The Pitfalls of Body Fat “Measurement”, Part 6: Dual-Energy X-Ray Absorptiometry (DEXA)

In the first five parts of this series, I discussed various 2-compartment models of estimating body composition.  These models divide the body into fat and fat-free mass.  Now let's talk about a common 3-compartment model:  dual-energy x-ray absorptiometry (DEXA).

DEXA

DEXA, once used for determining bone density, evolved into a technique for also estimating body composition.  DEXA represents a 3-compartment model for estimating body composition, because it can divide the body into 3 compartments:  fat, bone mineral, and all other fat-free mass that does not include bone.  Thus, unlike 2-compartment models, DEXA is not subject to errors caused by variations in bone density among different ethnicities.  However, there are other sources of error which I will discuss in this article.

DEXA offers a number of advantages.  Improved technology has dramatically decreased scan times (years ago, a DEXA scan would take 20-25 minutes; now scans can take 5-10 minutes), so the method is convenient and non-invasive.  DEXA can provide bone density estimates, and can provide regional estimates of body composition (meaning it can estimate the body composition of individual parts of your body).  DEXA works by measuring your body's absorbance of x-rays at two different energies.  Fat, bone mineral, and fat-free soft tissue have different absorption properties.  Thus, we can get estimates of your body composition by scanning your entire body.

DEXA Sources of Error

Like all other body fat estimation techniques, DEXA has numerous sources of error.  There can be inconsistent results between different machines from different manufacturers, and even different results between machines from the same manufacturer.  Software upgrades can change the algorithms that the device uses to calculate body composition.  Different hardware and software configurations can result in different interpolations of soft tissue over bone, and different treatment of pixels of which a small portion is bone.  The type of X-ray beam (fan beam or pencil beam) can also be a source of error; DEXA machines with fan beams can suffer from beam magnification (also known as parallax error).  A final source of error is the same error that all 2-compartment models also have:  the hydration of fat-free mass.  In fact, a 5% variation in fat-free mass hydration can change your DEXA-determined body fat percentage by nearly 3%.  This can be a problem when comparing different ethnicities or body types where fat-free mass hydration can vary.  It can also be a problem when trying to measure change over time.

DEXA vs. 4-Compartment Models

A number of studies have compared DEXA to 4-compartment models.  When looking at group averages, DEXA does pretty well, with errors of 1-2%.  However, like with all other body fat testing techniques, individual error rates can be much higher.  The error rates vary by what study you look at and which DEXA machine was used; error rates range from 4% in one study to up to 8-10% in another study. Also, the accuracy of DEXA can be affected by sex, size, fatness, and disease state of subjects.

There are also issues when trying to use DEXA for tracking change over time.  In one study, researchers wrapped lard around the legs of subjects to simulate weight gain.  However, this changed the apparent bone mineral content, questioning the ability of DEXA to accurately assess body composition during weight change.   In another study on bodybuilders, DEXA was reasonably accurate when looking at the group change in body fat percentage, but the individual error rate was as high as 4%.  Thus, you could decrease your body fat percentage by 4% yet DEXA might show no change, or DEXA might show a 4% change when you didn't really have a change.  Another study showed DEXA to overestimate decreases in percent body fat, while it underestimated increases in percent body fat.  In fact, the overestimations got as high as 5%.  Finally, here's a chart from a study that compared changes in body fat over time using DEXA to using a 4-compartment model:

Change in % body fat, comparing DEXA to a 4-compartment model

The X-axis of this chart shows the change in % body fat for DEXA, while the Y-axis shows the change for the 4-compartment model.  You can see there was very little agreement between the methods.  For example, one person showed an increase in body fat percentage by 5% with the 4-compartment model, but a decrease by 5% according to DEXA.  Another person had nearly a 10% loss of body fat according to the 4-compartment model, but only a 3% decrease according to DEXA.  These results question the reliability of DEXA for measuring change over time in individuals.

DEXA:  The Verdict

Despite the fact that DEXA represents a 3-compartment model, its error rates are no better than hydrostatic weighing, and in some cases is worse.  Like other techniques, DEXA does well when looking at group averages, but not so well when looking at individuals.  Individual error rates tend to hover around 5%, although some studies have shown error rates as high as 10%.  When looking at change over time in individuals, error rates have hovered around 5% in some research, although other research has indicated DEXA to perform much more poorly.  For these reasons, I do not recommend DEXA for tracking change over time in individuals.  If you do use DEXA for tracking change over time, I recommend very long time periods between measurements (a minimum of 3-6 months), as you will need a minimum of a 5% change in body fat to reliably detect a true change in body fat in most people.

That sums it up for the most widely available techniques for determining body composition.  In the final part next week, I will discuss the practical application of everything you have learned in this series.

104
Leave a Reply

avatar
64 Comment threads
40 Thread replies
0 Followers
 
Most reacted comment
Hottest comment thread
49 Comment authors
MattA Quick Guide To Estimating Body-fat Percentage – RippedBody.comBarryanonStefan Recent comment authors
  Subscribe  
Notify of
Jez
Guest
Jez

I had a full body DEXA scan back in March and it came out with a BF% of 31.9 which is much higher than an electrical resistance test (~22%) had showed the week prior, but I took the DEXA to be the ‘gold standard’ in BF measurement. A few months down the line, after increasing training intensity, I went back in for a repeat of the DEXA and it came out at 25.2%. However, I was told that the algorithm that had been used on this particular machine was now known to be incorrect so not to take either measurement… Read more »

SM
Guest
SM

As part of a weight/fat loss challenge, I had my first Dexa done 5 weeks ago. Last week I decided to repeat the test to track my progress. The results showed that I gained 1 pound of muscle and lost 1.5% fat in 4 weeks, almost proportionally in the entire body. However, I also gained 2% of fat in the right arm. I was on 34.5% and am 33% right now. I can tell by touching myself that my arms are super strong. In fact, right before the second test I asked my coach to change the exercise routine for… Read more »

Archie Williams
Guest
Archie Williams

Having performed nearly 3000 DEXA scans, the error rates stated are a load of rubbish. I have never done one scan where the results didn’t make sense (i.e no change in body composition but 4% down on scan results). This just doesn’t happen-not even anywhere close to it. I would put money on doing a back to back scan and the results being within grams difference (the bf% wouldn’t change more than 0.1%) You can only test how ‘out’ a method is by knowing the exact true value in the first place so how is this found to know if… Read more »

Walking Walnut
Guest
Walking Walnut

Hello? Can we still consider th DXA scan so wildly inaccurate since the tests sited are from 1999,2003,2006,2007? Technology does improve over time.

Douglas Ravski Pereira
Guest
Douglas Ravski Pereira

+1 to above comment. Would love to see the last article here that ties it up with a recommendation

Kevin
Guest
Kevin

Where is the article that ties all this together. If all these methods are so unpredictable, should I even bother with body fat monitoring. It is nice to know the error ranges for these – especially BIA because most health clubs use it. I don’t see any value in it now.

So, what should I do? Just measure my waist circumference and weight myself? Look in the mirror?

Patrick
Guest
Patrick

waist circumference is the most relevant measure for BF. For men, BF accumulates at the belly. Reducing your belly size or waist measure, regardless of general fitness or muscle development, is the most reliable measure of your actual BF%

For a typical male 5’10”, a 32″ waist equates to a 18-20% BF
32.5″ might be 21-22%
34″ would be closer to 28-30%
36″ would be 35-37%

regardless of muscle development, as fat pools mainly in the gut.
muscle development is in the limbs and trunk.

Joe
Guest
Joe

Not true . You can be densely muscled and have overly developed obliques and have a thicker midsection . Doesn’t mean your bodyfat is that high lol that’s stupid . Calipers are most reliable method in predicting subcontaneous boydfat.

John
Guest
John

Hello. I had a DEXA recently and it said my body fat was 39% (I’m male). This is very strange because, although I do have a bit of a belly, my BMI is normal, and my belly width is less than half my height. According to the internet (I know…), 39% is far into the obese category, but I do not look like the people in the pictures to represent even 30%, never mind almost 40. Based on the pictures I’m around 25%. And, if body fat is often measured by pinchers – that’s only subcutaneous fat, isn’t it? If… Read more »

Patrick
Guest
Patrick

John. The dexa is correct within 1-2% of your actual body fat. If you “have a bit of a belly”, then you will be above 30% BF and more likely in the 35-40% range. The dexa of 39% is probably correct. one year ago, I was 5’9.5″, approx 190lbs, waist size 36.5″, and estimated BF% of 37%. two months ago, I was 187lbs, significantly more muscle, waist size down to 34″, DEXA scans at 26%, bodpod 28.9%. What I do agree with you, is the internet categories are way off in terms of descriptives. They were likely formulated 40-50 years… Read more »

Marshall
Guest
Marshall

Would you consider the InBody 770 to now be the Gold Standard in body composition?

Kelly
Guest
Kelly

Hi Marshall,

The 770 or 720? As long as it is a Multi Frequency BIA then they appear to be accurate. However, this means you must keep the guidelines consistent. These include not eating, drinking, exercising, four hours prior to testing. You also must stay normally hydrated.

John
Guest
John

The accuracy of the Inbody BIA is measured against DEXA….the gold standard

Stacie
Guest
Stacie

I just had a dexa scan body fat analysis done. It says that I have 33% body fat. I am a size 4 so how is that possible? I think I just wasted my money.

John
Guest
John

it’s quite common, your weight and size could actually be very low but you could have low level lean mass. You didn’t waste your money but you probably didn’t have your results explained correctly. Bare in mind women have breasts, mainly fat tissue

james
Guest
james

Individualization!!!!! I don’t quite understand body fat %. I a weight lifter, have been for 25 years. I’ve been told I have 25% body fat, that means I have almost 50 lbs of fat on me. I’m 5’5″ and weigh 193 lbs. You can see all the muscle definition in my chest arms legs and back. I do have some fat on the waste. I’m 40 yrs old can can still bench 365lbs. If I’m carrying 48 lbs of fat, where is it. At most I’m carrying 15-20 lbs of fat. That puts me close to 10%. I just don’t… Read more »

John
Guest
John

Fat isn’t limited to subcutaneous, you carry fat in the walls of all cells, it’s marbled through your muscles and it lies throughout your abdominal cavity. Your brain is also about 20% fat