TomTeleRx said:

A sustained drop to 1,100 or 1,200 calories reliably under-delivers a predictable set: B12, vitamin D, iron, folate, zinc and magnesium.

Vitamin D levels and micronutrients: an often-overlooked connection. Obese individuals store vitamin D in adipose tissue, making it less bioavailable. As you lose fat, vitamin D can actually increase WITHOUT supplementation.

My trajectory: Baseline 18 ng/mL → Month 6 (with supplement) 42 ng/mL → Month 12 48 ng/mL. Target is 40-60 ng/mL. Adequate vitamin D supports immune function, bone health, and mood — all relevant during weight loss.

A narrower follow-up, since the general answer is now clear:

Was that from a primary source or from a summary of one?

quinn_sf said:

Vitamin D levels and micronutrients: an often-overlooked connection.

There is a second half to this that has not been said yet. The mechanism and the magnitude are separate questions. Agreeing that something happens says nothing about whether it happens enough to act on.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

Closing the loop on my own question.

Reporting back with a partial result, because a partial result is still better than silence. Two of the three things suggested helped; I cannot separate them and I am not going to pretend otherwise.

claudia_zurich said:

The mechanism and the magnitude are separate questions.

That is right, and it stops being right at the edges. The general case is well behaved; the interesting cases in this thread are all at the boundary where the general case breaks.