The precision question nobody puts to a body-composition report

The Journal has asked four separate imaging physicists the same question over the past year: given the best clinical DXA in routine use, what is the smallest change in appendicular lean mass you would report to a patient as real? The answers clustered between six hundred grams and one and a half kilograms, depending on the machine, the operator, the positioning protocol and whether the two scans were performed on the same device. Nobody said less than half a kilogram. That figure should be printed at the top of every body-composition report and is printed on none of them.

What the compartment called lean mass contains

Every widely used body-composition instrument partitions the body into compartments, and the compartment names do more work than they should. In the standard three-compartment DXA output, a body consists of fat mass, bone mineral content and lean soft tissue. The third of those is defined by subtraction: it is what remains once fat and bone are accounted for. It therefore includes skeletal muscle, cardiac and smooth muscle, the liver, kidneys, gut and other viscera, the skin, the blood, and all extracellular and intracellular water.

The water term is the one that causes the most confusion in the first weeks of treatment. Muscle glycogen binds water at roughly three grams per gram, so a shift in glycogen stores produces a change in lean mass measurement several times its own size. Reduced food intake, reduced carbohydrate intake and reduced training volume all lower glycogen. A person who reads a two-kilogram fall in lean mass across the first month of treatment may have lost very little muscle and a good deal of water, and no instrument in routine use can tell them which.

This is not a pedantic distinction. It determines whether an early reading is alarming or unremarkable, and it is the reason the Journal treats composition measurements taken inside the first eight weeks of treatment as close to uninterpretable.

What a DXA scan resolves

Dual-energy X-ray absorptiometry is the reference method in this field for practical rather than theoretical reasons: it is fast, the radiation dose is trivial, it is widely installed, and it reports regional as well as whole-body values. Its coefficient of variation for whole-body lean mass on a well-maintained clinical scanner with a consistent operator is on the order of one per cent, which sounds excellent until it is converted into kilograms. For a person with fifty-five kilograms of lean tissue, a one per cent coefficient of variation implies a least significant change — the smallest difference between two scans that can be distinguished from measurement noise with reasonable confidence — of roughly one and a half kilograms.

Appendicular lean mass, the arms-and-legs subtotal that is the closest DXA proxy for skeletal muscle, has a smaller absolute magnitude and a somewhat larger relative error, and the two effects roughly cancel. Regional values for a single limb are noisier again. None of this is a criticism of the instrument. It is the reason a body-composition report that changes by half a kilogram between visits has told the person nothing, and the reason the trial substudies report group means rather than individual trajectories.

Bioimpedance measures conductivity and calculates everything else

Bioelectrical impedance analysis passes a small alternating current through the body and measures the opposition to it. Lean tissue, being largely water and electrolyte, conducts; fat does not. From the measured impedance, a height term, a weight term and a set of population-derived regression equations, the device produces a fat mass figure. The impedance is measured. The body composition is computed from an equation fitted to somebody else.

The consequences are well documented. Agreement with DXA at the group level is often reasonable; agreement at the individual level is not, with limits of agreement for fat mass frequently spanning several kilograms in either direction, and the disagreement growing at higher body mass index — precisely the population of interest here.¹ Worse for our purposes, the measurement is sensitive to hydration status, recent exercise, recent meals, ambient temperature, skin moisture and time of day, all of which are changing during incretin treatment. A device that reads fat mass as a function of body water, used in a person whose body water is unstable, will report composition changes that are hydration changes. The Journal does not report BIA-derived composition changes from consumer devices, and would not treat them as evidence of anything.

Bone density responds more slowly than soft tissue, so a scan taken at the end of a rapid loss period is measuring a skeleton that has not finished. Reading it as a final result is a timing error rather than a measurement one.

The one-quarter rule and the paper that dismantled it

Clinical teaching has long held that approximately twenty-five per cent of the mass lost during weight reduction is fat-free tissue. The figure appears in textbooks, in review articles and in a great deal of consumer material, usually without a citation and always without an interval.

A critical review published in 2014 traced the rule to a limited number of older studies, examined the variation across the wider literature, and concluded that treating one-quarter as a constant is not defensible.² The fraction of loss that is fat-free tissue varies systematically with baseline adiposity — heavier people lose proportionally more fat — and with the rate of loss, the protein intake, the activity pattern and the measurement method. Reported values span from well under fifteen per cent to above thirty-five.

This matters for the current argument in a specific way. Both the reassuring and the alarming readings of the incretin substudy data are constructed by comparing an observed fat-free fraction against the one-quarter benchmark. If the benchmark is a loose average rather than an expectation, both comparisons are weaker than they appear, and the honest statement is that the observed fractions sit within the range that dietary weight loss has always produced.

What the substudies were never powered to detect

An imaging substudy inside a large trial is sized to describe rather than to test. The enrolment is set by how many participating sites have a scanner and by what the sponsor budgeted, not by a power calculation against a composition hypothesis, and the analysis is generally pre-specified as exploratory or descriptive. The consequence is that these substudies can report a mean change with a usable confidence interval and cannot support most of the questions asked of them.

They cannot, for instance, establish whether lean-mass change differs between dose arms, because the per-arm enrolment after splitting is in the low tens. They cannot establish whether it differs by age, sex, baseline adiposity or diabetes status, because those subgroups were not enrolled to be comparable. They cannot describe the distribution of individual responses, because the per-participant least significant change is a substantial fraction of the observed mean effect. And they cannot address function at all, because nobody measured it.

Nor was the imaging repeated when the programmes were extended. The two-year semaglutide extension reported weight, waist circumference and cardiometabolic parameters at week 104 and did not repeat the composition substudy, so there is no imaging at all beyond seventy-two weeks in this class.³ Whatever the trajectory of lean mass is in year two of treatment, nobody has measured it.

None of this is a scandal; it is the ordinary economics of trial substudies. It becomes a problem only when a descriptive group mean is quoted as though it characterised what will happen to an individual, which is now the normal register of coverage on this subject.

The soft-tissue artefact in bone densitometry

Densitometry infers bone mineral density from the differential attenuation of two X-ray energies, using the surrounding soft tissue as the baseline against which bone is distinguished. The algorithm assumes a soft-tissue composition, and that assumption is embedded in the calibration. When the thickness and fat fraction of the tissue overlying a measurement site change substantially, part of the apparent change in bone density is an artefact of the altered baseline.

The magnitude is contested. Phantom and cadaver work suggests errors of the order of one to three per cent for large changes in overlying fat, which is the same order as the real bone changes being reported over a year of rapid weight loss. In practice this means that a hip bone mineral density reduction of two per cent in a person who has lost a fifth of their body weight cannot be cleanly separated into a bone effect and a measurement effect, and the published analyses do not attempt it.

Quantitative computed tomography and high-resolution peripheral imaging are less vulnerable, measure geometry and microarchitecture rather than areal density, and have not been used in any trial in this class. The Journal regards that as the most easily closed gap in the whole body-composition literature.

Weight loss of any origin is associated with composition change, so without a matched comparator an observed finding describes losing weight rather than the agent used to do it. Nothing here is advice about either.

Correspondence on this subject reaches the Journal at a higher rate than on any other, and a striking proportion of it consists of readers reporting a number from a device and asking what it means. The honest answer, in most cases, is less than they hope. We would rather say that than supply a confident interpretation the instrument cannot support.