Collecting this in one place because it comes up every few weeks and the answer is always assembled from scratch. It is about micronutrient cover, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
A sustained drop to 1,100 or 1,200 calories reliably under-delivers a predictable set: B12, vitamin D, iron, folate, zinc and magnesium. Two of those are worth testing rather than guessing because the tests are cheap and the symptoms overlap with everything else — ferritin and B12. The rest a decent multivitamin covers, and there is no benefit in megadosing any of them.
The condition it depends on
The caveat that iron should be tested before supplementing, not after. Supplementing iron into normal stores is unpleasant at best and harmful in haemochromatosis.
The practical version
Cheap and worth having: ferritin, B12, vitamin D. Everything else on the deficiency list is adequately covered by a standard multivitamin at ordinary doses.
What I am not sure about
What I actually want to know is which deficiencies actually show up when intake drops this far, and which are worth testing rather than supplementing blind. I have searched first, so if this is covered somewhere point me at it and I will read it.
— kate.chem · corrections welcome and will be edited into this post with credit