What fraction of loss on semaglutide is lean mass according to SURPASS-3?

The case in front of me: semaglutide · SURPASS-3.

The figures are clear enough; the question is what they mean and what they do not.

I can supply the numbers if the specifics change the answer.

How should I read this, and where are the traps?

On the detail: start with the arithmetic, because the answer to the practical question is usually a number and the number is usually achievable.

DEXA precision is better than people assume for fat mass and worse than people assume for lean mass in a single scan — the least significant change for regional lean mass on a well-maintained scanner is on the order of a few per cent. That means two scans three months apart can differ without anything having happened, and it means a scan sequence needs to be at least three points before a trend is interpretable.

The underlying point is that absolute strength holds up better than scale weight during a deficit for a straightforward reason: strength is substantially neural and skill-based, and the contractile tissue you retain is being trained harder relative to its size. Grip strength and repetition maxima are therefore lagging indicators of muscle loss rather than leading ones, which is an argument for measuring both.

Adaptive thermogenesis — a fall in energy expenditure beyond that predicted by the change in body composition — is documented across weight-loss interventions and is the mechanistic basis for the plateau being expected rather than anomalous.

The plateau is arithmetic. Treat it as arithmetic and the response follows.

The commonly quoted figures for lean-mass loss are mostly measurement artefacts, and the artefact is well understood: fat-free mass as measured includes water and glycogen, both of which fall early and neither of which is contractile tissue.

Cardio does not interfere with lean-mass retention at the volumes anyone here is doing; the interference effect in the literature appears at high concurrent volumes in trained athletes. What cardio does at a large deficit is add to the deficit, which is either the point or a problem depending on the objective.

Fibre at very low total intake is a trap. Soluble fibre needs water and motility to work; insoluble fibre adds bulk to a slow transit. At 900 kcal a day with delayed gastric emptying, an osmotic agent is more predictable than a bulking one, and adequate fluid is doing more work than either.

The body-composition substudies in the major programmes consistently report that the proportion of weight lost as fat mass is approximately three quarters or better, with the lean-mass fraction falling within the range seen in dietary weight loss of comparable magnitude[1].

One qualification: none of this is a clinical assessment, and unexplained loss of function rather than of mass is a reason to see someone rather than to adjust a programme.

Train, eat the protein, measure something functional, and give the trend three months before you interpret it.

edited 3 Aug 2024 by w_okoye — corrected a unit error in the worked example

The distinction that resolves this is between proportional loss and functional loss. Losing lean mass in proportion to total mass is what happens in every weight loss intervention. Losing function is not, and function is measurable.

The minimum effective resistance-training dose in a deficit is lower than most programmes assume. Two sessions a week covering the major movement patterns, with loads taken close to failure, is sufficient to retain most of what would otherwise be lost. Volume beyond that adds recovery cost that a large energy deficit is poorly placed to pay.

Put another way, hydration state moves a DEXA lean-mass figure directly, because the algorithm assigns water to the lean compartment. Scanning fasted, at the same time of day, before training and without a recent high-carbohydrate day is the difference between a comparable sequence and a noisy one. Bioelectrical impedance is far more sensitive to hydration again, which is why its trend is unusable at this timescale.

The STEP 1 extension reported substantial regain in the year after treatment withdrawal, with weight and cardiometabolic variables trending back toward baseline[1].

The limitation of the arithmetic is that it assumes intake is being measured accurately, and self-reported intake is systematically underestimated by a substantial margin.

A maintenance plan written before you need it is worth more than a better loss plan.

It helps to be literal here: a plateau at four to six months is the expected shape of the curve, not a failure of it. Energy expenditure falls with mass, and the deficit closes itself unless intake falls further.

The first four weeks of loss is substantially fluid and glycogen. Each gram of stored glycogen carries roughly three grams of water, and total glycogen is on the order of 400 to 500 g, so the obligatory water shift alone accounts for a couple of kilograms. This is why the first month looks dramatic and the second looks like a plateau when in fact the fat-loss rate has not changed.

SURMOUNT-4 provides the cleanest maintenance-versus-withdrawal contrast available in the class, and it is the reference for any claim about what happens after stopping[1].

Two resistance sessions a week and a protein target you actually hit will do more than any refinement beyond them.