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.

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

  1. Great article series, thanks for publishing it and attempting to answer a lot of the misconceptions surrounding BF% assessment methods.

    I’m curious, have you looked at ultrasound analysis such as Body Metrix: http://intelametrix.com/? I’d like to see how it stacks up to the air displacement and DEXA results I’ve received.

    Thanks again.

      

  2. And what about CT and other 3D volume scans? If we ignore the cost of a full body pretty-high-res scan, would that be able to differentiate fat from other tissue, and thus be the gold standard?

      

  3. Can I have the price in US dollar for this DEXA

      

  4. Like Kyle, I am interested in a comparison of ultrasound technology (like Intelametrix’s Body Metrix) with other body comp methods.

    Thanks.

      

  5. Hello,

    I have performed research on the Body Metrix device and compared into to the BodPod and Tanita BIA Machine. They were highly correlated. To my knowledge there is no articles comparing ultrasound to dexa yet. My thesis is a cross validation of the BodPod, Body Metrix Ultrasound, DEXA, Skinfolds, BioSpace BIA Machine, and Anthropometrics. My paper is currently pending publication for the first research via the Body Metrix, Bodpod and Bia. Let me know if you have any questions. Thanks, Kelly

      

  6. A DEXA costs about $90k

      

  7. I wish I read this before getting a DEXA scan for body composition. I’ve always done hydrostatic testing and I hover around 14% BF. I read that the DEXA scan was more accurate than the dunk tank so I thought I’d give it a go. I did the DEXA test right after the holidays and figured with a little holiday padding I’d probably be more like 16% right now, but the scan results suggested I am 24% body fat with 21 lbs less lean mass than was reported after my last hydrostatic test. I was planning to do a dunk test on the same day so I could compare, but the facility was not available so I will do that before long. My coach says there is no way I could be 24% BF, and there is no way I could have lost 21 lbs of lean mass.

      

  8. What are the hydration instructions prior to the DXA body composition test?

      

  9. What are the hydration requirements (fasting or not drinking water for how long prior to the scan)?

    Thank you.

      

  10. Hydration can effect it but if you are overhydrated it can ovee
    er estimate %bf. Also remember generally the percent fat I have seen in my study thus far is is it overestimating but remember it takes into account anadroid and gynoid fat which could result in a higher percentage.

      

  11. From what I can tell, you can cheat the Dexa Scan Process by consuming mass amounts of water just prior to testing. Your weight may go up, but water is lean, so your percentage of lean mass will increase, lowering your body fat% No?

      

    • Hi Gary,
      Are you absolutely sure about your comment that consuming heavy amounts of water just before the DEXA will results in body fat percentage reduction?

      We are running a body fat reduction competition and don’t want people to cheat.

      Because there is a different view from Kelly who says it will overestimate????

        

      • Currently, conflicting opinions exist as to whether the DEXA is considered the “gold standard” in body composition assessment. Initially, DEXA was created to estimate bone mineral density, which it does accurately. It is still unclear whether the DEXA can account for differences in FFM due to hydration status (Rougbenoff, Kehayias, Dawson- Hughes, & Heymsfield, 1993). The DEXA assumes that hydration of FFM remains constant at 73%. If an individual is hyperhydrated, the DEXA may overestimate FM which could result in a larger %BF (Kelly, Berger, & Richardson, 1998; Pietrobelli, et al., 1999). These results suggest that in order to get accurate estimates of %BF, individuals must continue their regular patterns of hydration or results may be affected. Additionally, all of these methods are subjective to error because none of the proposed methods have been directly measured against cadavers (Brunton, Bayley, & Atkinson, 1993; Toombs, Ducher, Shepherd, & De Souza, 2012).

          

  12. I have been training ( bodybuilding ) for a couple of years and decided to have a dexa scan to determine my body fat %. My first result gave me a reading of 8.7% with total 6550g fat mass. After 4 weeks of very intense dieting I decided to have another dexa scan. This time I went to a different clinic with a different machine. My result was 4.2% with total 3090g fat mass. The result somewhat surprised me as I wasn’t expecting it to be that low. Visually, the change in my physique has been very noticeable.

    Is it possible to drop that much body fat over 4 weeks ? Is anyone sceptical on the accuracy of the Dexa scan? Do results vary between machines?

      

  13. I validated the Body Metrix Ultrasound machine (US) while comparing it to the DEXA and Bodpod. I measured Fat Mass Fat Free Mass and Percent Body Fat. My results where as followed. There was a significant difference in between all machines when measuring percent body fat, no significant differences between fat free mass between machines and there was a significant different in fat mass between the DEXA to US, and DEXA to Bodpod for Fat Mass. When the Ultrasound was compared to the Bodpod it seemed to be reliable, in fact with in 1-2 percent of the Bodpod, however when it came to comparing it to the DEXA which is considered the Gold Standard, when measuring percent body fat it varied by alot. This has to do with the assumptions made for a 2 compartment method i.e. the Bodpod and Ultrasound compared to a three compartment method such as the DEXA. I only used 85 healthy college-aged subjects, the Ultrasound machine has no been validated in other population, so whether it accurate in other populations is unknown. It has been shown to be accurate when measuring %BF, Fat Mass, and Fat Free mass, when compared to the Bodpod however, if it is accurate when looking at changes over time in different populations in unknown. This research is vitally important especially when a lot of the body composition is very expensive where this new ultrasound is inexpensive or relatively cheaper when compared to other known and validated methods to estimate body composition.

    Thanks, Kelly

      

  14. I did a Bod Pod and a DEXA scan within a month of each other and got two completely different results. Bod Pod says I’m at 25%, and the DEXA scan says I’m at 42%. DEXA scan is completely BS measuring body fat.

      

    • Sounds like your experience is in line with some of the individuals in the studies where DEXA results were way off.

        

    • A DXA Scan is absolutely not bullshit!!!!!!!! You must have gone to a facility that did not calibrate the machine. There is a company out of Chicago and Atlanta that uses it correctly. Try them you will not be let down.

        

  15. I had a dexa scan show 15.5% and a water test show 29% within two days of each other. The scan results better matches up to my small size and high level of muscle definition. Not sure what to think? I work out 2 hours a day and am very lean so the water results that put me at an ‘average’ level seemed weird.

      

  16. Here are the results of my Thesis.

    INTRODUCTION: Obesity has become a worldwide epidemic and is a major contributor to both cardiovascular disease and Type II diabetes. In result, there has been an increase in new technology being introduced into the field of body composition assessment. Recently, a new ultrasound (US) device was created to estimate body composition. However, to date, only one study exists comparing this device to other established and validated body composition assessment methods. PURPOSE: The purpose of this study was to investigate the validity of an Ultrasound System (US), and compare it to Dual Energy X-Ray Absorptiometry (DEXA) and Air Displacement Plethysmography (ADP) in estimating Percent Body Fat (%BF), Fat Free Mass (FFM), and Fat Mass (FM) in college-aged students. METHODS: Eighty-four participants volunteered for this study. All participants were instructed to adhere to the following pretest protocol: 1) No food or drink 2 hours prior 2) Avoid strenuous exercise 24 hours prior to the test 3) Wear tight fitting clothing (i.e. lycra or spandex material). Body composition was assessed using the US, DEXA, and ADP in which all measurements obtained were performed in accordance with manufacture’s guidelines. One-way repeated measures analysis of variance with Bonferroni adjustments to compare differences in %BF, FFM and FM between DEXA to ADP, DEXA to US, and US to ADP. The significance level for all comparisons was set to p < 0.05. RESULTS: The DEXA was greater (25.5 ± 8.5%) vs US (21.6 ± 6.29%) and ADP (19.4 ± 7.5%), p < 0.001, when measuring %BF and the US was greater (21.7 ± 6.6%) vs ADP (19.9 ± 7.9%), p < 0.03, when measuring %BF. The DEXA was greater (37.35 ± 14.97 pounds) vs US (39.38 ± 11.9 pounds) and ADP (30.58 ± 16.18 pounds), p < .000 when measuring FM. However, there was no significant difference in FM between US vs ADP.CONCLUSION: This study demonstrated significant differences between the DEXA, US and ADP when estimating %BF, and FM in college-aged students. This may be due to the measurement methodology, between machines. Therefore, further investigation is warranted.

      

  17. Kelly,

    Did you perform a Bland-Altman to measure the agreement between US and ADP with the gold standard of DEXA. This will compare the differences between each measurement for each person. Since you do not know the true value of the variables you are measuring you can determine the agreement between the devices. This provides more information than a repeated measures anova.

      

  18. Tyler,

    Yes, I did but didn’t include it in the abstract. Thanks, Kelly

      

  19. Hi,
    I have been having alot of problems with the DEXA scan. As an athlete I am forced to use it often to measure body fat and muscle mass. And many times I have been given negative results to my body fat or muscle mass changes. Just about every time I have taken it, it has been at the strongest point of my body and it may say I am down in 7 pounds of muscle and up 3 pounds of fat. Alot of times I say how can this be when I am stronger in the weight room than ever, and also faster and in my best shape. It can be sometimes discouraging when I am constantly working hard to only improve. Over time i have started to believe a lot of the information it gives me cannot be accurate. I really don’t know what to do about it, since it is one of the uses to measure success. Do you have any advice?

    Thanks alot for your time,

    Hilton

      

  20. I am doing a secondary data analysis from a large parent study with renal transplant patients. I cannot find any information that quantifies abdominal fat by DEXA scan.If I use a ratio of trunk fat to the sum of left & right leg fat, would that be accurate?
    I know the total body fat percentages that are considered overweight and obese, but I don’t know what amount of trunk fat is considered abdominal obesity. If a person has total fat of 28% and trunk fat of 6779.6g what calculation would I use to determine abdominal v lower body adiposity? ( this dexa shows right leg 3210.3 g of fat and left leg 3332.2g of fat)
    If you have any resources that you can point me to, I would greatly appreciate it.
    Cathy

      

    • Hi, Cathy,

      I havent’ actually performed a DEXA scan that looks specifically at abdominal fat so I would not be able to answer your question. Sorry I can’t be of more help.

      James

        

  21. 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 fat content and ratios.

      

    • “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.

        

  22. 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. Since my abdominals are flat, I can’t see how there’s room for all that plus my internal organs. Then my bone density is 2.008 g/cm2 which is almost off the chart for any age.

    Could there be an issue with my body position during the scan or possibly operator error in conducting the scan?

      

    • 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

        

  23. 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.

      

  24. 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.

      

  25. 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 exercise. Only about 1 1/4 liters of fat were removed but as a result, the trauma to the tissue may have caused water retention that has also increased my weight by 5-7 pounds overnight. Could water retention in the tissue and fat in these areas result in an overstated BF%?

      

  26. Grade A stuff. I’m unquestionably in your debt.

      

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