DXA precision and the least significant change for lean mass — the long version posts 91–112
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
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.
This is the sort of exchange that makes the archive worth searching.
Post #91 answers the question as asked. The question underneath it is different.
Where a scan disagrees with the mirror over a short period, the scan's precision limits are usually the explanation.
I would treat that as a working assumption and revisit it.
Post #91 put the caveat in the right place and I want to underline it.
Lean mass in these measurements includes water, so a change in hydration appears as a change in lean mass.
Noting that I have skin in this question and have tried to discount for it.
Preserving lean tissue during a substantial deficit depends on protein intake and a resistance stimulus, and the evidence for both is reasonably strong.
What I would tell a new member reading about DXA precision for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.
Where I part company with post #95, and it is a narrow parting.
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.
Flagging that the sources on this are thinner than the confidence in the thread suggests.
Post #99 is the version of this I will quote in future. One addition.
DXA precision looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
Coming back to post #99, because the follow-up matters more than the original answer.
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 is the distinction I keep failing to hold on to. Written down now.
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.
Not a conclusion. A place to stand while looking for one.
Visceral fat: DXA does not distinguish visceral (organ-associated) fat from subcutaneous fat. That distinction might matter but requires different imaging to measure.
I have written this out at length because the short version keeps being misread.
I read post #103 twice before replying, because I had assumed the opposite.
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.
Not a strong opinion, just a consistent one.
Post #103 answers the question as asked. The question underneath it is different.
Second-hand on DXA precision, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.
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?"
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.
Where a facility reports a precision figure for its own machine, that number is more useful than any published average.
I would rather post the uncertainty than round it away.
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.
Worth separating two things that post #108 runs together.
Air displacement plethysmography and hydrostatic weighing have their own precision limits and their own protocol sensitivities. None of these are exact.
Marking that as an opinion rather than a finding.
This topic was referenced in
- Interpreting a composition change that is within the error barsOutcomes › Body composition · 22 replies
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