We often hear people say, “I have a slow metabolism“… or “my resting metabolism is low so it is easier for me to gain weight”.
It has also become popular for people to have their resting metabolism (also known as basal metabolism) test and then use this as a focal point for your diet.
This has prompted us to get the question of “How do you guys measure resting metabolism for your clients and is it super accurate?”
This article is going to answer the following questions: 1) how is metabolism measured, and 2) does it really even matter?
To answer these questions, let’s start with a review of what your metabolism is.
Your “metabolism” is made up of a few major components:
- Your resting metabolism: this is the energy required to keep your body functioning
- Your thermic effect of food: this is the energy required to breakdown and use the food you consume
- Your structured exercise: this is the additional energy that you burn from exercise.
- Non-exercise activity: this is the energy that you burn moving around in a day that is unrelated to structured exercise.
Here is how much these things contribute to your overall metabolism.
Resting Metabolism: 50-60%Thermic Effect of Food: ~10%Exercise: ~5-15%Non-Exercise Activity: ~20%This means that the two most important things controlling your metabolism are your resting metabolic rate and your non-exercise activity.On the surface, we would think that our resting metabolism would be really important for weight loss… well, let’s dive into that.When you look at the science around weight loss, the last ~50 years or so has taught us something really interesting. While your Resting metabolic rate makes up the biggest part of your metabolism it has no real bearing on whether people gain or lose weight.Let me repeat that again… your resting metabolism has almost no predictive power on whether you gain or lose weight. So if your resting metabolism is a little lower than it should be you are no more likely to gain weight than someone who has a resting metabolism a little higher than it should be.That is pretty wild isn’t it?So what part of your metabolism does predict weight loss or weight gain? It is your non-exercise activity thermogenesis. So the moving around you do, outside of physical activity.So now that we know that your resting metabolic rate isn’t super important, let’s talk a little bit about it, how you can measure it and whether a rough prediction equation is enough to “get you started”.
There are really two main ways to “measure” resting metabolism… and then there are several ways to “estimate” it.The first and most common way to measure it is what we call indirect calorimetry. Essentially, you lay down on a table for a while (~ 20 minutes to an hour depending on the protocol) and then someone traps all the air that comes out of you. This air is then chemically analyzed and they calculate how many calories you burned based on some fancy equations.The second way is to use doubly labeled water (isotopically traced water) where they can measure how this water flows through the body and use some fancy math to calculate your resting metabolism.
So both ways of measuring it are based on equations of surrogate markers (not unlike body fat measurements).Then there are the equations that help us estimate your resting metabolism. There are several out there, with some slightly better than others, but in reality, they all have an error rate of ~0.5-15% of the measured values, with most being ~5% off.So this means that if you use our macro calculator, for 90+% of people, the estimated resting metabolic rate our calculator gives you is ~100 calories per day of your resting metabolic rate. with an average “error” of probably within 50 caloriesNow let’s put that into context… let’s say you want to lose weight and the calculator gives you ~1650 calories to lose weight on and this should be a 500 calorie a day deficit. Let’s say for argument sake that your resting metabolic rate estimate is as off as it can be… so you are really running a 400 calorie a day deficit. So instead of losing 10 pounds in 10 weeks you lose maybe 9-9.5 pounds in 10 weeks. Now, you could do a few things to make sure you are more accurate… you could go spend a lot of time and money getting an RMR test and maybe make a 50-100 calorie adjustment, or you could track for 3-4 weeks, see if you are on track and then make a minor adjustment.Also, we know that your resting metabolic rate is not super predictive of weight loss or weight gain… but your non-exercise activity is. And we know that the variance between people with their non-exercise activity is WAYYYYYY higher than the difference in resting metabolism between two similar sized people. Like we are talking ~1000 calories per day difference based on size. So, what is more important in being spot on precise with? Probably your non-exercise activity.
This is why as coaches we don’t get super worked up if a resting metabolic rate estimate is slightly off, why we don’t recommend or require clients to go get their resting metabolic rate measured, and why we tell people to track their steps.