hard science of metabolism, exercise and overall health, there is absolutely no merit to using calories burned as a judgement of workout quality.  Learn why below.

Reason #1: The technology just isn’t there yet.

This is going to come as a surprise and a bummer to those who have spent several hundred dollars on the latest Apple watch hoping the watch is the key to their weight loss:  The technology that is found in most consumer available models of smart watch simply is not advanced enough to give us an accurate measurement of caloric expenditure.  How can this be?  

To understand the inaccuracies associated with “calories burned” as recorded by your smart watch, we have to peel back several layers of data and also understand that Apple, like most commercial brands, do not make their exact caloric expenditure algorithm publicly available.  Apple’s website only goes so far as to say “Every full minute of movement that equals or exceeds the intensity of a brisk walk counts toward your daily Exercise and Move goals. With Apple Watch Series 3 or later, your cardio fitness levels are used to determine what is brisk for you. For wheelchair users, this is measured in brisk pushes. Any activity below this level counts only toward your daily Move goal.”

So when we break this down into primary components, what we’re left with is an actual measurement of two variables: heart rate (HR) and arm movement.  Again, the exact algorithm behind the scenes is proprietary and likely varies from brand to brand, but those two variables are the primary driving factors behind the “calories burned” estimate you see on your watch. This presents us with our first clear evidence that having calories burned as a focal point during exercise is flawed reasoning.  

The problem with heart rate: 

Heart rate detection, even on our most expensive smart watches, has been shown to be highly variable and unreliable.  Smart watches such as the Apple Watch use technology which estimates heart rate using optical sensors to collect pulse rate or volumetric changes in blood profusion that act as a proxy for heart rate (HR).  In other words, a smart watch isn’t directly measuring HR, it’s creating an approximation.  In order to directly measure heart rate, we’d need to hook up to an Electrocardiogram (ECG):

If this sounds like a lot of extra time and effort just to get an accurate measure of HR, that’s because it is.  This type of testing is typically done in a doctor’s office or research lab.  Looking through the research comparing smart watches to actual ECG HR measurement, we see a striking amount of variance and unreliable measurements.  The scatter plots below come from a 2020 article titled “Accuracy of wearable heart rate monitors in cardiac rehabilitation,” comparing ECG-tracked HR to several commercial brands of smart watch:

You’ll notice in the graphs above that the messier the plot looks, the more each device varied from what the ECG was actually reading.  You might also notice that the very first graph shown above looks pretty tightly wound, and that is because this graph represents a comparison of ECG to a Polar Chest Strap device.  This very neat cluster of data points tells us that the chest strap significantly out-performed watches, which rely upon optical sensors.  Several other studies have drawn similar conclusions, and the Cleveland Clinic summarizes some of this work with the following statement, “The researchers found that the most accurate readings ⁠— as measured against the EKG ⁠— were from the chest-worn monitor, Dr. Gillinov says. Other studies have also found that fitness devices slightly underestimate heart rate, and that their accuracy diminishes during exercise.”

You may be thinking “well, isn’t my watch close enough?”  

If you’re using your watch to make decisions or judgements about your exercise routine, no, I don’t think your watch is close enough.  At least not with where the tech is at the time of this writing.  The accuracy of optical-sensor driven HR data gets worse as exercise intensity increases.  The research done to date suggests that at best, we can expect that heart rate (measured in beats per minute or BPM) could be anywhere from 27 BPM below to 29 BPM above what an ECG would measure during intense exercise.  In other words, if we were to hook you up to ECG leads and measure your HR during intense activity, we might see that your heart rate sits at an average of 150 BPM for 30 minutes.  If you did that same 30 minute workout, instead using your Apple Watch to monitor heart rate, your average HR could show up as low as 123 BPM, or as high as 179 BPM.  This is the level of inaccuracy you must understand and expect when you’re using technology that simply isn’t as evolved as we’re led to believe.  While this difference in average HR might not sound meaningful, if we were to use a simple calories burned equation based on age, weight, activity duration and average heart rate (again, we don’t know the exact formula that Apple or other brands are using), this would mean the difference in “calories burned” as a simple result of measurement error could be alarming.  

Practically speaking, let’s say a 50 year old 200-pound male runs on the treadmill for 30 minutes at a 6mph pace on Monday (session 1 ), and then repeats the exact same workout one week later (session 2).  The “measurement error” could lead him to believe that he burned as many as 615 calories, or as few as 363 calories.  It would be well within the realm of possibility that his watch could tell him that he burned almost TWICE as many calories during session 1 compared to session 2, despite them being the exact same workout simply due to the fact that his watch has a built-in degree of error of (close to) plus/minus 30 BPM.

The bottom line when it comes to tracking heart rate, particularly during moderate to intense physical activity:  If you’re not wearing a chest strap or hooked up to an ECG machine, don’t bother.  There are other, more accurate ways to assess exercise intensity level which will be discussed later.

This brings us to our next issue with the inaccuracy of calories burned as told by your favorite watch:  Estimating calories based on heart rate.

So far what we’ve learned is that your smart watch is estimating your heart rate based on an optical sensor.  This HR goes into an equation which estimates your caloric expenditure during a given session, which you then use to estimate the effectiveness of a workout.  We’re talking about the estimate of an estimate being used to estimate an outcome.  Here’s the thing about estimates: they can be very helpful under the right circumstances, but if used incorrectly, they become useless or even potentially harmful.  If you look at your weather app and see a forecast calling for 80 degrees and sunny next week, you’re looking at an estimate which uses advanced scientific data that I won’t pretend to be smart enough to understand.  Based on this forecast, you start planning a pool day for the following week.  You call off work.  You buy snacks and beverages and invite all of your friends to play marco polo.  On your big day, you walk out to your pool expecting to catch some rays but are instead greeted by thunderstorms and 60 degrees.  Could you blame the original forecast or the weather guy for your misfortune?  Or was the problem that you didn’t use any other tools or common sense to evaluate the situation more critically?  You see, the problem I have isn’t with the estimates themselves, but rather the decisions we make as a result of those estimations, when the estimations themselves may be flawed.  

Another massive glitch in the “calories burned” component of smart watches is that even if we were to assume that our heart rate data is accurate (it’s not), we have to then assume that HR is a valid part of the calories burned equation.  Unfortunately for some of the commercial brands out there touting “increased calorie burn,” but fortunately for everyone else, we can actually measure caloric expenditure using a process called direct calorimetry, another process which is very expensive, time intensive and usually performed in a lab.  When compared to directly measuring caloric expenditure under laboratory conditions, using heart rate to estimate caloric expenditure is simply not accurate on the individual level.  Most studies which have compared the difference between a direct measurement of caloric expenditure to that of a heart-rate estimated number have found that individuals can vary by around 10%, with heart rate- based methods tending to overestimate actual caloric expenditure.  

To again put this into perspective, let’s say you read the first half of this article and took my advice to go out and buy a chest strap to monitor heart rate during your workouts.  You then have to deal with the fact that if your watch tells you’ve burned 500 calories, there is still up to a 10% error rate in how heart rate contributes to that number. And this error rate seems to be greater the more complicated the activity is, or the more different the activity is from walking.  This is again a case of the tech just not being where we think it is.  Apple and other trackers attempt to estimate and make up for the difference by allowing you to choose from different “activity modes.”  The very fact that you could see a completely different caloric output as a result of choosing between “high intensity training,” vs. “Strength training,” should be all of the evidence you need to conclude that your watch is not actually showing you calories burned. It is showing you some bastardized, convoluted estimate using data that is not even reliable from arm to arm (seriously, that study has been done and shows that wearing a watch on each hand can yield completely different calories burned metrics), let alone person to person, day to day, or session to session.  

One last thought on using heart rate to measure calories burned, and using calories burned to assess workout quality:  If all of the numbers and research above doesn’t speak your love language, let’s think about this from a logical standpoint:  Is a higher heart rate (and therefore higher caloric expenditure) even a good thing?  Not necessarily.  I’ll explain this point using a real world example:  A client of mine recently bought her first smart watch and began tracking every workout in terms of calories burned.  One Monday (call it session 1) a few weeks ago, she looked down and got excited because her “closed an exercise ring” and her watch told her she’d burned 200-something calories despite being only about halfway through her workout.  “We need to workout like this more often,” she told me.  10 days later,  we did the exact same workout (call it session 2) and she burned a total of around 200 calories by the 60-minute point.  She was frustrated and annoyed because “this is why I’m not losing weight, I’m not burning any calories.”  After asking her a few questions, I came to find out that session 1 had the following surrounding circumstances:  She went out for the Browns game the day before and got horrible sleep.  She was rushed to get to the gym and got pulled over by the police on her way.  Her resting HR when she came in that day was 92, about 30 beats higher than her normal resting, as a result of stress of Sunday Funday, bad sleep, and the fight or flight response that comes with getting pulled over.  Her average HR during that workout was in the 160s.  

Logically, we can all conclude that none of the circumstances involved with session 1 have anything to do with getting a “better workout,” than what she got in session 2.  So even though her HR was through the roof and her caloric expenditure was up, this does not mean that anything she did during session 1 pushed her any closer to her goals compared to session 2.  She didn’t lift more weight.  She didn’t have better technique, she didn’t increase her volume or decrease her time between sets.  She didn’t do any of the things that we know results in better fitness.  She just had a lousy night’s sleep and ran a red light.  

I’ll finish this conversation on heart rate by providing a caveat:  I still recommend tracking heart rate at certain times, in populations with specific goals.  For instance, if a client is recovering from a stroke or heart attack and is beginning a new exercise program, tracking heart rate makes a lot of sense.  If you’re new to aerobic conditioning work, it could make a lot of sense to track your heart rate during your cardio routine and ensure that you’re keeping it below a certain threshold until your fitness level improves.  I also like to see how an athlete’s heart rate may respond differently to a given workout structure as they go from untrained to trained.  For example, if I run a 10-minute mile with an average heart rate of 180 during week 1 of a phase and then run that same 10-minute mile with an average heart rate of 160 during week 10, I can start to make some conclusions that I have made my cardiovascular system more efficient through training.  However, under no circumstances can I think of any reason to view calories burned as a measure of workout quality.

Reason #2: Your body doesn’t care how your jeans fit.

Let’s enter an alternate universe for a moment.  In this alter universe, we as humans have evolved our physiology to make things like “getting toned not bulky,” fitting into skinny jeans and “losing this dad bod,” actual priorities.  The body prioritizes how many selfies we can take in the gym mirror and post to Instagram, showing off our amazing, fat free bodies. This is going to be a very quick trip to the alternate reality because here, everyone who tries to get “sculpted” during their 30 minute hot cycle class would eventually cease to exist as their body would shunt resources away from irrelevant bodily functions.  Forget about digestion, brain function and reproduction….we want abs baby!

Luckily for our current reality, the body doesn’t care how your jeans fit or how many likes you get on the Gram.  From an evolutionary standpoint, the only thing your body gives a shit about is allowing you to live long enough to pass on your genes.  

In order for you to live long enough to pass on your genes, the body has several mechanisms at its disposal, all of which boil down to the fact that you need a certain amount of bioavailable energy in order to find a mate and reproduce.   The amount of energy a human needs varies greatly on an individual level and includes all of the functions which keep us alive.  Every function our body performs on a daily basis requires some amount of energy, and this daily energy requirement is most commonly referred to in calories per day.  Oftentimes we are so focused on the number of calories we’re burning during a workout, that we forget everything else going on in the body.  The sum of all energy expended throughout our day is called Total Daily Energy Expenditure (TDEE) and is summarized in the image below:

The numbers listed above are estimates based on population averages, so keep that in mind as we walk our way through what all of these acronyms refer to:

1. Total Daily Energy Expenditure (TDEE)- the total number of calories you expend in any given day. This number is broken up into two primary segments: Resting Energy Expenditure (REE) and Non-Resting Energy Expenditure (NREE).  As you can see, REE makes up quite a bit of TDEE, around 70% on average.
2. Basal Metabolic Rate (BMR)- the number of calories your body will expend just to keep you alive.  This includes all bodily functions that are required to keep you alive if you never got out of bed: each heartbeat, each breath, the functions of your liver, kidneys, muscles, fat tissue, etc.  Again, this makes up the vast majority of our daily TDEE.
3. Non-Exercise Activity Thermogenesis (NEAT): refers to all of the energy we expend through movement outside of a structured exercise session.  This makes up about 15% of TDEE.
4. Thermic Effect of Food (TEF): This is the amount of energy we expend digesting food all day long.  This varies some by individual and by the type of foods in your diet, but in general will account for about 10% of TDEE.
5. Exercise Activity Thermogenesis (EAT):  Lastly, the thing that everyone obsesses over despite the fact that EAT has the least amount of total impact on TDEE, making up just about 5% of TDEE.  This area likely receives so much focus because the perception is that this is the area of our day that we can change the most, however as I will illustrate below, this notion is misguided.  

ADVERTISEMENT

about:blank

The best ways to increase TDEE go beyond the scope of this article, but suffice it to say the obvious levers for us to pull here focus on BMR and NEAT.

Metabolic flexibility:

Your body isn’t stupid. 

The chart listed above represents average percentages, but it’s important to note that these things are always dynamic.  It’s an amazing evolutionary trick that we have:  Remember, your primary objective from an evolutionary standpoint is to live long enough to reproduce.  If you spend a long enough time restricting the amount of energy you’re making available to do those things (via restrictive dieting and/or exercising to maximize “calories burned”), your body will simply shift it’s resources around to find a place for homeostasis.  Why do seemingly healthy young women develop amenorrhea (loss of their menstrual cycle) when they’ve been over-exercising and/or restricting their caloric consumption?  Simply put, the body has shifted its resources away from reproductive functions in order to preserve enough energy to survive.  

Have you ever had an absolute beast of a workout?  I’m talking about dripping sweat, unable to walk, maybe some puking in the corner kind of crushing workout where the only thing you can think about is how awesome it’s going to be to post about it all over social media to prove how tough you are?  And then because you’re trying to “get toned” you also make sure you cut out carbs and really watch what you eat because you don’t want to “undo what you did in the gym, bro.”  Next time you do something awesome like that again, I want you to think about what the 24 or 48 hours looks like following your workout.  Were you “so sore from those killer squats,” that you didn’t manage to get your 10,000 steps in the next few days?  Did you not sleep all that well because you were so jacked up on dopamine from all the IG likes from your post-puke selfie?  You didn’t poop for 2 days because that’s just the gym life?  

Nah.  You crushed your body’s energy availability during your stupid workout and then you denied it exactly what it needed to fuel itself because you didn’t want to undo your burn zone.  You gave your body a giant middle finger, and guess what– it gave you one right back.  By decreasing the amount of energy you normally spend on eating, recovery, digesting, and being physically active outside of the gym, your body found its way to homeostasis without you even realizing it.  Yep, it’s a wonderful self-correcting mechanism where if you over-do EAT and/or suppress your intake hard enough for long enough, your body will simply decrease your BMR, TEF, and NEAT to compensate.  More calories burned during a workout, fewer burned during the rest of the day.  

And this brings me to my final point on metabolism and calories burned during exercise:  BMR is always running in the background.  Your watch telling you that you burned 300 calories during a 60-minute workout does not mean that you’ve suddenly made a 300 calorie deficit in your day.  Let’s say your BMR is around 1500 calories/day or roughly 62 calories/hour.  So even if we make the faulty assumption that your watch is giving you an accurate measure of calories burned, your 60-minute 300 calorie workout has actually only burned about 238 more calories than you would have had you not even gotten out of bed.  Take that one step further.  That 60-minute bootcamp class that you suffered through also needs to be compared to you just doing NEAT activities like gardening, walking around your house, folding laundry, etc.  Your NEAT activities are generally about 2-3 times more metabolically demanding than being at total rest, or between 120-180 calories/hour.  So when you break it all down, your “HIIT class” at the rec center might have burned 150 more calories than you would have if you had stayed home and walked your dog for an hour.  

All of this is not at all to say that you shouldn’t work hard, or shouldn’t be proud of pushing yourself during workouts.  Quite the contrary.  My point here is that if your gauge of workout quality starts and stops with the number of calories you burned, you’re fighting an uphill battle that will inevitably lead to burn out and frustration.  

A solution for better results and using tech for good

One thing I’ve always hated about long-winded health & fitness articles like this one is that the author spends the entire length of the article talking about why something doesn’t work or isn’t a good idea, but provides no alternative solutions or context that would be useful to the reader.  Let’s do that here:

1. When you’re evaluating the quality of a long term training plan, judge it’s effectiveness based on the variables that impact the goalset you’re trying to achieve.  We have an entire article devoted to different goals requiring different programming types, click here.  In short, are you seeing progress from week to week, month to month, year to year?  Progress could mean adding weight to the bar, adding reps or sets, improving technique, or other measurable, physical qualities.  If you and your coach have selected the appropriate training block based on your goals, simply measure and track your progress over the course of the program and you’ll have a clear answer as to its effectiveness.
2. When performing cyclic aerobic work such as running, biking, rowing, etc., get a high quality chest strap to monitor the intensity of the exercise.  Staying within particular heart rate zones, especially if you are new to aerobic training, can be a great way to ensure you’re getting the intended results.  Let your performance over time be the ultimate judgement of the program’s effectiveness.  Did your mile time improve?  Is your heart rate recovering quicker following a hard run?  
3. When performing strength training or mixed modal training, forget about heart rate.  Even when paired with a chest strap, HR isn’t going to be a good measure of intensity.  Instead, get really good at using the Rating of Perceived Exertion (RPE) or Reps in Reserve (RIR) scales.  For very high intensity intervals, RPE and the talk test (the number of words you can say at a time) are going to be more accurate than heart rate.  If you’re a more experienced lifter, tracking your bar speed is likely the best use of tech you’ll find available right now as far as tracking and improving performance as well as monitoring load over time.  
4. If overall health, weight loss, or body composition change are your goals, do you need to throw out your $500 Apple Watch? No.  As mentioned earlier, one of the best ways to impact TDEE is to manipulate NEAT, and one really powerful way to do so is to use your watch to encourage daily movement.  Track your steps and increase your daily step count slowly over the course of weeks and months. Set an alarm on your watch to encourage you to stand or move regularly throughout the day.  If you’re considering your first smart watch, I’d suggest buying the cheapest watch that tracks your steps because all of the major brands do so with pretty good accuracy.  Granted, if you’re more experienced with distance running or cycling and want to utilize your watch’s ability to map, track cadence, etc., those factors are all important as well.  

Sources:

1. H J Kalkwarf, J D Haas, A Z Belko, R C Roach, D A Roe, Accuracy of heart-rate monitoring and activity diaries for estimating energy expenditure, The American Journal of Clinical Nutrition, Volume 49, Issue 1, January 1989, Pages 37–43, https://doi.org/10.1093/ajcn/49.1.37
2. https://health.clevelandclinic.org/your-fitness-tracker-isnt-the-best-way-to-measure-heart-rate/
3. Nelson BW, Allen NB. Accuracy of Consumer Wearable Heart Rate Measurement During an Ecologically Valid 24-Hour Period: Intraindividual Validation Study. JMIR Mhealth Uhealth. 2019;7(3):e10828. Published 2019 Mar 11. doi:10.2196/10828
4. Wang R, Blackburn G, Desai M, et al. Accuracy of Wrist-Worn Heart Rate Monitors. JAMA Cardiol. 2017;2(1):104–106. doi:10.1001/jamacardio.2016.3340
5. Wasserfurth P, Palmowski J, Hahn A, Krüger K. Reasons for and Consequences of Low Energy Availability in Female and Male Athletes: Social Environment, Adaptations, and Prevention. Sports Med Open. 2020;6(1):44. Published 2020 Sep 10. doi:10.1186/s40798-020-00275-6