Vitamin D is one of the most commonly taken supplements in the U.S., consumed by an estimated one-third of people age 60 and older. For decades, doctors have touted the vitamin’s ability to help the body absorb and retain calcium and phosphorus, which help bone growth, reduce cancer cell growth, and control infections.
But a growing body of evidence shows that vitamin D doesn’t provide the other benefits doctors once thought. A large, new study published last week in the New England Journal of Medicine finds vitamin D (taken with or without calcium) has no effect on bone fracture rates, even for participants with low levels of vitamin D.
This news follows results from the same study finding that vitamin D does not significantly lower the risk of cardiovascular events or cancer. Data also show it does not prevent cognitive decline or reduce the risk of falling in older adults. An editorial about the findings makes a stark recommendation: vitamin D supplementation is not necessary for healthy adults.
To understand how we arrived at such ubiquitous supplementation, it helps to understand the three main ways that people obtain vitamin D: by eating foods rich in vitamin D, by ultraviolet rays from the sun reaching the skin, and by taking a supplement. In recent decades, doctors have worried that people living in northern latitudes don’t absorb enough sunlight to prompt their bodies to manufacture their own vitamin D, so vitamin D testing and supplementation became commonplace.
This recent study, called VITAL, involved more than 25,800 participants, including more than 13,000 women. Participants were either given vitamin D at a dose of 2,000 international units (IUs) or a placebo pill, and then reported health outcomes.
VITAL was funded by the federal government after the U.S. Institute of Medicine convened a panel of medical experts in 2011 to take a careful look at the benefits of vitamin D supplements. The panel concluded there was not enough evidence to prove that vitamin D supplements improved health.
What about previous evidence that vitamin D reduces the risk of dying from cancer? A systematic review published in the British Medical Journal reported that vitamin D supplements did reduce the risk of cancer death by 15 percent. The review included 52 randomized-controlled trials with a total of more than 75,400 participants.
The reduction in deaths from cancer occurred among participants taking supplements of vitamin D3, a type found only in animal-sourced foods, such as oily fish, egg yolks, and butter. This is the type of vitamin D that the human body produces naturally when exposed to sunlight. The health benefits of the study did not hold true for participants taking supplements of vitamin D2, a type found in mushrooms and most fortified foods. (As a side note, vitamin D2 is less expensive to produce and is more common in foods fortified with vitamin D.)
The take-home message: Vitamin D supplements are unlikely to prevent bone fractures later in life. There is conflicting evidence about whether vitamin D helps reduce the risk of dying from cancer, but it’s unlikely to help prevent other medical problems.
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