What does the lean mass number on a DEXA scan mean?

The lean mass number on a DEXA scan covers everything that is not fat and not bone mineral: muscle, organs and water. Scanners print it for the whole body and for each limb, and the limb version is the one with thresholds attached. It is usually labelled appendicular lean mass index, meaning the muscle of your arms and legs in kilograms divided by your height in metres squared [8].

In 500 postmenopausal women from a Brussels cohort, median age 77, the cut-off for low muscle mass was 5.5 kilograms per square metre [14]. Just over a third of them, 35.6%, had osteoporosis, the condition in which bone thins enough to break easily [14]. The same criteria set two further conditions no scanner can measure: a hand grip under 16 kilograms, and a walking speed of 0.8 metres per second or slower, which is about 2.9 kilometres an hour [14].

More muscle is not automatically better. The Vietnam Osteoporosis Study scanned 1,372 adults aged 50 and over and X-rayed their spines, finding a vertebral fracture, meaning a collapsed spinal bone, in 173 of them [9]. Set against people in the middle of the range, those in the lowest quarter for arm-and-leg muscle had about 1.7 times the odds of that fracture, and those in the highest quarter about 2.6 times [9]. Fat mass showed no such pattern [9].

Pooling 56 studies and 796,914 people, those with sarcopenia were about three times as likely to also have osteoporosis, and the link held in reverse [15]. Each sizeable step up in arm-and-leg muscle relative to body size went with roughly 35% lower odds of osteoporosis [15]. Among 719 Spanish adults aged 18 to 30, the groups carrying more lean mass had clearly better bone readings than those carrying less [2].

Most reports also give a Z-score, which places you against the average for your age and sex: zero means average. In 77 adults with coeliac disease, the muscle and fat Z-scores sat near zero, so their body composition matched the American reference group the scanner compared them with, while their bone readings came in below it [11].

Bone grows in response to being pulled on, and muscle does most of the pulling. That is why lean mass is treated as a marker of the mechanical stimulation bone receives, and why shifts in lean mass track shifts in bone in young adults [2].

Shared mechanics is the likeliest reason the two problems turn up together. Muscle loss and bone loss have overlapping causes in ageing bodies [12], and the pooled figures show the overlap running both ways, at about three times in one direction and two and a half in the other [15].

The data do not explain the other end of the curve. The Vietnamese study measured each person once, at a single moment, so it cannot say whether a high muscle reading contributes to spinal fractures or simply keeps company with something that does [9].

The headline threshold comes from one narrow group. The 5.5 kilograms per square metre cut-off was applied to postmenopausal women with a median age of 77 [14], and none of these papers give the equivalent number for men, for younger women, or for populations outside Europe.

The evidence here does not describe:

• children and teenagers, whose reference curves are built separately and shift depending on which database sits behind them, with curves from a low-obesity cohort landing well below the American national curves at the upper end [1][8]
• people with conditions that reshape body composition, such as coeliac disease [11] or inflammatory spine disease [7]
• anyone scanned on a machine whose reference data came from a different population than their own

The scan's own precision limits how much any single reading is worth. Measured against a four-part reference model combining body mass, body volume, bone mineral and body water, DEXA read body fat about 1.9 percentage points too high on average in 75 adults with overweight or obesity, and was out by as much as 5 percentage points for individuals [4]. Bioimpedance devices, the scales and handsets that pass a small current through you, drift further: their whole-body bone density readings landed between about 0.29 grams per square centimetre below the DEXA figure and 0.17 above it in 318 Taiwanese adults [3].

The number also diagnoses nothing by itself. The European group that wrote the muscle criteria recommends grip strength and a four-metre walk for that reason: mass does not tell you whether the muscle works [13].

Someone in their fifties opens a body composition report and reads the whole-body lean mass figure first, which is the wrong line to anchor on. The one carrying published thresholds is the arm-and-leg total divided by height in metres squared [8], and what turns it into a verdict is the comparison group: whether the report matches them against their own age and sex, and whose national reference data the scanner holds [1][11]. If it comes back low, the criteria built around it still call for a grip test and a timed walk before anyone draws a conclusion [13].