[kpfleger]

Nonesense. It's well established that dietary D3 (or D2, but D3 is better) clearly incleases 25(OH)D serum levels. It is that serum biomarker whose deficiency (levels below 20ng/ml or 30ng/ml) that has the largest association with higher risk of dozens of diseases as well as all-cause mortality. You can defend your phrasing by saying that "not well" just means one has to take a lot of it. Yes, there is a wide variety of dose response, which is why it's best to test blood levels to titrate supplementation amount.

[elric]

Is the blood level diagnostically significant with vit D? Or is this something like magnesium where serum levels can be normal while having a magnesium shortage? IIRC vit d is fat soluble, does the body store any excess quickly, or does it linger around?

[canucker2016]

Yes, from https://pmc.ncbi.nlm.nih.gov/articles/PMC5577589/

  The goal of this study was to investigate whether the relationship between body composition, serum 25-hydroxyvitamin D (25OHD), vitamin D in subcutaneous (SQ) and omental (OM) adipose, and total adipose stores of vitamin D differ among OB and C. ... In summary, although OB had significantly greater total vitamin D stores than C, the relationship between serum 25OHD and fat vitamin D and the overall pattern of distribution of vitamin D between the OM and SQ fat compartments was similar.