emma_london 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.

Iron studies on micronutrients — important especially for women:

My ferritin cratered at month 6 from reduced dietary intake. Iron bisglycinate 36mg with vitamin C brought it back up. Don't wait for anemia symptoms — test proactively.

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

What did you change at the same time, and can you separate the two now?

PeptideSynthNJ said:

Iron studies on micronutrients — important especially for women: Marker Baseline Month 6 After Supplementation Ferritin 69 27 49 Iron 99 54 97 TIBC…

Coming at PeptideSynthNJ’s question from a different direction. Start from the measurement rather than the conclusion. Almost every disagreement here turns out to be two people measuring different things and comparing the numbers anyway.

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.

Dr.PathRoch said:

Start from the measurement rather than the conclusion.

Dr.PathRoch has the substance of this right. The condition it depends on is worth stating. Yes, and the corollary is less popular: if the reasoning holds, it also holds where it produces an answer we do not like.