Adding the clinical framing, because it changes how the question reads. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.
OP back with an update, since a thread like this is useless without one.
Closing this out. What actually changed my mind was the point about waiting for a trend instead of reading a single measurement, which is embarrassing in hindsight and worth saying anyway.
NurseLeah_Nash said:The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either.
Adding the part of the answer the thread has not reached. Worth answering the question that was asked rather than the one behind it. The narrow version usually has an answer; the broad version usually does not, and answering the broad one is how a thread stops being useful.
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Browse GL BiochemAdding the numbers, since they settle part of this. One habit that pays for itself: post the method alongside the number. A figure without its method cannot be checked, and an unchecked figure is how this community accumulates folklore.
Following on from NurseLeah_Nash — and this may be the naive question:
How would you tell the difference between that and the alternative explanation?