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.
Understandably, there is no perfect machine. However, if you take two test within a short time period and the results are relatively close you should know your rough stats with the Dexa Machine. I had a Dexa Scan twenty days apart recently. I lifted really heavy almost everyday in between. According to the scan I gained 1.8lbs in lean mass. I lost about 2.5lbs overall. About 30 days later (which is today), I’m about 7lbs lighter and a bit stronger, so i want to go back soon. My Z-Score bounced from 2.4 to 2.7 in 20 days (which is a… Read more »
Guys, I don’t see the problem.
Dexa gets two things absolutely correct: absolute body fat weight and skeleton weight. This is all I would want to know, because I just want to know how much fat I can realistically lose from point A to point B without going, say, under 8% of my total weight for my total fat weight.
Your ‘LBM’ will change always with hydration and stool. This would scew the percentage of body fat but that really does not bother me so much: the absolute fat weight can be utilized.
Just for the record Ralph DEXA does not get body fat mass absolutely correct. It measures bone mineral content very accurately because that is what they were designed to do. Then it estimates lean body mass based on the volume of lean mass it scans. Then finally it estimates body fat mass by subtracting the bone mineral content and lean body mass from your total weight. It does not specifically measure fat. None of the tools do. Hence the fluctuations between methods because of variations in bone density and muscle hydration levels.
I work in radiology, and I’ve done dexa scans with brand new 2014 machines, and they are nearly spot on. It depends on the body fat location for the machine. It will go by percentages of each body part and make a total assessment. It is about 1%-2% off usually for water weight changes but there is nothing you can really do about that.
Okay I’m convinced. Let’s put it to action.
Hi Kelly, I will participate in a research that includes body composition (ow and obesity) in children (mostly 0-36 mo), but also for other studies we will include older children (comparisons between groups but also longitudinal measurements since it would be a cohort study). We would like to buy the best equipment for this purpose but we are still not sure if the DEXA or BodPod. I thought BodPod was not that good for young infants and DEXA not for changes in time. So, what would you recommend? and please if you have some references from your papers I would… Read more »
Hello Carolina, If your using 0-36 months. I might recommend a Peapod from Cosmed if your using children 0-36 months. The issue with the Bodpod for children is stature. However, the do have a booster seat for the Bodpod now. See links below. As far as DXA being used. I know it is not good for tracking changes over time. However, this will only be good if the software that is used is for pediatrics and changes overtime to account for differences as children age would be best. In my professional opinion, I feel that deuterium oxide dilution (D20) may… Read more »
DEXA is ideal for changes over time but not so much so for repeated testing i.e. daily.
DEXA is a medical device used in hospitals including on children. You just need to make sure the normative data and correct software is on the machine
My recent scan indicated my body fat to be 24% which seems very high considering I work out daily, intense strength training twice a week with very heavy weights. The scan did indicate less than 1 lb of visceral fat so all the fat is subcutaneous and it seemed to be spread all over my body fairly evenly. My diet is extremely clean and consistent. The one thing I thought could throw the results off would be a smart lipo procedure I had abut 3-4 months prior on a couple of trouble spots I could not address with diet or… Read more »
No, the only thing water retention could do is UNDER estimate body fat.
24% body fat isn’t that bad for a female
It is not the amount of sport you do that determines your weight/fat, but the daily netto calories you eat.
I agree with Ralph. 24% is considered “fit” for a female, anything under 32% is healthy (unless it is too low i.e. less than 13%).
You are probably thinking about subcutaneous fat, you need to account that this machine measures everything, your brain is about 20% fat, fat is marbled through out muscles, fat is in our abdominal cavity, in our organs and females have breasts, mainly fat tissue
Is the iDXA (from GE) more accurate for body composition than the DEXA? I understand it has a “Core Scan” function that quantifies visceral abdominal fat.
Can you provide a.link to the article? I saw something that made a vague statement about tall individuals but nothing quantitative. Also what about lying on ones side instead of supine? Thanks.
I would like to ask if body position during a DEXA scan can affect results. I am very tall, 195cm, and lean 56 year old with almost zero subcutaneous fat. I am very active working out 3 or more days/week. I recently had a DEXA examine and because of my height, I was instructed to lie in the fetal position rather than supine. My results indicated 16,732 grams of fat (37 pounds) on my 195 pound frame. That is the equivalent of over 4.5 gallon jugs of fat the vast majority of which would have to be in my gut.… Read more »
Tom,
See if you can get this article. It should answer your question. The issue is there is not set guidelines for the DEXA! Techniques for Undertaking Dual-Energy X-Ray
Absorptiometry Whole-Body Scans to Estimate Body
Composition in Tall and/or Broad Subjects
My DEXA tech said for very tall individuals, they will either skip the head, or the feet.
Skipping the feet eliminates less mass from the test.
If testing more than once, he will test the feet on a 2nd or 3rd test to get the data for the feet, after the head data has already been inputted into the system.
testing in a fetal position is crazy. It would throw off all the testing as the person is supposed to be testing lying down on their back. doing anything else would throw off the diagnostic software.
Hmmmm . . . conclusions drawn from old studies without referencing any data gathered from the deployment of new DXA technology begs one to consider the possibility that the article is, aside from being inaccurate (ok, ok, maybe accuarate 2 decades ago), slanted – why? Here is the honest truth regarding precision and accuracy: Precision BMC/20g, FAT/100g, LEAN MASS/200g, TOTAL MASS/200g Accuracy Actual mass vs CT SCAN statistically insignificant CT SCAN vs DXA .8% difference DXA vs other Hydrostatic weighing is 4x less accurate BTW: new, FDA approved software from top DXA equipment suppliers measure and report android and gynoid… Read more »
“Old” studies? Some of the studies referenced are only 5 years old or so.
If you are going to comment on my blog, then I expect you to provide scientific evidence to support your comment, complete with scientific references. What you have provided is not “honest truth”…you have provided random data, with no reference where it came from. It is also apparent that you do not understand that CT scans are no more a gold standard for determining body comp than DEXA is. So your CT scan/DEXA comparison is not valid.