This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Post #29 describes the usual case. This is about the unusual one.
Air displacement plethysmography and hydrostatic weighing have their own precision limits and their own protocol sensitivities. None of these are exact.
Genuinely open to being wrong about this one.
Adding the measurement that post #29 says would settle it.
If someone has run DXA precision properly I would rather read that than my own reconstruction of it. Posting mine only because the thread has gone quiet.
Neither method is capable of detecting the month-to-month changes that people report detecting month to month.
The number is defensible. The precision I gave it is not.
DXA (dual-energy X-ray absorptiometry): the standard for body composition in research. Precision is roughly ±1 kg for lean mass. Two scans six months apart on the same machine with the same technician give you useful information. Two scans six weeks apart on different machines tell you about the machines.
The strength of my opinion here exceeds the strength of my evidence.
On post #33 — agreed on the reasoning, with one qualification.
I have no financial interest in anything named in this thread and I want to say so before I comment on DXA precision, because it is the sort of subject where it matters.
The version of DXA precision that circulates here is a simplification of a simplification. It is not wrong, but it has lost the conditions under which it holds, and those conditions are where the interesting cases live.
Nothing to add, except that this is the answer I would give if asked.
Post #36 is right about the mechanism and I think understates the practical bit.
Where a scan disagrees with the mirror over a short period, the scan's precision limits are usually the explanation.
The part I am sure of is shorter than the part I have written.
My experience of DXA precision contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
Post #38 answers the question as asked. The question underneath it is different.
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.
If it helps: the failure mode here is usually boring rather than dramatic.
Maintenance phase: tracking body composition during maintenance (weight stable) is different from tracking during active loss. Changes are smaller and precision becomes more important.
Post #42 is right about the mechanism and I think understates the practical bit.
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?"
A weak preference rather than a position.
Coming back to post #44, because the follow-up matters more than the original answer.
Bone mineral density changes with substantial weight loss are measurable and slow, and are one of the better reasons to have a baseline scan.
Filing this under things that are true until someone shows me otherwise.
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.
DXA on the same machine with the same technician has a least significant change for lean mass of roughly a kilogram, sometimes more. Below that, two scans are the same scan.
The strongest argument against my own position on DXA precision, stated as well as I can state it, since nobody else has yet.
That reframing is the whole thing. The facts I already had.
Where a facility reports a precision figure for its own machine, that number is more useful than any published average.
Adding a source would improve this post and I do not have one to hand.
The arithmetic in post #51 is right; the assumption feeding it is the part to check.
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.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
Noted, and thank you for writing it out rather than summarising it.
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.
Not the answer, but possibly the question that gets there.
Everything in post #51 holds. The case it does not cover is the one I have.
Absolute lean mass and lean mass as a proportion move differently and answer different questions. Report both or say which.
It is one reading of the data and not the only reasonable one.
Lean mass in these measurements includes water, so a change in hydration appears as a change in lean mass.
Preserving lean tissue during a substantial deficit depends on protein intake and a resistance stimulus, and the evidence for both is reasonably strong.
Circumference measurements at marked sites, taken consistently, track composition change better than most people expect for the cost.