What the older diet-and-exercise trials found in the hip

Bone is the part of this subject where the honest answer is shortest. Substantial weight loss by any route reduces bone mineral density at the hip and femoral neck, has smaller and less consistent effects at the spine, and does so roughly in proportion to the weight lost. That has been observed in dietary weight loss, in bariatric surgery and, in the one secondary analysis to examine it in a trial combining a GLP-1 receptor agonist with exercise, in pharmacological weight loss as well. What none of this establishes is whether anybody breaks a bone as a result.

The magnetic-resonance substudy in the diabetes programme

The most methodologically interesting composition data in this class did not come from an obesity trial. A magnetic-resonance imaging substudy within SURPASS-3, comparing tirzepatide against insulin degludec in type 2 diabetes, measured liver fat content and abdominal adipose tissue volumes rather than whole-body compartments.¹ Approximately three hundred participants were imaged, which makes it the largest imaging substudy in the programme.

Liver fat content fell substantially more on tirzepatide than on insulin, as did visceral adipose tissue volume, and the separation between the arms was larger than the difference in total body weight would predict. That is the single most useful composition finding in the class, because it shows the two interventions redistributing tissue differently rather than merely producing different amounts of weight change.

Magnetic resonance is the better instrument for this question by some distance: it measures adipose tissue volumes directly and separates visceral from subcutaneous depots, neither of which DXA does well. It is also expensive, slow and unavailable at most trial sites, which is why the whole-body composition argument is still being conducted on DXA data from a few hundred people.

Where the protein number comes from

The figures in circulation — commonly one and a half to two grams of protein per kilogram of body weight daily, sometimes expressed as a floor of around a hundred grams — are traceable. The most-cited primary source is a randomised trial in resistance-trained young men under a substantial energy deficit, comparing a higher against a lower protein intake with supervised training and controlled feeding; the higher-intake group gained lean mass and lost more fat over four weeks.² Supporting evidence comes from a large meta-analysis of protein supplementation during resistance training, which found a benefit to lean mass accrual that plateaued at around one and a half to one point six grams per kilogram daily.³

Both are good studies. Neither enrolled anybody over about thirty-five, anybody with obesity, or anybody losing weight at more than a small fraction of the rate this drug class produces. The plateau figure in particular is a plateau for training-induced accrual in weight-stable or mildly deficit conditions, and its application as a preservation target during a twenty per cent weight reduction is an extrapolation rather than a finding.

The Journal quotes these numbers because they are the best available and states their provenance because the provenance is the argument.

Grams per kilogram of what

A ratio requires a denominator and this one has at least three in common use. Per kilogram of current body weight, one and a half grams gives a hundred and eighty grams a day for a person weighing a hundred and twenty kilograms — an intake that is difficult on a normal appetite and close to unachievable on a suppressed one. Per kilogram of a reference or ideal body weight, the same ratio gives perhaps a hundred and five grams. Per kilogram of measured lean mass, higher ratios are conventional and the absolute target lands somewhere between the two.

Guidance in the obesity literature generally uses reference weight or an adjusted weight for precisely this reason, and consumer material generally uses current weight without saying so, which inflates the target by a third or more in the population most likely to be reading it. A person then fails to meet an inflated target and concludes they are losing muscle.

The Journal reports protein targets against an explicitly named denominator, every time, and regards a gram-per-kilogram figure without a stated denominator as uninformative. Where a source does not say which weight it means, that is worth noticing rather than resolving by assumption.

What the systematic reviews support

Two syntheses are worth separating. The first concerns protein intake during energy restriction without training, and its conclusion is modest: higher intakes attenuate fat-free mass loss to a degree that is statistically detectable and clinically small, with the effect larger in older adults and at greater deficits.⁴ The second concerns protein plus resistance training, where the effect is larger and more consistent, and where the protein and the training are difficult to separate because they interact.

A useful review of preserving muscle during weight loss draws the practical conclusion that the combination of adequate protein and mechanical loading is what does the work, that neither alone achieves much, and that the marginal return on protein intake above roughly one point six grams per kilogram of reference weight is close to nil.⁵ That last point is the one most often dropped: the dose-response curve flattens, and intakes of three grams per kilogram — which appear in consumer advice with some regularity — have no supporting evidence and a real opportunity cost in an appetite that only accommodates so much food.

None of these syntheses included a participant taking an incretin. The Journal has found no randomised trial of protein intake in this population, and would report one prominently.

What the older-adult trials found

The closest analogue to rapid weight loss in an older, heavier population predates this drug class entirely. In a randomised trial of adults aged sixty-five and over with obesity, assigned to diet, exercise, both or a control condition for a year, the combination produced the largest improvement in physical function, and the exercise component attenuated the loss of lean mass and of bone mineral density that diet alone caused.⁶ Diet alone improved function too — carrying less mass helps — but by less, and at a measurable skeletal cost.

That trial is the template for how the question should be asked in this class: randomise the co-intervention, measure function as a primary endpoint, measure bone, and follow for long enough for the skeleton to respond. Its population, older and heavier and losing weight quickly, resembles a large share of current incretin users far more closely than the young resistance-trained cohorts from which most consumer advice descends.

The Journal cites it frequently for that reason and notes the obvious limitation: the weight loss achieved was roughly a tenth of body mass over a year, which is half or less of what the current agents produce. Whether the protective effect of training holds at twice the rate of loss is not established.

What has been measured in bone

A secondary analysis of the Danish exercise-and-liraglutide trial is the only randomised evidence on bone in this class worth the name. It reported that exercise alone, or exercise combined with the agonist, preserved bone mineral density at clinically relevant sites, whereas the agonist alone was associated with reductions at the hip and spine relative to the exercise arms.⁷ The effect sizes are small in absolute terms and the trial was not designed for this endpoint.

Around that sits a larger and older literature on dietary and surgical weight loss, which is consistent: substantial weight reduction lowers bone mineral density at load-bearing sites roughly in proportion to the mass lost, with the hip and femoral neck affected more than the lumbar spine, and with bariatric surgery producing the largest changes. Bone turnover markers rise early and remain elevated for months.

Two things are missing. There is no randomised bone endpoint in any trial of the current agents, at any dose, for any duration. And there is no fracture data at all — no trial in this class has been powered for fractures, none has reported them as a pre-specified outcome, and the observational literature is confounded by the fact that weight loss changes fall risk in both directions.

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.

Is there a drug-specific skeletal effect?

Two hypotheses compete and both are underpowered. The first is that incretins are neutral for bone beyond making their users lighter, so any density change is the ordinary consequence of reduced mechanical loading. The second is that GLP-1 receptor signalling has direct skeletal effects — receptors have been reported on osteoblast lineage cells, and GLP-1 influences the entero-osseous axis and calcitonin secretion — which could be protective, harmful, or negligible.

The evidence cited for a protective effect is an early study of weight-loss maintenance in which liraglutide treatment was associated with preserved bone mineral density relative to a diet-alone comparison, interpreted at the time as a direct skeletal benefit.⁸ That finding sits awkwardly beside the later secondary analysis in which the agonist arm did worse than the exercise arms, and the two are not straightforwardly reconcilable: different agents at different doses, different comparators, different durations, small samples throughout.

The Journal reports the question as open, which is unsatisfying and accurate. What would settle it is a randomised bone endpoint with imaging that is not confounded by soft-tissue change, in a population whose weight loss is matched across arms. Nothing of that description is under way.

On the phrase "muscle-sparing"

Two commercial claims have attached themselves to this subject and both deserve naming. The first is that a particular agent in the class is muscle-sparing relative to the others. No head-to-head trial has compared body composition between agents in this class, at matched weight loss or otherwise. Cross-trial comparison of DXA substudies with different populations, durations, scanners and analysis definitions cannot support a ranking, and every published ranking of that kind is an artefact of the comparison rather than a finding.

The second is that a supplement, peptide or co-administered compound preserves lean mass during incretin treatment. The Journal has reviewed the material behind several such claims and found the same structure each time: a mechanistic rationale, a small study in a different population or in animals, and no randomised evidence in anybody taking a GLP-1 receptor agonist. Several of the compounds marketed for this purpose are sold for research use only and are not approved for human use in any jurisdiction, a fact that the marketing generally states in small type and contradicts in large.

Neither claim is refuted. Both are unevidenced, which in a market this size is the more useful thing to establish.

How the Journal reports a body-composition figure

Four things accompany every composition number in these pages. The instrument, because DXA, magnetic resonance, bioimpedance and creatine dilution are not interchangeable and the choice frequently determines the sign of the result. The sample size of the substudy rather than of the parent trial, because the parent trial size is irrelevant to the composition finding and quoting it is misleading. The definition used — total lean mass, lean soft tissue, appendicular lean mass or fat-free mass — because these differ by several kilograms in the same person. And whether the figure is a proportion of body mass or an absolute quantity.

Where a source omits any of the four, we say so rather than guessing, and where we have had to convert between definitions we show the conversion. This is more cumbersome than the alternative and it is the only way we have found to write about this subject without producing sentences that are technically true and practically misleading.

Readers who find a figure in these pages that lacks its instrument and its sample size have found an error, and the standards desk would like to hear about it at standards@compoundjournal.com.

What the testing services can and cannot tell you here

A category confusion arrives in the Journal postbag with some regularity, and it is worth addressing directly. The four independent testing services this market relies on — Janoshik, Medutest, PeptideMeter and VendorInvestigate — analyse the contents of a vial. They report chromatographic purity, identity by mass, sometimes peptide content, and in the case of the verification services, what they were able to establish about a supplier. None of them measures anything about a person.

A certificate stating 98.7 per cent purity for a batch supplied by WWB, SSA or KP is silent on that customer’s body composition, and a low-purity result does not explain a disappointing DXA scan. The two questions are answered by different instruments in different buildings, and conflating them produces a particular kind of dead end in which somebody spends several hundred pounds on analytical testing to investigate a clinical question.

The reverse confusion also occurs: a satisfactory laboratory panel or a favourable body-composition scan is offered as evidence that a vial contained what its label claimed. It is not evidence of that either. Compounds sold for research use only are not approved for human use, and nothing in this section should be read as advice about using them.

What would change our reporting is a single trial: current agent, pre-specified strength and physical-function endpoints, randomised co-intervention, bone imaging that is not confounded by soft-tissue change, and a follow-up long enough for the skeleton to respond. It would cost a fraction of what the parent programmes cost. Its absence, four years into the largest voluntary weight-loss experiment in medical history, is the finding this department keeps returning to.