Where I part company with post #27, and it is a narrow parting.
Circumference measurements at marked sites, taken consistently, track composition change better than most people expect for the cost.
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Outcomes · Body composition · continued
DXA precision and the least significant change for lean mass posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
s.cardoso, post #23: Percentage body fat is a ratio, so it changes when either component changes. A rising percentage during weight loss is arithmetic before it is physiology. That is the version I would defend. It is not the version I started with. Go to post
Post #31 is the version of this I will quote in future. One addition.
The claim about DXA precision upthread is stronger than its source supports. I have read the source. The source says "associated with" and the post says "causes".
sharps_bin, post #13: Useful. I have added it to my own notes with the date on it. Go to post
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 holds under the stated conditions and I have stated them.
Where a facility reports a precision figure for its own machine, that number is more useful than any published average.
Same conclusion as the reply above, reached differently, which is mildly reassuring.
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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.
That has been true for the cases I have seen and I have not seen many.
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n.stanescu, post #22: Post #20 and I disagree about the size of the effect, not about the direction. Visceral fat: DXA does not distinguish visceral (organ-associated) fat from subcutaneous fat. That distinction might matter but requires different imaging to measure. The rule of thumb is fine; the edge cases are where it earns its keep. Go to post
Building on post #35 rather than restating 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.
I had read the opposite somewhere and cannot now find where, which tells me something.
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n.petrov, post #6: Grateful for the specificity. Vague answers to this question are what sent me looking. Go to post
Post #35 and I disagree about the size of the effect, not about the direction.
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.
t.wojcik, post #36: Building on post #35 rather than restating 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. Go to post
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.
A modest claim, modestly supported.
Post #38 put the caveat in the right place and I want to underline it.
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?"
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n.petrov, post #6: Grateful for the specificity. Vague answers to this question are what sent me looking. Go to post
Appreciated. The plain phrasing does more work here than a longer post would.
The most useful reply I ever got about DXA precision was a request to state my units. It sounds like pedantry and it has saved me twice.
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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.
This is where my knowledge stops and I would rather mark the edge than blur it.
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Since DXA precision keeps coming up, it should probably be a maintained page rather than a recurring thread. I am happy to draft it if someone with more direct experience will review it.
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z.cardoso, post #1: Posting this under the heading it deserves: DXA precision and the least significant change for lean mass Everything below is what sits behind that. 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… Go to post
Post #44 answers the question as asked. The question underneath it is different.
I would call the community position on DXA precision likely rather than established, and I would be comfortable defending that hedge.
se.okafor, post #12: 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?" For what it is worth, the same held on the two occasions I checked. Go to post
Circumference measurements at marked sites, taken consistently, track composition change better than most people expect for the cost.
Someone will know this better than I do and I hope they say so.
My position on DXA precision is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.
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e.ferreira, post #17: Answering the question post #14 raises rather than the one it answers. 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. Go to post
Where a scan disagrees with the mirror over a short period, the scan's precision limits are usually the explanation.
On post #49 — agreed on the reasoning, with one qualification.
The confident answers on DXA precision and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
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Picking up post #53: that is the part I would want checked first.
Adding the boring version of DXA precision, because the interesting version keeps getting posted and the boring one is usually right.
Check the ordinary explanations, in order, and stop when one of them accounts for what you are seeing. Most of the time the second one does.
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Coming back to post #53, because the follow-up matters more than the original answer.
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 conclusion is tentative; the arithmetic underneath it is not.
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Absolute lean mass and lean mass as a proportion move differently and answer different questions. Report both or say which.
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Collapsed as off-topic by two members at trust level 3 or above
Lean mass in these measurements includes water, so a change in hydration appears as a change in lean mass.
That much is documented. The rest is how I have interpreted it.
This follows post #56 rather than contradicting it.
Small correction to my own earlier position on DXA precision. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
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r.erdogan, post #20: Post #17 is right about the mechanism and I think understates the practical bit. What I can speak to on DXA precision is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know. Go to post
A request rather than an answer: could whoever has the primary source for DXA precision post it? I have seen the claim three times this month and each version had lost a qualifier.
b.nilsen, post #7: Counterpoint on DXA precision, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out. Go to post
Adding a data point of agreement rather than a data point.