Health professionals often recommend vitamin D to help with a variety of ailments because of previous research that found it can improve bone health and lower the risk of developing cancer, heart disease and more.
But new research published this week in Annals of Internal Medicine suggests that while vitamin D is vital to health, the appropriate amount is up for debate, since data from existing studies are inconclusive. Before experts can make recommendations about what the proper dosages of are for optimum health, more consistent studies are needed.
In one research review, the authors analyzed 19 clinical trials and 28 observational studies to determine the effects of vitamin D with and without calcium supplementation on the risk of cancer and fractures in adults.
"Vitamin D and calcium supplementation reduced the risk of total fractures, but this effect seems to be largest among institutionalized elderly than among older adults in the community," said lead author Mei Chung, assistant director of the Tufts Evidence-based Practice Center at Tufts Medical Center in Boston. They also found that vitamin D alone did not reduce risk.
The studies they reviewed found that vitamin D doses between 300 international units (IU) per day and 1,100 IU per day combined with calcium doses between 500 to 1,200 milligrams per day for older adults, but Chung explained that range is very wide and more research is needed before it can be narrowed down.
Chung and her colleagues also said they cannot make any conclusions based on existing data about how much vitamin D will help reduce cancer risk.
"Direct evidence from RCTS [randomized controlled trials] of vitamin D (with or without calcium) supplementation on cancer outcomes is limited and does not agree with data from observational studies," they wrote.
Experts not involved with the research said the relationship between vitamin D and bone health is the only definitive one.
"All other outcomes -- heart disease, immune function, cancer -- the data are not sufficient to establish an optimal dose," said Dr. Steven Clinton, professor of internal medicine at Ohio State University in Columbus. "Different cancers, for example, can have different relationships to vitamin D. There isn't any real evidence of what the right dose is to lower risk."
Chung's research was used by the U.S. Preventive Services Task Force to draft recommendations on vitamin D, which will be available for public review in January.