Don Matesz at Primal Wisdom put up a post a few days ago that I think is worth reading. It follows an e-mail discussion between us concerning a paper on magnesium restriction in rats (executive summary: moderate Mg restriction reduces the hormone form of vitamin D by half and promotes osteoporosis). In his post, Don cites several papers showing that vitamin D metabolism is influenced by more than just vitamin D intake from the diet and synthesis in the skin.
Celiac disease patients have low 25(OH)D3, the circulating storage form of vitamin D, which spontaneously corrects on a gluten-free diet. There are numerous suggestions in the medical literature that overweight and sickness cause low vitamin D, potentially confounding the interpretation of studies that find lower levels of illness among people with low vitamin D levels.
Don't get me wrong, I still think vitamin D is important in preventing disease. But it does lead me to question the idea that we should force down huge doses of supplemental vitamin D to get our 25(OH)D3 up to 60, 70 or even 80 ng/mL. When the dosage of supplemental D goes beyond what a tan Caucasian could conceivably make on a day at the beach (4,000 IU?), that's when I start becoming skeptical. Check out Don's post for more.