More cautious public-health doctors, as well as a federal panel that updated guidelines in November, note that evidence of the vitamin D/heart-health link comes from a small number of studies. And they say megadoses are potentially risky.
Both sides agree there is no solid evidence that vitamin D supplements can prevent heart disease from developing or worsening. Since the evidence of harm from taking too much is squishy as well, for many patients a decision may come down to their comfort level with experimentation and risk.
Although the issues are complex, some statistics are tantalizingly simple. A recent Drexel University study, for example, cross-referenced vitamin D blood levels against heart-failure deaths over eight years in a national database of 13,131 adults.
After adjusting for age, sex, race and medical conditions at baseline, they found that people whose vitamin D levels were "deficient" were 3.4 times more likely than those with "normal" levels to die of heart failure prematurely (before age 75); people whose vitamin D levels were merely "insufficient" were two times more likely to die.
The critical importance of vitamin D for bone development has been clear for nearly a century, but nearly all the other potential benefits - for flu, diabetes, multiple sclerosis and numerous cancers, as well as heart disease - were discovered in the past two decades. With no records dating back to when humans lived outdoors and produced vitamin D the natural way, from sunlight, there is no consensus on how much is normal.
The Drexel researchers used common, but by no means universal, definitions for blood-serum levels measured by a 25-hydroxyvitamin D test: "Deficient" was less than 20 ng/mL, "insufficient" was 20-29 ng/mL, and "normal" was 30 ng/mL or more.
By those definitions, more than 55 percent of whites in the national sample were below normal, including 20 percent who were deficient; among blacks, nearly 90 percent were below normal, including more than 60 percent deficient.
Blacks have far higher rates of heart disease than whites. Still, a correlation does not prove cause and effect; some third factor could play a role in both heart failure and vitamin D levels.
"And we have no evidence that treating them [with vitamin D] offers any benefit," said Howard Eisen, chief of cardiology at Drexel's College of Medicine.
Eisen led the study and presented the findings in September at the Heart Failure Society of America's annual meeting in San Diego. Yet he has not changed his practice at Hahnemann University Hospital - ordering vitamin D tests, for example, only for heart patients with conditions for which the link is clearer, such as osteoporosis.
Vitamin D, which is actually a hormone, has recently been found to regulate thousands of genes in different ways, offering several potential mechanisms for an effect on the heart. A key one is reducing inflammation. It also tends to inhibit deposits that stiffen arteries, and it improves insulin sensitivity. So it could play a role in various cardiovascular conditions.
When doctors at the Intermountain Medical Center in Utah searched their records for patients who had been tested for vitamin D between 2000 and 2009, they found strong correlations between lower blood-serum levels and greater likelihood of being diagnosed with coronary artery disease, heart failure, atrial fibrillation and peripheral vascular disease, as well as having had a previous heart attack and stroke.
Conditions considered risk factors for heart disease - hypertension, diabetes, high cholesterol - were similarly related, they reported in October in the American Journal of Cardiology.
Although this study also could not prove cause and effect, lead author Jeffrey Anderson said he has seen enough evidence of benefit for overall health that he recommends his patients take 1,000 to 2,000 IU of vitamin D a day, which is higher than new federal recommendations.
"Supplementation may not be universally accepted for CV health," said Anderson, associate chief of cardiology at the hospital and a professor of medicine at the University of Utah, "but benefit to bone health of normal levels generally is accepted, and any CV benefit should be frosting on the cake."
The federal recommendations issued in November tripled outdated guidelines for supplements to 600 IU a day for ages 1 through 70, and 800 IU for older people. But they also set upper limits of 4,000 IU for everyone over 8.
The committee of outside scientists that wrote the guidelines for the Institute of Medicine, part of the National Academy of Sciences, was openly skeptical of vitamin D hype. Their 1,000-page report noted the lack of agreement on vitamin D "insufficiency" and said that most Americans probably get enough.
And they said there was no conclusive evidence of benefit for cardiovascular disease because there has been no examination specifically for heart conditions that meets the standard for medical research.
"We have to be cautious," said JoAnn Manson, a Harvard Medical School professor and member of the Institute of Medicine panel. She cited past "cautionary tales" of supplements - vitamin C, vitamin E, selenium - that became popular based on promise but turned out to have no benefits, and even had risks, after intensive study.
Manson is principal investigator for just such a study for cardiovascular disease and cancer. Designed to test two supplements, it is enrolling 20,000 people who will be placed in one of four groups - vitamin D, fish oil, both and placebos - and will follow them for five years.
Manson said that some earlier research has suggested a U-shaped curve, with increased risk when vitamin D levels are either too low or too high. Study participants will be given 2,000 IU a day. The dose is projected to raise blood levels to between 32 ng/mL and 36 ng/mL - high enough to tease out a benefit suggested by earlier studies but low enough to stay within federal guidelines even if people get additional vitamin D from multivitamins and the few dietary sources, mainly fatty fish and fortified dairy products.
Until this and other research provides more evidence, Manson does not recommend that cardiologists even screen for vitamin D unless their patients have known risk factors for deficiency, including advanced age, obesity, lack of sun exposure and dark skin.
Meanwhile, her study is seeking healthy men (age 60 and over) and women (65 and over), regardless of where they live. For information, go to www.vitalstudy.org or call 800-388-3963.
Contact staff writer Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.