Resting Metabolic Rate Test Result

I am posting about my results from my Dexafit visit today. I previously posted about what I was having done and why.  This post is about the results of my resting metabolic rate testing.  Other procedures:

Body Composition through dexa (DXA) scan which you can find here.

I also had a Fit3D scan done which is here.

In the post I did on my body composition and body fat percentage I said that I was surprised about the result.  However, I indicated that it was not the most surprising result from the testing.  The most surprising result was that which showed my resting metabolic rate.  I was actually gobsmacked by the result.

But, before I get to that:

Why Do a Resting Metabolic Rate Test

Online you can find a lot of calculators that can tell you what your resting metabolic rate (RMR) is (or your basal metabolic rate – they are not identical but are treated as interchangeable for this post).  These calculators are useful since they are free and for most people are probably reasonably accurate.  The one I see used most often uses the Mifflin St Jeor equation.  And, that is the equation that Fitbit uses to estimate your resting metabolic rate which is the number that underlies all of the calorie burn information you get from Fitbit.  That is, how many calories you burn during exercise is based upon a factor applied to your Mifflin St Jeor RMR.

Another calculator that I like is the Katch McArdle calculator which uses weight and body fat percentage to determine RMR.  Most calculators, including Mifflin St Jeor, use age and height instead of body fat percentage since most people don’t know their body fat.  Age and height act as a proxy for body fat.

Calculators are useful but are not accurate for everyone.  The Mifflin St Jeor equation (or similar equations) don’t really deal well with the fact that some people burn less calories than expected because they have unusually high body fat.  And, they may not be that accurate for those with unusually low body fat.  Katch McArdle rectifies that problem, but it necessarily requires an accurate body fat percentage.

And, there are unknown factors that impact RMR other than the factors used by the calculators.  This study sought to determine what factors influence the variation in basal metabolic rate.  Basically they found 63% of the difference in BMR between subjects was due to differences in fat free mass, 6% was due to differences in fat mass, and 2 % was due to differences in age.  For women, 26% of the variance in BMR was unexplained.  That is, it wasn’t explained by the factors that are used in the typical RMR calculators.  They did look at some other possible factors (such as age, bone mineral content, and leptin) but those factors didn’t explain the difference.  Calculators can be useful, but I felt that actually testing my RMR would give me a more valid result given the individual variance that can exist for metabolism.

It is useful to get a valid RMR because that would help me to know how many calories I burn each day and would help me to make sure I have a calorie deficit.

The Test Itself

I had previously done an RMR test a couple of years ago.  This one was quite similar.  True RMR tests use indirect calorimetry. Basically you breathe into a face mask for a period of time and a machine measures the amount of oxygen your body uses and the amount of carbon dioxide your body produces and, from that, can determine your resting metabolic rate.

I did this test after doing the dexa scan.  Because I was doing this test, I had been told not to eat or drink anything after midnight other than water.  There are also restrictions on activity before hand.  Basically they didn’t want you to have eaten or exercised within a certain amount of time before the test because those activities increase your metabolism.

They had me sit in a recliner and breathe into a face mask for 18 minutes staying as still as possible.  It was actually very relaxing as the recliner was reclined back and was super comfortable.

This is how the mask looked:


The mask fit over both my nose and mouth.  It fit snugly and I had mask marks on my face for a couple of hours afterwards.  But, the mask itself was comfortable.  I was told that I could either breathe through my nose or my mouth, but that it was preferable to pick one and keep it consistent through the test.  I chose to breathe through my nose and stuck to that throughout the test.  The test was over way sooner than I thought it would be.  I was trying to stay very still so I just was there in the chair by myself during the test (my husband took the picture before leaving the room before the test started).  I was actually surprised when the tester came in and told me the test was done.

This was the same basic test as I had done somewhere else 2 years ago, but overall this was a way more pleasant process.  Here is a picture of when I had the test done the first time:

The big difference in that test and this one is that for that one I had to breathe through my mouth.  A nose clip was used to make sure I breathed only through my mouth.  My lips were around a piece of plastic that I breathed into.  I was sitting upright in an office chair.  I was warned that I would start drooling toward the end of the test, which I did.

So, yes, reclining in a very comfortable chair with a face mask and no nose clip was much more comfortable.  And, I was glad not to have to drool.  I did wonder after I left if breathing through the nose was as valid a test as breathing through the mouth.  I looked online and didn’t find anything that addressed the question.  I did see that there are a variety of different ways of doing the test and didn’t see anything which said that that the face mask I used was less valid than using the device I used before.  I assume that the face mask I used is just as valid.  It did seem to be a newer piece of equipment, so I assume it was as good or better than the above.

RMR Results

Back in 2015, when I was tested, my RMR was 1120 calories.  On the day of testing I weighed 147.8 pounds and was, of course, 2 years younger.  My Bod Pod body fat percentage on that day was 45.3%.  Today, I weighed within half a pound of that weight (when I got up before drinking a bottle of water on the way to the test).  My dexa scan says my body fat percentage is 45.4%.  Due to differences in how a Bod Pod and dexa scan determine body fat percentages, the two methods are not really comparable.  I talk about this in my prior post about my dexa scan results.

At the time, a Mifflin St Jeor calculator would have predicted my RMR (actually the calculator uses BMR but for these purposes that is close enough) at 1220 calories, exactly 100 calories a day more than the RMR test found.  At that time, Katch McArdle would have calculated my BMR at 1162.1.  That was still 40 calories a day more than my tested RMR.

But, remember, as I noted above, not all variation in RMR can be explained by variations in the factors that are in the Mifflin St Jeor or Katch McArdle calculators.  Having an RMR of 1120 meant my metabolism was a little slower than could be explained by body fat and weight alone.  And, that assumes my body fat percentage was actually 45.3%.  In reality, body fat test methods available to use don’t necessarily allow for the degree of precision.  There is a margin of error and it can make a big difference.  For example, if that day the Bod Pod had said my body fat was 42%, it would have given me a BMR of 1209.89 calories.  On the other hand, if my body fat percentage was 48.2%, my BMR would have been 1120.11, very close to my tested BMR of 1120.

There is also adaptive thermogenesis, which is where your body burns fewer calories than would be expected based upon your body weight, fat free mass, and fat mass.  There is some indications that this can occur after prolonged dieting.  And, some suggest that it can persist for years, or even permanently, after completing weight loss. I have discussed this before while talking about the study of past Bigger Loser contestants.  Suffice it to say, that the impact of adaptive thermogenesis and whether (and how long) it persists is a matter of controversy and the research does not seem to be dispositive on this subject.

In any event, when I did my RMR test in 2015, I had recently gotten to goal.  Before that I had been closely following Weight Watchers (aka restricting intake and calories) for almost 2 years.  And, I have had many other periods of dieting during the years before.  There was certainly room for possibility that my very low RMR – below that suggested by Katch McArdle – was due to adaptive thermogenesis based upon my previous calorie restriction over a long period of time.

Given everything, I was expecting to get RMR results today fairly close to that in 2015.  My weight was very similar.  I did not feel I had appreciably added to my lean body mass.  I was still overfat.  I was 2 years older.  I was expecting my RMR to be 1120 or possibly a little lower.

OK, so here is where I was utterly gobsmacked.

Yes, you are reading it correctly.  My measured RMR today was 1211.  A full 91 calories a day more than 2 years ago.  I am still trying to come to grips with it.  Can it possibly be right?

Bear in mind, that for this calculation it doesn’t matter what body fat percentage I got on the dexa scan versus the Bod Pod.  That doesn’t mean that my lean body mass or my fat mass don’t matter to my RMR.  They do matter.  But, the test is actually measuring my RMR so you don’t have to know my lean body mass or fat mass numbers to get the result from the test.   My RMR depends in part on my lean body mass and fat mass, but you don’t have to know their numbers in order to measure my RMR.

This result today is so different I’ve spent a good part of the day wondering if it could possibly be true.  Of course, I really want it to be true because it means I am burning more calories than I thought was. That doesn’t mean I should suddenly start eating more food.  But, I must admit I was worried about what would happen if I lost another 20 pounds or so.  Would my RMR be so low that I couldn’t maintain if I ate more than 1200 calories a day?

So, hmm.  I do know that there are things that can make an RMR test result less valid.  However, those things deal with doing things before the test that increase RMR.  But, I didn’t do any of those things.  I intentionally didn’t even exercise on Saturday or Sunday because I didn’t want exercise to interfere with the calculation.  I didn’t have caffeine or eat after about 11:00 PM Sunday night.  I had water on the way to the test which was allowed. I didn’t take any medications before hand.  I did walk into the building after an hour long car ride (I was a passenger).  But, I did the dexa scan before the RMR test so I had plenty of time to get rested before the test.  I mean I abstained from all of that for longer on this test than I did when I was tested in 2015.

There is a possibility that, nonetheless, one of the tests was not correct. Awhile ago I compared my actual weight loss/gain in 2016 and this year versus what would be predicted by Fitbit with my having tricked Fitbit into using an RMR of 1120 instead of the 1220 Fitbit would have used (I did this by making myself shorter in Fitbit).  And, it was pretty close.  My actual change over that period (adjusting for the 6.2 pounds lost through surgery) was a gain of 1.8 pounds.  Using my Fitbit calorie burn information, the predicted gain was 2.3 pounds.  So, I did a bit better than Fitbit predicted which might suggest that at some point anyway, my RMR was higher than what I was using in Fitbit.  And, in fact, my predicted weight gain/loss based upon the Fitbit calorie burn was more off in 2017 than in 2016. That would suggest that my metabolism had increased since September, 2015.

This suggests one of two possibilities.  It is possible that, for whatever reason, the test in 2015 understated my RMR.  RMR tests are considered very accurate.  That said, I did find at least one study which compared various testing devices and found varying reliability among them.  So, it is possible that the device being used at my 2015 test was less accurate than the one used on me today.

Another possibility is that my RMR has changed for the better in the last 2 years.  That article I linked to above basically links over 70% of the variation in RMR to things that I don’t think have appreciably changed in the last 2 years.  That is, my weight, fat free mass and fat mass haven’t changed a lot in that time (at least I don’t think they have although it is difficult to know exactly given the fact I was tested with a Bod Pod then and with a dexa scan today).

But, something that could have possibly changed is my adaptive thermogenesis.  During most of the last 2 years I was not really restricting calories as I had been while losing weight.  I did restrict part of the time, but I deliberately maintained during a good part of these 2 years.  During 2016, for example, I averaged about 184 calories a day more than I averaged during 2015.  And, it was basically the same this year through July (I have been eating less since then as I am now actively trying to lose more weight).  It is perhaps possible that my metabolism was slowed at the time of the first RMR due to my dieting efforts over the 2 years before the test, but that the result then was a temporary slowing that has since rebounded.

Of course, the possibility exists that despite my care to follow all instructions today that this test was simply wrong and my RMR is not really 1211.  Under the Katch McArdle formula, I would have an RMR of about 1211 if my body fat percentage was about 42%.  Of course, I tested today at 45.4%.  But, the dexa scan usually gives a higher body fat percentage than other methods.  I don’t know what method of body fat calculation was used in validating Katch McArdle.  In other words, I don’t know if a dexa scan body fat percentage gives a valid Katch McArdle result.

It is interesting that if I plug into the Mifflin St Jeor calculator my height as 5’4″ and put in my weight as measured at home this morning it comes out to exactly 1211 calories which is what I tested at.  The predicted RMR of 1191 on my test summary sheet is based upon the Mifflin St Jeor calculation using 5’3″ and a weight of 147 pounds.  I had indicated on the form at Dexafit that my height was 5′ 3 1/2″.  I used to be a quarter inch taller, but I lost a little height somewhere.  Apparently Dexafit’s software rounded down to 5’3″.  And, the software used my reported weight not that measured at Dexafit.  I apparently wrote down 147 instead of 148 which is what it was at home this morning.

And, here is the irony in all this.  I had changed my height on Fitbit to 4’10” so I could have an RMR there of 1120 to match my RMR test 2 years ago.  Today, I simply changed my height back to 5’4″ as that will give me an RMR of 1211 (of course, that will change a bit each day as my weight fluctuates).

So, Which RMR is Correct?

Which RMR test is correct?  Beats me. At the moment, I think I need to go with the one that is most recent.  RMR is governed by some known factors but that study I mentioned above couldn’t find what factors explained about a quarter of the variation between people.  So, maybe I just can’t explain the difference between the test result then and the one now.  But, the current number seems plausible.  And, the fact that I have been doing better this year in terms of weight loss/gain than would be predicted by the “old” RMR test suggests to me that maybe at some point my metabolism has gotten faster than it was 2 years ago.

The tester said that usually people wait a year or so before retesting RMR (if they plan to test again) unless there is a big change in weight or body composition.  I think that sounds reasonable.  I do want to lose 20 pounds of fat to improve my body composition.  If I do that, then I would undoubtedly get a new RMR test.  But, until that happens, I will just monitor how my actual loss compares to what would be predicted using Fitbit calorie burn which is based upon an RMR of 1211.

Leave a Reply

Your email address will not be published. Required fields are marked *

CommentLuv badge