One thing that is still open after traveltech_sara’s answer:

What actually distinguishes 503A from 503B, in terms of what each may make and from what starting material?

dave_SLC said:

Worth stating the units and the reference range whenever you post a number here.

Pushing back on dave_SLC here. This treats a plausible mechanism as a demonstrated one. Plausible is where you start looking, not where you stop.

dave_SLC said:

Worth stating the units and the reference range whenever you post a number here.

Agreed, with the usual condition: that holds for the average and this board is a collection of individuals. A population claim and a personal prediction are different things and get quoted interchangeably.

That is the short version; the long version is somebody else's post.

dave_SLC said:

Worth stating the units and the reference range whenever you post a number here.

This answered a question I did not know how to ask. Adding it to my notes with a link back to this thread.

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.