Lean mass is a compartment, not a tissue

Almost every disagreement about muscle loss on incretin therapy turns out, on inspection, to be a disagreement about measurement rather than about physiology. One party is quoting dual-energy X-ray absorptiometry from a trial substudy; another is quoting a bioimpedance readout from a gym scale; a third is quoting the difference between two bioimpedance readouts taken at different times of day and in different states of hydration. These are not three estimates of the same quantity. They are three quantities, and the spread between them is comfortably wide enough to accommodate any conclusion a person arrives wanting.

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.

The method that measures muscle directly, and why nobody uses it

There is a technique that estimates whole-body skeletal muscle mass rather than inferring it from a subtraction. Deuterated creatine dilution involves an oral dose of labelled creatine, which distributes into the total creatine pool — almost all of which sits in skeletal muscle — with the enrichment of labelled creatinine in a subsequent urine sample giving an estimate of pool size and therefore of muscle mass.¹ It is not an imaging measure and it does not depend on regression equations fitted to a reference population.

Comparisons with DXA are instructive and slightly deflating. The two methods correlate only moderately in older adults, and where they disagree the creatine-dilution figure has been the better predictor of physical function and of incident disability. That is an argument that DXA appendicular lean mass, the standard proxy, is measuring something adjacent to what matters rather than the thing itself.

The method has been available for more than a decade. It has been used in no trial of any drug in this class. It requires a timed urine collection and a mass spectrometry laboratory, which is a modest imposition set against the volume of argument the absence of good muscle-mass data has generated.

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.

Proportion of loss against absolute kilograms

There is a rhetorical move available to both sides of this argument and it works by choosing a denominator. Report lean mass as a proportion of total body mass and it rises during successful treatment, because fat is falling faster; the treatment looks composition-improving, which it is. Report lean mass in absolute kilograms and it falls; the treatment looks muscle-costing, which it also is. Both statements can be made from the same scan pair without either being false.

The Journal reports both, in that order, and thinks anybody presenting only one should be asked why. The proportional figure is the right one for questions about metabolic quality: a body with a higher lean fraction handles glucose better and carries less ectopic fat. The absolute figure is the right one for questions about function and reserve, because a hip fracture at seventy-eight is not prevented by a favourable ratio.

The two framings also diverge most sharply exactly where the stakes are highest. A person losing twenty-five per cent of their body weight will show an excellent proportional result and the largest absolute lean-mass reduction in the cohort. Selecting the framing selects the conclusion, which is why the trade has settled on whichever one suits it.

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.

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.