Dr.SportsMedIN 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.

I read this differently from Dr.SportsMedIN, on substance rather than tone. Pushing back on the consensus forming here. Consistency is not the same as correctness, and a board that agrees with itself quickly is usually agreeing with the first person who sounded certain.

The figures, for anyone assembling their own picture. Cheap and worth having: ferritin, B12, vitamin D. Everything else on the deficiency list is adequately covered by a standard multivitamin at ordinary doses.

InsuranceTom said:

Pushing back on the consensus forming here.

Adding the part of the answer the thread has not reached. Most of what circulates confidently in this community traces back to one summary of one study, and the qualifier was dropped somewhere in the third retelling.

Following on from BenResearch_OR — and this may be the naive question:

Which deficiencies actually show up when intake drops this far, and which are worth testing rather than supplementing blind?

Closing the loop on my own question.

Update, and it is the boring outcome: nothing was wrong. The measurement I was reading had moved less than its own week-to-week variation, which somebody said upthread and I did not believe.