DEXA and Body Composition
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The number that tells you almost nothing
Body weight is a single number describing a mixture of things that behave very differently: muscle, fat, bone, water, and the contents of your digestive tract.
Two people can weigh exactly the same and have entirely different metabolic risk, fracture risk, and functional capacity. More to the point, one person can weigh the same at 40 and 55 while having quietly traded a meaningful amount of muscle for fat. The scale registers nothing. The consequences are substantial.
This is why I would rather measure composition than weight.
What DEXA measures
DEXA — dual-energy X-ray absorptiometry — passes two low-energy X-ray beams through the body. Different tissues attenuate the beams differently, which allows the scan to separate the body into three compartments: bone mineral, fat mass, and lean soft tissue.
It reports these regionally, not just as a total. You get numbers for each arm, each leg, and the trunk. That regional detail is clinically useful — a left-right asymmetry in leg lean mass after a knee injury, for example, quantifies something you can otherwise only guess at.
The scan takes about ten minutes. You lie still, fully clothed, and nothing touches you. Radiation exposure is very low, in the range of a few micro-sieverts — comparable to a day or so of ordinary background radiation, and a small fraction of a chest X-ray.
The three numbers worth caring about
Appendicular lean mass. The muscle in your arms and legs, usually indexed to height. This is the measurement used to define sarcopenia, and it is the number I care most about in older adults and in anyone recovering from injury or illness.
Visceral adipose tissue. Fat around the organs, as distinct from fat under the skin. This distinction matters enormously. Visceral fat is metabolically active and is far more strongly associated with insulin resistance, cardiovascular risk, and systemic inflammation than subcutaneous fat. Two people at identical body fat percentages can carry very different visceral loads, and waist measurement is only a rough proxy.
Bone mineral density. Reported as T-scores and Z-scores at the spine and hip, and this is the same measurement used to diagnose osteoporosis. Given that fracture risk in older adults is a genuinely serious matter, having a baseline is valuable — particularly for postmenopausal women, anyone on long-term steroids, and anyone with a family history.
What it is good for, and what it is not
DEXA is the practical reference standard for body composition in a clinical setting. It is far more accurate than bioimpedance scales, calipers, or the estimate your smartwatch offers.
It is not perfect. Hydration status affects lean mass readings, which is why measurement conditions should be consistent. It cannot distinguish between muscle and other lean tissue with complete precision. And absolute values differ between machines and software versions, which means a comparison across two different facilities is not reliable.
The correct use is serial measurement on the same machine. A single scan gives you a baseline. Two scans six to twelve months apart tell you the direction you are traveling, which is the actually useful information.
Where this connects to what I treat
This is not a wellness add-on disconnected from musculoskeletal care.
If you have knee arthritis, the quadriceps mass in that leg is directly relevant to how the joint is loaded, and quantifying the deficit tells us how much work there is to do. If you are recovering from a lower limb injury, regional lean mass tells us whether the leg has actually rebuilt or whether you have simply stopped limping. If you are over 60 and losing function, distinguishing muscle loss from deconditioning changes the plan.
And if you are training hard and not making progress, knowing whether you are gaining muscle, losing fat, both, or neither is the difference between adjusting intelligently and guessing.
How to use the numbers without becoming neurotic
A caution worth stating. Body composition data can feed unhelpful preoccupation, and for some people a number on a report becomes something to chase rather than something to inform a plan.
The framing I prefer: this is a measurement of whether your training and nutrition are producing the tissue changes you want. If lean mass is rising and visceral fat is falling, the plan is working, regardless of what the scale says. If lean mass is falling, something needs to change — usually load, protein, or sleep.
It is a feedback instrument. It is not a scorecard, and it says nothing about your worth.
Who should get one
Worth considering if you are over 50 and want a baseline; if you have noticed strength or function declining; if you are postmenopausal or have osteoporosis risk factors; if you are recovering from significant injury, surgery, or hospitalization; if you carry metabolic risk factors and want to know your visceral fat rather than infer it; or if you are training seriously and want objective feedback.
DEXA is on the list of services we are bringing online. In the meantime, strength testing, gait speed, and simple functional measures give me a great deal of the same directional information without any equipment at all.