Feb. 3, 2014 -- Vitamin D had been gaining a reputation as a ''wonder supplement." Studies have suggested it can help bone and heart health, ease mild depression, and lower the risk of cancer. Others have suggested it might help people with fibromyalgia, multiple sclerosis, and other chronic diseases.
Now comes a different finding. Researchers who looked at dozens of studies say that vitamin D supplements do not lower the risks of heart attacks, strokes, cancer, or fractures by more than 15% in generally healthy people. This was true whether or not the supplements included calcium.
Bottom line: For most healthy adults, vitamin D supplements are not worth it, the researchers say in The Lancet Diabetes & Endocrinology.
Not everyone agrees, and the debate is far from done. Here, two experts address the most common questions about vitamin D supplements.
Are vitamin D supplements losing their luster?
"I believe so," says Doug Campos-Outcalt, MD, of the University of Arizona College of Medicine, Phoenix. He recently wrote a review of vitamin D for The Journal of Family Practice.
Evidence shows that vitamin D helps bone health, he says. But early studies that show vitamin D may help in other areas, such as heart health and cancer prevention, are not convincing.
"Information on the health benefits of vitamin D is difficult to sort out," he writes in the review. He cites a report from the Institute of Medicine, an independent organization that provides health advice. The institute looked at studies of vitamin D to protect against cancer, heart disease, diabetes, and autoimmune diseases such as lupus. Except for bone health, it found no evidence that vitamin D helped with any other diseases.
Robert R. Recker, MD, director of osteoporosis research at Creighton University School of Medicine in Nebraska, disagrees. He cites research finding vitamin D lowers the risks of colon, breast, and other cancers, and improves how the immune system works.
On the other hand, other experts say low vitamin D levels may be a result of illness, not the cause.
What do we know for sure about vitamin D?
What it does: Experts agree on the basics. Vitamin D helps the body absorb calcium, and that is good for bone health. Vitamin D also helps reduce inflammation in our cells. Inflammation can trigger disease.
What are the main areas of disagreement about Vitamin D?
How much is needed: At the center of the debate is how much vitamin D is enough. "We need more vitamin D than what we are getting [from diet and sun exposure]," Recker says. "What is not agreed upon is how much more."
The Institute of Medicine recommends that most Americans need no more than 600 international units (IU) of vitamin D a day. People 71 and older may need 800 IU, it says. This level is enough for bone health, it says.
Vitamin D is found in some foods, including fatty fish like salmon and tuna, beef liver, fortified dairy products, cheese, and egg yolks. Except for those, getting enough vitamin D from your diet isn’t easy. As examples, a 3-ounce serving of salmon provides 447 IU, and 3 ounces of tuna fish offer 154 IU.
Meanwhile, our skin makes vitamin D when exposed to natural sunlight. This helps vitamin D levels in our blood. But Recker says only people who live at the equator get a large amount of D from sunlight.
Testing: Experts disagree on whether healthy people need routine testing to detect low vitamin D blood levels.
How much is enough: Experts also disagree on how much vitamin D we need in our blood to be healthy.
Which groups of people might benefit more from higher levels of D?
Older adults who are frail, Campos-Outcalt says. Getting 800 IU a day may help them prevent falls and fractures.
Recker says older people who are healthy can also benefit from the higher levels, ''because the skin loses the ability to make vitamin D" as people age. Some older people also stay indoors more as they age, he says.
Other people may also need to pay close attention to vitamin D in their foods. Among them are people on corticosteroids and other medications that can affect bone health, Recker says.
What are the potential harms of excess vitamin D supplements?
Very high doses of vitamin D can cause extremely high levels of calcium in your blood, which can lead to heart rhythm problems, kidney stones and damage, and severe muscle weakness. This calcium excess usually happens if you take 40,000 IU per day for a couple of months or longer, or take a very large one-time dose.