The evidence seems to support that higher serum levels (greater than 30 ng/mL) are associated with reduced fracture and fall risk, but that at super-high levels, vitamin D actually increases falls and fractures, said Dr. Binkley. It’s not clear why that is. In the study, women were given a megadose of 500,000 IU of vitamin D once a year; findings suggested that a big blast of the vitamin is not helpful, he said (JAMA 2010:303:1815-22).
The data show that for each additional 1,000 IU daily, the serum level goes up by about 6 ng/mL. Increases will be greater for patients who start with lower serum levels. To determine what’s optimal, it’s probably necessary to measure serum levels periodically, Dr. Binkley said.
If you write a prescription, you’re likely to get the less optimal form of vitamin D, that is, vitamin D2. Humans make vitamin D3 when they are exposed to sunlight, but only D2 is available in prescription form. Dr. Binkley said that he prefers patients to take D3, which is available over the counter. But if he’s going for a big repletion – say a typical 50,000 IU dose – he has to use the D2 form.
It is possible to have vitamin D intoxication. Most studies have never documented anyone with serum levels greater than 70 ng/mL. Patients who have levels above 70 are probably getting too much vitamin D, he said.
Equally intoxicating is the hype surrounding vitamin D, which recently has been touted as a veritable fountain of youth. Almost every tissue in the body has a vitamin D receptor and it is produced locally by many tissues. But the evidence does not support the notion that low levels are responsible for a laundry list of diseases, Dr. Binkley said. It is plausible that low vitamin D contributes to impaired immune function, diabetes, vascular disease, and a higher risk for bone loss and fractures, among other conditions. Sufficient vitamin D may also reduce cancer risk.
But so far, too many studies show association, and not causation. "We need to stay tuned," said Dr. Binkley, noting that there need to be more randomized, prospective studies and no more meta-analyses.
The IOM report "is too weak," but trying to get patients to serum vitamin D levels greater than 40 ng/mL is not responsible, Dr. Binkley stressed.
Dr. Binkley has consulting arrangements with Eli Lilly and Merck, and grants from those companies as well as Amgen and Tarsa, but none relate to vitamin D.