Taking vitamin D, along with calcium supplements, may reduce your risk of breaking a bone, but there's not yet enough evidence to say whether it may lower your risk of cancer, a new analysis concludes.
People who were taking vitamin D and calcium supplements were 11 percent less likely to fracture a bone than people not taking the supplements, according to the study.
There was an even larger reduction in fractures — about 30 percent — among elderly people living in institutions who were taking vitamin D, said study researcher Mei Chung, a nutritional epidemiologist and assistant director of the evidence-based-practice center at Tufts Medical Center.
Chung's analysis was requested by the U.S. Preventive Services Task Force, and was the only study the group requested be done in advance of their draft statement on recommendations for vitamin D intake, set to be issued in January, she said.
As to the studies examining the vitamin's role in cancer prevention, "We just don't have good enough information," Chung said, and factors such as how much vitamin D people were getting in their diets, and how much sun exposure they got were not well-controlled in the previous studies she reviewed for her analysis.
The results of the new study are published online today (Dec. 19) in the journal Annals of Internal Medicine.
Vitamin D in the body
Vitamin D is present in very few foods, though some foods are fortified with it, according to the National institutes of Health. Fish such as salmon and tuna and egg yolks are good sources of it. It's also synthesized by the skin when we're exposed to ultraviolet rays in sunlight.
Research has shown that vitamin D is involved with the depositing of mineral in bone, Chung said, but its potential role in cancer is less clear. Some research has suggested it may promote cell division and other processes that may lead to cancer, but other work has shown it may have the opposite effect, she said. The vitamin's effects may vary across different parts of the body — it could promote some cancers, but inhibit the development of others.
Chung's analysis included 19 studies examining the effects of vitamin D on bone fractures, and 28 studies of its effects on cancer.
In terms of reducing fractures, vitamin D only reduced the risk when taken in conjunction with calcium, the study showed. The benefit was seen among people taking from 300 International Units (IU) to 1,100 IU daily, according to the study.
The studies she examined conflicted in their findings about whether vitamin D might prevent cancer, Chung said. Three of the studies were prospective randomized controlled trials — considered the strongest type of scientific evidence, in which participants are divided into two groups at the study's start and asked to either take vitamin D or a placebo — and these studies suggested that high doses of vitamin D (1,000 IU a day) may reduce cancer.
However, the levels of vitamin D in the blood of participants in those studies were not measured, Chung said, and without such measurements, conclusions cannot be drawn. Some people in the placebo group may have in fact been taking vitamin D supplements, perhaps as part of a multivitamin, and study participants could vary greatly in terms of the levels of vitamin D in their diets, and their sun exposure.
Some of the prospective, observational studies Chung analyzed — in which researchers did measure blood levels of vitamin D, and tracked those levels with cancer cases — suggested that people with higher levels may have a lower risk of colorectal cancer, but the data for breast cancer and prostate cancer were mixed, and higher vitamin D levels brought an increased risk of having any type of cancer in general. Therefore, a general conclusion about vitamin D and cancer risk could not be made, she said.
More studies needed
To better determine whether vitamin D lowers cancer risk, controlled trials are needed in which participants' blood levels are measured, and other data on diet and supplements are collected, Chung said.
Taking vitamin D along with calcium has been associated with an increased risk of kidney stones, she noted.
In the body, almost every organ has receptors for the active form of vitamin D found in our blood, Chung said, which is known to affect many cellular processes. Better understanding of the vitamin's role in all such processes would be helpful in assessing the vitamin's potential in preventing diseases, she said.
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