Experts say the results shouldn't be seen as a reason to start popping vitamin D pills or to spend hours in the sun, which is the main source for vitamin D.
For one thing, megadoses of vitamin D pills can be dangerous and skin cancer risks from too much sunshine are well-known. But also, it can't be determined from this type of study whether lack of vitamin D caused the deaths, or whether increasing vitamin D intake would make any difference.
Low vitamin D levels could reflect age, lack of physical activity and other lifestyle factors that also affect health, said American Heart Association spokeswoman Alice Lichtenstein, director of the Cardiovascular Nutrition Laboratory at Tufts University.
Still, she said the study is an important addition to an emerging area of research.
"This is something that should not be ignored," Lichtenstein said.
The study led by Austrian researchers involved 3,258 men and women in southwest Germany. Participants were aged 62 on average, most with heart disease, whose vitamin D levels were checked in weekly blood tests. During roughly eight years of follow-up, 737 died, including 463 from heart-related problems.
According to one of the vitamin tests they used, there were 307 deaths in patients with the lowest levels, versus 103 deaths in those with the highest levels. Counting age, physical activity and other factors, the researchers calculated that deaths from all causes were about twice as common in patients in the lowest-level group.
Results appear in Monday's Archives of Internal Medicine.
The study's lead author, Dr. Harald Dobnig of the Medical University of Graz in Austria, said the results don't prove that low levels of vitamin D are harmful "but the evidence is just becoming overwhelming at this point."
Scientists used to think that the only role of vitamin D was to prevent rickets and strengthen bones, Dobnig said.
"Now we are beginning to realize that there is much more into it," he said
Exactly how low vitamin D levels might contribute to heart problems and deaths from other illnesses is uncertain, although it is has been shown to help regulate the body's disease-fighting immune system, he said.
Earlier this month, the same journal included research led by Harvard scientists linking low vitamin D levels with heart attacks. And previous research has linked low vitamin D with high blood pressure, diabetes and obesity, which all can contribute to heart disease.
The new research "provides the strongest evidence to date for a link between vitamin D deficiency and cardiovascular mortality," said Dr. Edward Giovannucci of the Harvard study of 18,225 men.
Low vitamin D levels also have been linked with several kinds of cancer and some researchers believe the vitamin could even be used to help prevent malignancies.
It has been estimated that at least 50 percent of older adults worldwide have low vitamin D levels, and the problem is also thought to affect substantial numbers of younger people. Possible reasons include decreased outdoor activities, air pollution and, as people age, a decline in the skin's ability to produce vitamin D from ultraviolet rays, the study authors said.
Some doctors believe overuse of sunscreen lotions has contributed, and say just 10 to 15 minutes daily in the sun without sunscreen is safe and enough to ensure adequate vitamin D, although there's no consensus on that.
Diet sources include fortified milk, which generally contains 100 international units of vitamin D per cup, and fatty fish — 3 ounces of canned tuna has 200 units.
The Institute of Medicine's current vitamin D recommendations are 200 units daily for children and adults up to age 50, and 400 to 600 units for older adults. But some doctors believe these amounts are far too low and recommend taking supplements.
The American Medical Association at its annual meeting last week agreed to urge a review of the recommendations.
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