Vitamin D doesn’t prevent heart attack or cancer
Among seniors with a high risk of bone fractures, taking vitamin D or calcium pills has no impact on their chances of dying from cancer or vascular disease, researchers say in a new study.
Vitamin D is considered beneficial for bone health, and earlier studies have found that having low vitamin D levels in the blood is tied to a greater chance of dying from heart problems (see Reuters Health reports of November 25, 2011 and June 24, 2011).
The thinking, therefore, has been that taking extra vitamin D might cut that risk.
“There’s a lot of interest in vitamin D preventing heart disease and cancer, but the evidence from randomized trials is weak,” said Dr. Alison Avenell, the lead author of the study and a researcher at the University of Aberdeen in England.
In the latest study, published in the Journal of Clinical Endocrinology and Metabolism, the researchers tracked the health of nearly 5,300 people over age 70 who had had a bone fracture.
The participants were randomly divided into four groups: one took 800 IU (International Units) of vitamin D daily, the second group took 1000 milligrams of calcium each day, a third group took both supplements, and a fourth group took fake pills that looked like the supplements.
Vitamin D is essential for the formation, growth, and repair of bones and for normal calcium absorption and immune function. It is obtained primarily through exposure of the skin to ultraviolet radiation in sunlight, but it can also be obtained from some foods and dietary supplements.
Some studies suggest that higher intakes of vitamin D from food and/or supplements and higher levels of vitamin D in the blood are associated with reduced risks of colorectal cancer; however, the research results overall have been inconsistent.
Whether vitamin D is associated with reduced risks of other cancers, including breast, prostate, and pancreatic cancers, remains unclear.
The National Cancer Institute (NCI) does not recommend for or against the use of vitamin D supplements to reduce the risk of colorectal or any other type of cancer.
People in the study took the pills for two to five years, and were followed for up to three years afterward.
Among people who took vitamin D, 32 out of every 100 died during the study, while 33 out of every 100 people who did not get the supplement died. That small difference could easily have been due to chance, the researchers found.
There were no differences in deaths from cancer or heart disease either. Calcium also proved unhelpful.
A recent analysis of 50 studies on vitamin D and heart health found no impact from taking the vitamin (see Reuters Health report of July 11, 2011).
Still, Avenell said her study doesn’t provide the final answer on whether vitamin D can help stop heart disease or cancer.
Is there a role for vitamin D in reducing cancer risk?
A large number of scientific studies have investigated a possible role for vitamin D in cancer prevention.
The first results came from epidemiologic studies known as geographic correlation studies. In these studies, an inverse relationship was found between sunlight exposure levels in a given geographic area and the rates of incidence and death for certain cancers in that area. Individuals living in southern latitudes were found to have lower rates of incidence and death for these cancers than those living at northern latitudes. Because sunlight/UV exposure is necessary for the production of vitamin D3, researchers hypothesized that variation in vitamin D levels accounted for the observed relationships.
Evidence of a possible cancer-protective role for vitamin D has also been found in laboratory studies of the effect of vitamin D treatment on cancer cells in culture. In these studies, vitamin D promoted the differentiation and death (apoptosis) of cancer cells, and it slowed their proliferation.
Randomized clinical trials designed to investigate the effects of vitamin D intake on bone health have suggested that higher vitamin D intakes may reduce the risk of cancer. One study involved nearly 1,200 healthy postmenopausal women who took daily supplements of calcium (1,400 mg or 1,500 mg) and vitamin D (25 ?g vitamin D, or 1,100 IU-a relatively large dose) or a placebo for 4 years. The women who took the supplements had a 60 percent lower overall incidence of cancer (6); however, the study did not include a vitamin D-only group. Moreover, the primary outcome of the study was fracture incidence; it was not designed to measure cancer incidence. This limits the ability to draw conclusions about the effect of vitamin D intake on cancer risk.
A number of observational studies have investigated whether people with higher vitamin D levels or intake have lower risks of specific cancers, particularly colorectal cancer and breast cancer. Associations of vitamin D with risks of prostate, pancreatic, and other, rarer cancers have also been examined. These studies have yielded inconsistent results, most likely because of the challenges of conducting observational studies of diet (7). Information about dietary intakes is obtained from the participants through the use of food frequency questionnaires, diet records, or interviews in which the participants are asked to recall information about their dietary intakes. Information collected in this manner can be inaccurate. In addition, only recently has a comprehensive food composition database with vitamin D values for the U.S. food supply become available. Other dietary components or energy balance may also modify vitamin D metabolism (8).
Measuring blood levels of 25-hydroxyvitamin D to determine vitamin D status avoids some of the limitations of assessing dietary intake. However, vitamin D levels in the blood vary by race, with the season, and possibly with the activity of genes whose products are involved in vitamin D transport and metabolism. These variations complicate the interpretation of studies that measure the concentration of vitamin D in serum at a single point in time.
Finally, it is difficult to separate the effects of vitamin D and calcium because of the complicated biological interactions between these substances. To fully understand the effect of vitamin D on cancer and other health outcomes, new randomized trials will need to be carried out (9). However, the appropriate dose of vitamin D to use in such trials is still not clear (10).
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“People often stopped taking their tablets, so we might not have had enough people taking tablets to find effects,” Avenell wrote in an email to Reuters Health. “The dose of vitamin D might not have been high enough.”
Peggy Cawthon, a researcher with the California Pacific Medical Center Research Institute who was not involved in the new work, said people should be cautious regarding information on vitamin D’s alleged heart and cancer benefits.
“A supplement or vitamin might not have the magic bullet to prevent the next disease,” Cawthon told Reuters Health. “We’ve had a lot of examples, and vitamin D is just the latest showing it has no effect on these health issues.”
Vitamin D is formed in the skin when it is exposed to sunlight. Though higher levels of the molecule are linked to better heart health, it could be that the vitamin D is only a sign of general health, and not something that actually improves the heart’s function.
“My thought is that people who are healthier get out more and would produce more vitamin D,” speculated Cawthon.
Avenell said she is looking forward to two other studies in the Unites States and the UK that will help confirm whether vitamin D has benefits beyond boosting bone strength.
SOURCE: Journal of Clinical Endocrinology and Metabolism, online November 23, 2011.