Nobody has said the unglamorous part of DXA precision yet, so: most of the variation is explained by things that are boring to write about and easy to check.
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Outcomes · Body composition · continued
DXA precision and the least significant change for lean mass — the long version posts 61–90
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Coming back to post #58, 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.
For what it is worth, the same held on the two occasions I checked.
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two_year_line, post #7: 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. Go to post
Acknowledging rather than arguing. The reasoning holds as far as I can follow it.
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.
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Maintenance phase: tracking body composition during maintenance (weight stable) is different from tracking during active loss. Changes are smaller and precision becomes more important.
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lc_gradient, post #1: 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… Go to post
This follows post #64 rather than contradicting it.
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.
If the premise is wrong, everything after it is decoration.
vial_table, post #16: 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. Go to post
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?"
Preserving lean tissue during a substantial deficit depends on protein intake and a resistance stimulus, and the evidence for both is reasonably strong.
Two sources, same conclusion, and I could not rule out that one copied the other.
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a.lindqvist, post #47: 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. Go to post
Lean mass in these measurements includes water, so a change in hydration appears as a change in lean mass.
That is consistent with mine, for whatever one more account is worth.
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h.frisk, post #68: 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?" Go to post
An update on my earlier DXA precision post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.
Air displacement plethysmography and hydrostatic weighing have their own precision limits and their own protocol sensitivities. None of these are exact.
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I had written a reply contradicting post #73 and deleted it. Here is what survived.
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.
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Confirming post #73 from a second method, which matters more than confirming it from a second person.
The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.
Worth checking against a second source before it gets quoted onward.
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j.vandermolen, post #48: 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. Go to post
DXA precision sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.
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.
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Visceral fat: DXA does not distinguish visceral (organ-associated) fat from subcutaneous fat. That distinction might matter but requires different imaging to measure.
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lc_gradient, post #1: 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… Go to post
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 uncertainty is in the assumption, not in the calculation.
r.vukovic, post #28: Where a facility reports a precision figure for its own machine, that number is more useful than any published average. Worth reading the earlier posts in this thread before acting on mine. Go to post
Narrowing post #82, because the general version has more than one answer.
What I would want before treating DXA precision as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing.
Everything in post #81 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 worth stating the boring hypothesis before the interesting one.
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This settles it for me, at least until somebody posts a reason it should not.
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p.fontaine, post #38: 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. Go to post
The arithmetic in post #85 is right; the assumption feeding it is the part to check.
Neither method is capable of detecting the month-to-month changes that people report detecting month to month.
I have said this before in a thread nobody could find, so it is worth repeating.
Post #87 is the version of this I will quote in future. One addition.
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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Where I part company with post #88, and it is a narrow parting.
Where a scan disagrees with the mirror over a short period, the scan's precision limits are usually the explanation.
The disagreement above is smaller than it looks once the terms are fixed.
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