DEXA Bone Density Scans are the gold standard for assessing the quality of the inside of bone, and determining whether a person has osteopenia or osteoporosis. More on those scans, soon.
A DEXA Body Composition Scan is designed for assessing the amount of body fat and the distribution of that fat, yet often come with a report that includes “bone mineral density” information. It is essential to understand that “bone mineral density” on a DEXA Body Composition Scan is estimated, not measured.
This article is about the DEXA Body Composition Scan and what information it reliably provides.
NOTE: (February 14, 2024) If you are thinking of having a DEXA Body Composition Scan, be sure the facility you choose is accredited by the College of Physicians and Surgeons in your area. More on this in the next article!
DEXA Body Composition Scan
The DEXA Body Composition Scan measures
(a) total amount of fat mass in grams, and
(2) total amount of Lean Body Mass plus bone, in grams.
It does not measure bone mass, but adds both lean body mass and bone mass together. This is important because reports that often accompanying DEXA Body Composition Scans include information about “bone mineral density”, but this information is estimated, rather than assessed.
The DEXA Body Composition Scan reliably indicates how much total fat someone has, and the distribution of that fat over the body. It does not differentiate between sub-cutaneous fat (the fat under the skin) and visceral fat (the fat around the organs) which is associated with increased health risk.
Since fat mass is what is being directly evaluated in a DEXA Body Composition Scan, the total amount of body fat determined by this method is accurate.
DEXA Body Composition Scan Data of Fat and Lean
Below are two pages from a DEXA Body Composition Scan report.
This above page from a DEXA Body Composition Scan report is mostly related to what it assesses directly, which is Total Fat Mass in grams, and Lean Body Mass plus bone in grams, and evaluates the distribution of that fat.
While a DEXA Body Composition Scan does not differentiate between subcutaneous fat and visceral fat, the report indicates “Estimated Visceral Adipose Tissue (fat)” in the table of adipose (fat) indices. It is important to note that visceral fat is not assessed, but estimated.
DEXA scanners (both GE and Hologic brands) have the National Health and Nutrition Examination Survey (NHANES) data integrated into their software, this which enables them to generate Z-scores for total amount of fat in grams, as well as localized Z-scores for fat in arms, legs, and trunk (1).
Z-scores compare the an individual subject’s results to those of an aged-matched population, and since a DEXA Body Composition Scan measures total fat directly, the z-score in this report for adiposity (fat) is valid.
T-scores compare an individual’s results to how many standard deviations it is from the results of a 30-year old young adult, and since a DEXA Body Composition Scan measures total fat directly, the t-score for adiposity (fat) is also valid.
DEXA Body Composition Scan Data on Bone Mineral Density and Bone Mineral Composition
This is where reports that may be provided with a DEXA Body Composition Scan can get really crazy.
A DEXA Body Composition Scan does not differentiate between Lean Body Mass as muscle and bone, so any information about “bone mineral density” and “bone mineral composition” is based on estimations!
Since there is no measurement of bone mass in grams separate from Lean Body Mass (muscle), z-scores for “Bone Mass Density” from a Whole Body Composition Scan make no sense. This is a comparison of estimated bone data to actual data from an aged-matched population!
Have a look at the table below from a 55 year old woman whose DEXA Body Composition Scan report indicates that she had osteoporosis based on estimated bone density numbers. Without having a DEXA Bone Density Scan, of both hips and lower spine she really doesn’t know if she has osteopenia or osteoporosis or not. A DEXA Body Composition Scan is designed to assess fat mass and the distribution of that fat, not bone.
Final Thoughts…
If you have had a DEXA Body Composition Scan and been told that you have osteopenia or osteoporosis remember that this is based on estimates of total amount of bone, and not actual measurement of bone. In such a case, I would recommend discussing with your doctor having a DEXA Bone Density Scan of both hips and lower spine.
If the DEXA Bone Density Scan indicates that you meet the criteria for osteopenia or osteoporosis, then meet with you doctor to discuss the results and their recommendations. In some cases, a doctor may recommend medication to keep bone from breaking down too quickly, and/or a program designed by a Physical Therapist to enable you to safely exercise and retain as much of bone mass you still have, while minimizing the risk of fractures.
Take Away Message
Remember, that estimated data of “bone mineral density” from a DEXA Body Composition Scan is not the same as data from a DEXA Bone Mineral Density scan which is based on direct assessments.
Getting accurate information using the right diagnostic tool is essential.
How I Can Help
If you are an older adult who wants to optimize your diet and lifestyle to retain as much bone mass as possible as you age, I can help.
To your good health,
Joy
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Reference
- Shepherd JA, Ng BK, Sommer MJ, Heymsfield SB. Body composition by DXA. Bone. 2017 Nov;104:101-105. doi: 10.1016/j.bone.2017.06.010. Epub 2017 Jun 16. PMID: 28625918; PMCID: PMC5659281.
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