Body composition scans and bone mineral density scans are both performed using the same DEXA machine, but they answer very different questions. If you’ve come to this page, you are likely trying to understand what the difference is.
A body composition scan measures how your body weight is distributed between fat, lean tissue and bone. It is generally used to monitor health, fitness, weight loss and muscle development.
A bone mineral density scan, sometimes called a BMD or bone density test, specifically assesses the density of certain bones to help identify low bone density, osteopenia and osteoporosis.
Although the same type of scanner may be used, these tests have different scan procedures, reports and clinical purposes. One cannot not be substituted for the other.
What is a DEXA scan?
DEXA stands for Dual-Energy X-ray Absorptiometry, although it may also be written as DXA.
The scanner sends two very low-energy X-ray beams through the body. Because bone, fat and lean tissue absorb these beams differently, the scanner can distinguish between these different types of tissue. DEXA scans are non-invasive, painless and do not require an injection.
The way the scan is performed and analysed depends on whether you are having a body composition assessment or a diagnostic bone density study.
What does a body composition scan measure?
A DEXA body composition scan examines the whole body. It provides considerably more information than body weight or Body Mass Index, commonly known as BMI.
Your report will include:
- Total body and tissue fat percentage
- Total fat mass
- Lean mass
- The distribution of fat and lean tissue throughout your arms, legs and trunk
- Differences between the left and right sides of your body
- An estimate of visceral adipose tissue around the abdominal organs
- Your android/gynoid ratio of your fat distribution
- A DEXA-based estimate of your resting metabolic rate
- Whole body bone mineral content
- Your relative skeletal muscle index (for diagnosing sarcopaenia)
- Comparisons with age and sex-related reference data
DEXA distinguishes between fat mass, lean soft tissue and bone mineral. It can also report these measurements for different regions of the body rather than providing only one whole-body number.
It is important to understand that lean mass is not exactly the same as muscle mass. Lean mass includes muscle, organs, connective tissue and body water. However, changes in regional lean mass can provide useful information when tracking muscle development, rehabilitation or changes associated with ageing or weight loss medications.
Who might benefit from a body composition scan?
A body composition scan may be useful when you want to:
- Establish a baseline before beginning a weight-loss or exercise program, including when taking GLP-1 medications
- Determine whether weight change is coming from fat or lean tissue
- Monitor muscle gain during strength training
- Track changes during rehabilitation or return to exercise
- Measure differences between your arms or legs
- Assess the distribution of body fat
- Monitor visceral abdominal fat
- Evaluate progress when the number on the scales has changed very little
For example, someone may lose body fat while gaining muscle and therefore see only a small change in body weight. A body composition scan can identify these changes separately.
At Riverside BodyScan we perform this scan, and provide you with the results on the spot, including an explanation of what they mean.
What does a bone mineral density scan measure?
A bone mineral density scan has a narrower but clinically important purpose: it measures the mineral density of particular bones.
The most commonly assessed areas are the:
- Lumbar spine
- Hip
- Femoral neck, which is the upper part of the thigh bone
- Forearm in most circumstances
These areas are examined because fractures associated with osteoporosis commonly affect the spine, hip and wrist.
Rather than analysing fat and lean tissue across the entire body, the scan carefully measures bone mineral density at these clinically recognised sites. The report is usually sent to your referring doctor and includes measurements such as your BMD, T-score and Z-score.
What are T-scores and Z-scores?
A T-score compares your bone mineral density with the average peak bone density of a healthy young adult of the same sex.
For postmenopausal women and men aged 50 and older, results are generally classified as:
- –1.0 or higher: normal bone density
- Between –1.0 and –2.5: osteopenia or low bone density
- –2.5 or lower: osteoporosis
A Z-score compares your bone mineral density with people of a similar age and sex. It is particularly important when assessing younger adults, premenopausal women and men under 50. A Z-score of –2.0 or lower is described as being below the expected range for age and may require further investigation by the referring doctor.
Bone density results should always be considered alongside other factors that affect fracture risk, including age, previous fractures, medical conditions, medications and family history.
Can a body composition scan check my bone density?
A body composition scan does provide information about the amount of bone mineral across your body. This can be useful as part of an overall body composition assessment.
However, the bone measurement produced by a whole-body composition scan is not a substitute for a diagnostic bone mineral density study.
Diagnosing osteopenia or osteoporosis requires a dedicated scan of clinically recognised sites (usually the lumbar spine and hip) with the appropriate positioning, analysis and reference data. Whole-body bone density values are not used to diagnose osteoporosis in adults.
Therefore:
- Choose a body composition scan when your main questions relate to fat, lean mass, muscle development, weight management or physical performance.
- Speak with your doctor about a bone mineral density scan when the concern is osteoporosis, low bone density or fracture risk.
Some people may benefit from having both scans because each provides different information.
What happens during the scan?
For either examination, you lie on a padded scanning table while the arm of the DEXA machine moves above you. The machine is open rather than enclosed, so you do not enter a tunnel.
Metal objects, zippers, buttons and jewellery need to be removed because they interfere with the scan. Riverside Body Scan provides a gown where required.
The scan itself takes 5 minutes or less, and is painless. Additional appointment time allows for positioning, checking your medical and scan history, preparing your report and, during an initial body composition appointment, explaining your results.
Which scan should I book?
Book a DEXA body composition scan when you want to understand:
- How much of your body is fat or lean tissue
- Where fat and lean tissue are distributed
- Whether your training or nutrition program is changing your body composition
- Whether weight loss includes fat loss, muscle loss or both
- How your results have changed since a previous scan
A bone mineral density scan is more appropriate when your doctor wants to:
- Investigate possible osteopenia or osteoporosis
- Assess your fracture risk
- Monitor previously identified low bone density
- Review the effects of a medical condition or medication on your bones
If you are uncertain which examination you need, contact Riverside BodyScan or discuss the question with your health professional.
Understand what is happening inside your body
Body weight alone cannot show whether you have lost fat, gained muscle or experienced a change in bone density.
DEXA technology can provide detailed and objective information, but it is important to select the correct type of scan. A body composition scan helps you understand your fat and lean tissue, while a bone mineral density scan is a targeted medical assessment of your bone health.
Riverside Body Scan provides both DEXA body composition scans and referred bone mineral density studies at West Gosford on the Central Coast.
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