With millions of Americans taking daily vitamin D supplements to prevent cancer and bone fractures, a federal task force announced Tuesday that there’s “insufficient evidence” to recommend that Americans take these supplements for such purposes, but some experts disagree with that assessment.
After reviewing the latest research, the US Preventive Services Task Force, an independent panel of experts, concluded in a draft recommendation that taking vitamin D and calcium to maintain sturdy bones or ward off cancer isn’t warranted and that side effects such as kidney stones need to be weighed against any uncertain benefits. Last month, the same panel of experts recommended in favor of vitamin D supplements to prevent falls in seniors at high risk of falling.
“Vitamin D remains an essential part of a healthy diet,” said task force member Dr. Kirsten Bibbins-Domingo, an epidemiologist at the University of California, San Francisco. “This really pertains to supplementing a person’s diet with vitamin D for the purpose of preventing fractures and cancer in the future, and there’s no evidence that it helps.”
She and other task force members based that assessment on several clinical trials comparing vitamin D supplements, often taken along with calcium, against placebos. One trial, called the Women’s Health Initiative, didn’t find any bone or cancer preventive benefits in women ages 50 to 79 who took 400 international units of vitamin D per day and a 1,000 milligrams of calcium, according to the panel. That led the experts to issue a strong recommendation against postmenopausal women following that practice to prevent fractures.
But Dr. JoAnn Manson, a WHI principal investigator who heads the preventive health department at Brigham and Women’s Hospital, said while she agrees with the panel’s uncertain conclusions on whether to take vitamin D for cancer prevention, she believes the experts failed to delve into the nuances of the WHI study data to discern bone benefits. For women age 60 and older, Manson said, WHI found a significant increase in bone mineral density and reduction in hip fractures in those who took calcium and vitamin D supplements compared with those who took placebos.
(Getting a perfect read on the benefits of supplementation in the trial was tough since many women took supplements on their own or failed to take those pills assigned to them, which is why Manson said the researchers did subanalyses on those who reported being very compliant.)
“I think our study points to the importance of following the Institute of Medicine recommendations to get 600 to 800 IUs of vitamin D per day and 1,000 to 1,200 milligrams of calcium,” Manson added. “Often that can’t be achieved through diet alone.”
The task force, however, raised the issue of adverse effects from supplements, specifically an increased risk of kidney stones, which was found in the WHI trial. Kidney stones occurred in 2.1 percent of women who didn’t take supplements compared with 2.5 percent of those who did. “Of 273 post-menopausal women taking vitamin D plus calcium each year to prevent fractures, 1 will have a kidney stone as a result of this supplementation,” said Bibbins-Domingo.
Manson acknowledged these risks, but said they primarily stemmed from women taking too high a daily dose of calcium, which is the primary culprit of kidney stones. “More isn’t necessarily better when it comes to calcium and vitamin D supplements,” she said. “Many women take 1,200 to 1,500 milligrams a day of calcium and if you add in what they get from their diet, they can easily reach 2,000 milligrams a day, which is the tolerable upper limit.”
Vitamin D increases the absorption of calcium in the body, which can increase the risk of kidney stones, particularly in those exceeding the calcium RDA of 1000 to 1200 mg for adults. While the task force didn’t mention other adverse effects of excess calcium, Manson said recent research indicates it could increase the risk of calcification in the arteries, raising the risk of heart problems.
The ideal dose of vitamin D -- and whether it exceeds what most Americans get in their diet and manufacture through sun exposure -- remains unknown. Manson is conducting a clinical trial in 20,000 healthy people to see whether taking 2,000 IUs a day of vitamin D for five years will lower their risk of developing cancer, heart disease, and stroke, but results for that aren’t expected until 2017.
“There are several areas where we need more studies,” said Bibbins-Domingo, a sentiment echoed in the task force recommendations, which found that “the current evidence is insufficient to assess the balance of the benefits and harms of vitamin D supplementation, with or without calcium” for cancer and bone fractures.
The final recommendation, slated to be issued later this year, could be altered after public comments are submitted and reviewed.