Dr.Martinez 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.

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

How would you tell the difference between that and the alternative explanation?

PeptideChemSF said:

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

There is a second half to this that has not been said yet. The useful move here is to separate what is established from what is widely repeated. Those two sets overlap less than the confident tone of most write-ups suggests, and the second set is where nearly all the disagreement on this board comes from.

Closing the loop on my own question.

Update: the thing I was blaming turned out not to be the cause. Leaving the original post as written rather than editing it, so the mistake stays visible for whoever searches this next.

lisa_labSD said:

The useful move here is to separate what is established from what is widely repeated.

That is correct as far as it goes, and here is where it stops going. Yes, and the corollary is less popular: if the reasoning holds, it also holds where it produces an answer we do not like.