Most people in the research the IOM assessed had what the group considered sufficient levels of 25-hydroxyvitamin D (50 nanomoles per liter as measured in the U.S. and 20 nanograms per mole as measured in Canada) despite not meeting daily intake recommendations. This incongruity suggests that despite increased caution about sun exposure to avoid skin cancer, people are still getting a substantial amount of vitamin D from natural synthesis. To be safe, however, the committee made their intake suggestions based "on the assumption of minimum sun exposure," Ross said.
Hanley worked with researchers from Osteoporosis Canada who published recommendations this summer in the Canadian Medical Association Journal, and they recommended 400 to 1,000 IU per day for optimum bone health in those 50 and younger. But the two recommendations fall within a comfortable range of one another. "We're not talking about major differences here," Hanley says. "These recommendations would be an improvement on what we have now," he notes. "Where the argument might come in is: What's optimal for bone?" His group's recommendations are likely higher because they were focused on bone health—as well as a population that generally receives less sunlight in winter months.
Despite having assessed 1,000 studies and reports and "listened to testimony from scientists and stakeholders" the IOM committee found that most published health benefits (aside from bone strength) provided "mixed and inconclusive results and could not be considered reliable," according to the report, which was commissioned by the U.S. and Canadian governments.
Evidence that vitamin D plays a role in fending off some cancers or preventing cardiovascular disease has emerged "largely through association, identified through population studies and not through controlled trials," Hanley says. "So it's not high-level evidence." Nevertheless, he notes, "it's probably true that vitamin D is important in many body systems" Hanley points out that some studies have uncovered a link between very high levels of 25-hydroxyvitamin D and pancreatic cancers, whereas other studies have found very low levels of the vitamin associated with the cancer. "None of the findings in relationship to cancer—both benefit and harm—are consistent," he says.
It may well be a matter of research catching up to biology. To really home in on the ideal levels of vitamin D, Hanley recommends studies that follow people deficient in vitamin D who are randomly assigned to a supplement or placebo group. "What were missing were studies of cause and effect," Ross explained.
Too much of a good thing
The IOM conclusions at least may stop those who may be getting excessive vitamin D and calcium via supplements. Too much vitamin D hurts the heart and kidneys, and too much calcium can lead to kidney stones. Other side effects include increased risk of death from any cause as well as increased risk of fractures, falls and possibly some cancers, the authors of the report noted in a summary.
So one of the committee's changes was trying to figure out "how much is too much" of these crucial nutrients, Ross explained. Previous evidence has shown definitive increase in harm for those taking more than 10,000 IU of vitamin D a day, but "we took a cautious approach" in assigning new upper limits, Ross said.
Nevertheless, the upper limit of 4,000 IU/day of vitamin D should not be a goal, Jones noted. "We don't recognize any benefit from taking a dose" between the recommended daily allowance and the top levels considered safe, he said.
Ross noted that the new recommendations will "have a shelf life of many years" in terms of setting health recommendations—influencing everything from nutritional labeling to school lunch composition.
In the meantime, there is "an urgent need" to standardize blood tests, Ross said. Currently, one person might be declared vitamin D deficient or sufficient depending on which test they are given. And for now, some experts recommend scaling back on the amount of screening for vitamin D deficiency. Such tests "probably should not be part of routine care," Steven Clinton, a professor of hematology and oncology at the Ohio State University and review committee member, said at the briefing. In the next decade, as tests become more consistent and more studies are done, he said, doctors will likely be able to provide patients personalized recommendations for vitamin D and calcium intake based on their family history and genetic profile.
But many more tests and trials remain to be done, not only to zero in on optimum doses of the nutrients, but also to explore their role in the human body. "As a basic scientist, I am still very excited about this molecule, which regulates hundreds—maybe thousands—of genes in the body," Jones said.