Dr.SleepRoch 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 NurseLeah_Nash’s answer:

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

Dr.GutHealth said:

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

Adding the part of the answer the thread has not reached. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.

Closing the loop on my own question.

For the archive: it took about six weeks rather than the two I expected, and the difference between those two numbers was the whole of my problem.

Dr.MetabolicMD said:

The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either.

Agreed, and one caution: a single measurement is not a measurement. Anything that moves day to day needs a trend before it means anything at all.

Happy to go further on any of that.