On the subject in the title: DXA precision and the least significant change for lean mass — the long version Working notes rather than a conclusion.
A question about measurement precision rather than about physiology.
Two readings, some weeks apart, on the same equipment, differing by an amount that feels meaningful and may not be. I do not know what this instrument's least significant change actually is.
How much difference does there have to be before it is a difference?
Two sentences on DXA precision and then I will stop, because the rest is speculation and the thread is better without mine.
What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.
This is the first time the answer has come with its own limits attached. Appreciated.
Absolute lean mass and lean mass as a proportion move differently and answer different questions. Report both or say which.
That matches what I was told, which is not the same as knowing it.
The arithmetic in post #6 is right; the assumption feeding it is the part to check.
What I would check first on DXA precision is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.
Answering the question post #5 raises rather than the one it answers.
Filing a mild objection to the consensus on DXA precision. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit.
Visceral fat: DXA does not distinguish visceral (organ-associated) fat from subcutaneous fat. That distinction might matter but requires different imaging to measure.
Written in the hope of being told what I have missed.
On post #8 — agreed on the reasoning, with one qualification.
Resting metabolic rate: body composition affects metabolic rate. As fat mass decreases and lean mass is maintained, metabolic rate is usually maintained better than with simple weight loss alone.
The arithmetic in post #11 is right; the assumption feeding it is the part to check.
Comparing a body composition result against a population reference is a different exercise from tracking your own change, and the second is what most people want.
That is where I would start, not where I would stop.
I would keep DXA precision and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
Trying to state the DXA precision position in a way that someone who disagrees would recognise as fair, because I do not think the version in this thread passes that test.
Adding the measurement that post #15 says would settle it.
The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.
A qualification I should have led with rather than closed on.
Where I part company with post #15, and it is a narrow parting.
Lean mass in these measurements includes water, so a change in hydration appears as a change in lean mass.
This is the version I would want a new member to read first.
Preserving lean tissue during a substantial deficit depends on protein intake and a resistance stimulus, and the evidence for both is reasonably strong.
Where a scan disagrees with the mirror over a short period, the scan's precision limits are usually the explanation.
Comparison to published data: the trials reported mean fat and lean mass changes. Individual results vary around that mean. Being above or below average is not a sign of something wrong.
Maintenance phase: tracking body composition during maintenance (weight stable) is different from tracking during active loss. Changes are smaller and precision becomes more important.
That holds for the case as described. Change the assumptions and it may not.
Picking up post #20: that is the part I would want checked first.
Lean mass loss during rapid weight reduction: depends on protein intake, resistance training volume, and total energy deficit. Adequate protein and maintaining training intensity both help preserve lean mass.
On post #22 — agreed on the reasoning, with one qualification.
Precision versus accuracy: the precision you need depends on what question you are answering. "Am I losing fat?" requires less precision than "Is my lean mass preservation within the expected range?"
The bit of DXA precision that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
Adding the measurement that post #24 says would settle it.
Bioelectric impedance analysis: less precise than DXA and strongly affected by hydration state. Not reliable for detecting month-to-month changes. Useful for trend over years if done consistently.
Ultrasound: thickness of subcutaneous fat at specific sites. Less standardised than DXA and operator-dependent. Useful as a supplementary measure but not as a primary method.
I looked this up rather than remembered it, which is the right order.
Where I part company with post #27, and it is a narrow parting.
Practical experience of DXA precision, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.