Caloric Deficits and Basal Metabolic Rate (BMR) Test Accuracy
There is a common concept in weight management that “calories are calories”. Beyond getting core nutrients, this is true. But that is for the calories in portions of things and is only one side of the calories in calories out equation. When it comes to burning calories everyone is different. So does your smartphone, Apple Watch, or Fitbit estimate calories correctly?
The answer is, it depends. Everyone is different and estimates are averages. One of the more impactful examples of this is how much energy you burn at rest. The body uses energy to live, and the baseline rate of energy it uses is called Baseline Metabolic Rate, Basal Metabolic rate, or Resting Metabolic rate. Fitness trackers like Apple Watch, which uses Resting Energy, and MyFitnessPal estimate this rate using height and weight to make a guess.
So, what is a normal resting energy in calories burned. Taking a lab test, where you breath into a tube for some period of time to calculate how much CO2 you are expelling can provide this basic metabolic rate. When I took one I was shocked to learn that my baseline was more than 500 calories lower than predicted. For clarity RMR (Resting Metabolic Rate) and BMR (Basal Metabolic Rate) are effectively the same thing and used as synonyms in most places.
Some of this baseline deficit can be masked by exercise, and thankfully I was running 5-6 hours per week. But overall a daily fluctuation of 500 calories is killer when most people aim for a caloric deficit of 300-500 calorie. 500 calories a day equates to 3500 a week which is the rule of thumb of 1 pound of body weight.
Getting this measurement helped to show why standard online food trackers would lead me to sub optimal results- even with full food tracking I would likely be at maintenance calories even when showing a substantial deficit. This becomes an even bigger issue when you realize that some popular diet trackers include crowd sourced food estimates.
MyFitnessPal has questionable calorie estimates because of this, and coupled with the results of the test revealed a real challenge to tracking and then seeing the expected results. If you have similar fluctuations from what a Watch estimates it can be a big deal.
Accuracy of Resting Energy for Apple Watch
A quick scan of the above shows how tough it can be when estimates are off. So how accurate are different measurements. Keep in mind this is highly personal, but using three different options resting calories estimated were vastly different.
Maintenance calories are supposed to be what you eat to maintain weight. This should be a break even point that includes a BMR + Minimal energy used. Below are a few measurements and calculator outputs, the Italic version are just basal rate and include no activity estimates.
- Dexafit Lab Test – 1326 calories
- Dexafit Estimate – 1853 calories
- MyFitnessPal – 1850 calories
- Apple Watch – 2320 calories
- Mayo Clinic Calculator – 2750 calories
- TDEE Calculator – 2857 calories
- Healthline Calorie Accuracy – 2857 calories (uses same equation as TDEE)
So which one is it? The short answer is they are all somewhat useful, but none of them are spot on. The best thing to do is try out a level you think works and monitor weight daily to see which one matches with what you see on the scale.
Apple Active Energy Vs. Measured RMR
It is worth noting that Apple does not reveal exactly their methodology of predicting BMR, but the Apple forums suggest it is based on a fairly common equation, similar to what the lab test used for Predicted RMR. Still, they then further make things confusing by reporting a Resting Activity which is not exactly a RMR or BMR as measured. The difference here is that RMR should be really that, at rest, compared to just minimally active. Apple takes into account some movement, but it is unclear what constitues Active Energy vs. Resting Energy.
When it comes to the that means that the basal metabolic rate of apple watch was off by nearly 1000 calories, enough to totally kill a weight loss plan. One thing that this type of testing an inaccuracy reveals is that everyone is unique and since the margin for error on a diet can be sometimes as small as 200 calories, the need for some understanding is critical. The best approach is to not just measure one day, but to look at longer term trends and see if they make sense. If over the course of a month sometime does not add up, it may be time to explore further on some of the assumptions and estimates made.
CICO – Calories In Calories Out
There is a simple equation that needs to be solved to gain or loose weight. (Calories In) – (Calories Out) = (Eventual Weight Difference). This is sometimes referred to by the acronym CICO. The outcome is noted as “eventual” since there are many day to day changes that can alter this, which is sometimes just referred to as water weight. It’s true that you can gain weight by just drinking water or eating a heavy amount of a low or no calorie food, but in two to three days your body will sweat or otherwise dispose of this with minimal effort.
The calories out portion of the equation is controlled by two things, exercise and time. Sure you can burn some calories moving or working out, but a huge amount of the weight or calorie you burn is just living. Even lying in bed all day will burn calories, and the amount is called your basal metabolic rate (sometimes “base” metabolic rate). This can vary from 500-2000+ calories depending on your size. A 220lb person with a lot of muscle (muscles burn more calorie even just resting) can burn 2200 calorie sleeping all day, while someone who is 120lbs might only burn 1300. There are calculators available to determine your BMR.
Another option to determine your BMR is to take a lab test as shown above and is helpful since online calculations are averages. If you have a condition that changes your metabolism (like hypothyroidism, which can drop your BMR by 10-20%, enough to elongate any weight loss from a small 200 calorie estimated deficit), then finding your actual BMR can help with maintaining a proper diet. Similarly if you have significant muscle or a general build that is not average for the calculations the. A test will help.
Once you know how many calories you burn as a baseline just sitting around, you need to add in how many calorie you burn. Just walking to the bathroom, getting dressed, or picking up your phone can burn energy with a strenuous job that has you on your feet this can be 500-800 calories a day. A step tracker, like the Apple Watch or Fitbit, can help estimate how much energy this is. Going to the gym, hiking, swimming, or jumping rope (especially with a fun jump rope tracker this can be one of the best calorie burning activities) will amp this calories burned number up.
Once you have both numbers, add them up for your calories out. Now if you eat less than this by 3500 calories you will loose a pound! Most people suggest spreading this average over a week so you have a 500 calorie average daily deficit (7 days x 500 calories a day = 3500 calories). If this zaps energy levels then eating a little more and having a 100 calorie daily deficit in your CICO calculation will have you losing a pound a month (or 35 days).