Second pass at: Circumference measurements as a poor man's composition estimate posts 61–69

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.

On post #59 — agreed on the reasoning, with one qualification.

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.

post #63 answers the question as asked. The question underneath it is different.

Visceral fat: DXA does not distinguish visceral (organ-associated) fat from subcutaneous fat. That distinction might matter but requires different imaging to measure.

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.

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.

Worth separating two things that post #63 runs together.

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

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?"