Posting this under the heading it deserves: Ultrasound and skinfolds: operator dependence in practice — what changed since Everything below is what sits behind that.
What changes if the standard account of Ultrasound and skinfolds is wrong? I ask because I have been treating it as settled and I noticed this week that I could not say why.
Working through the consequences rather than the evidence, since others here are better placed on the evidence.
Small correction to my own earlier position on Ultrasound and skinfolds. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
The opening post is the version of this I will quote in future. One addition.
The confident answers on Ultrasound and skinfolds and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
Before the thread moves on from Ultrasound and skinfolds — what is the sample size behind the claim? I am not being difficult; I have seen the same figure quoted from an n of four and from an n of four hundred.
Answering the question post #2 raises rather than the one it answers.
Bioimpedance is considerably less precise and is strongly affected by hydration state, which is why readings move overnight without anything having changed.
I would not lead a decision with this, but I would not ignore it either.
Building on post #4 rather than restating it.
I keep a log for Ultrasound and skinfolds specifically because my memory of it turned out to be systematically wrong in one direction. Six weeks of notes cost nothing and settled it.
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.
For what it is worth, the same held on the two occasions I checked.
Narrowing post #8, because the general version has more than one answer.
Ultrasound and skinfolds is one of those subjects where the general answer and the answer for a specific case diverge, and the thread will go in circles until someone says which one is being asked for.
This settles it for me, at least until somebody posts a reason it should not.
On post #9 — agreed on the reasoning, with one qualification.
Neither method is capable of detecting the month-to-month changes that people report detecting month to month.
The rule of thumb is fine; the edge cases are where it earns its keep.
Picking up post #9: that is the part I would want checked first.
Where I have landed on Ultrasound and skinfolds, having got it wrong once in public: the direction is clear, the magnitude is not, and anyone quoting a precise magnitude has borrowed it from somewhere that did not measure it.
The question underneath Ultrasound and skinfolds is usually "how would I tell?" rather than "what is true?", and that one has a method attached to it.
Write down what you would expect to see under each hypothesis before you collect anything. If they predict the same observation, collecting it will not help.
Bone mineral density changes with substantial weight loss are measurable and slow, and are one of the better reasons to have a baseline scan.
Post #13 describes the usual case. This is about the unusual one.
Adding the boring version of Ultrasound and skinfolds, 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.
The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.
Useful. I had the fact and not the reason, which turns out to be the important half.
For anyone finding this later: the short answer on Ultrasound and skinfolds is that it depends on one thing, and the rest of the thread is people identifying which thing.
Practical note on Ultrasound and skinfolds: write down what you expect before you look. The number of times I have found what I went looking for is higher than chance would allow.
Bone mineral density changes with substantial weight loss are measurable and slow, and are one of the better reasons to have a baseline scan.
Ultrasound and skinfolds came up in a thread eighteen months ago and was answered well. I cannot find it, which is itself the problem, so here is the reconstruction.
A note on how Ultrasound and skinfolds gets discussed rather than on Ultrasound and skinfolds itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
I had written a reply contradicting post #20 and deleted it. Here is what survived.
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.
I am confident about the direction and much less about the magnitude.
On Ultrasound and skinfolds 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.
Reporting rather than recommending, on Ultrasound and skinfolds. What happened is above. Whether it should have is a different question and not one I am qualified to answer.
Where I part company with post #24, and it is a narrow parting.
Photographs under fixed lighting and posture are a genuine record and are the hardest of these to standardise.
Flagging that the sources on this are thinner than the confidence in the thread suggests.
Offering a way to settle Ultrasound and skinfolds rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
Worth separating two things that post #28 runs together.
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.
Filing this under things that are true until someone shows me otherwise.