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 not know what this instrument's least significant change actually is.
How much difference does there have to be before it is a difference?
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.
I would put the burden of proof on the interesting explanation, not the dull one.
Adding the measurement that post #4 says would settle it.
Where a scan disagrees with the mirror over a short period, the scan's precision limits are usually the explanation.
On DXA precision I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
Neither method is capable of detecting the month-to-month changes that people report detecting month to month.
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.
The answer changed when I changed how I was measuring, which was informative.
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.
Adding it in case it saves somebody the afternoon it cost me.
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.
Useful. I have added it to my own notes with the date on it.
Adding the measurement that post #12 says would settle 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.
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 practical version. The rigorous version is longer and says the same thing.
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.
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.
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.
Lean mass in these measurements includes water, so a change in hydration appears as a change in lean mass.
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.
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.
Distinguishing three things in the DXA precision discussion that keep getting used interchangeably: the observation, the proposed mechanism, and the recommendation that gets attached to both.
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.
A qualification I should have led with rather than closed on.
DXA precision is worth one more sentence than it usually gets, and the sentence is the one about how the number was arrived at.