Most dietary supplements do nothing to protect against heart attacks, stroke or deaths from heart disease, and some may even cause harm, according to a newpaper published Monday in the Annals of Internal Medicine.
Researchers at West Virginia University analyzed data on the effects of dietary supplements and dietary approaches from 277 previous studies that included almost one million people in total.
They found scant evidence that supplements improve cardiovascular outcomes, and the evidence that they did find was lacking in quality.
"The majority of supplements have no effect on improving survival or reducing the risk of heart attack or stroke," said study author Dr. Safi Khan, an assistant clinical professor of internal medicine at West Virginia University.
Please stop spending money on these supplements for the sake of improving cardiovascular health. Because they don't.
Still, the findings may do little to shake Americans' strong faith in supplements and vitamins. An estimated 3 out of 4 people in the United States take at least one dietary supplement, and Americans are projected to spend $32 billion on them this year alone.
But, according to Khan's review of the available science, it's largely a waste of money — at least when it comes to heart health.
"Please stop spending money on these supplements for the sake of improving cardiovascular health. Because they don't," Khan told NBC News.
Little benefit, but may cause harm
Though the researchers did find that fish oil offered some benefit in reducing the risk of heart attack and overall heart disease, and that folic acid was associated with a lower risk of stroke, they said the majority of supplements analyzed offered no benefits at all.
For example, they said, there was no evidence that multivitamins, antioxidants, iron or vitamins A and B had any impact on heart health.
And some of the research suggested the potential for harm. Taking calcium plus vitamin D, the researchers found, was associated with an increased risk for stroke.
It's not clear whether this increased risk is due to the supplements on their own, or the add-on of the supplements to a diet already rich in calcium and vitamin D. The Western diet, for example, "already has significant dietary fortification with calcium plus vitamin D," researchers from the Scripps Research Translational Institute in La Jolla, California, wrote in an accompanying editorial.
But this isn't the first time scientists have identified a link between calcium and cardiovascular risk: An earlier study from Johns Hopkins University found people who took calcium pills were more likely to develop plaque in their arteries than people who did not take them.
Outside experts said the overall findings on supplements were consistent with what they're already telling patients.
"Except to prevent or correct specific deficiencies, or in specific circumstances such as folic acid supplements in early pregnancy to prevent neural tube defects, there is generally good agreement that dietary supplements should not be recommended to the general population," Susan Jebb, a professor of diet and population health at the University of Oxford, wrote in an email.
You can't just eat a crummy diet and then pop a pill and assume that's going to solve all the problems.
Dr. Pieter Cohen, an associate professor of medicine at Harvard Medical School, who was not involved in the new research, noted that in general, it's better to get nutrients from foods, not supplements, to avoid any potential risks.
But Steve Mister, president of the Council for Responsible Nutrition, a lobbying group that represents the supplement industry, said that people turn to nutrients in a capsule to fill gaps in their diet, not prevent heart disease.
"Taking dietary supplements and practicing healthy dietary patterns are essential ways for consumers to assure they are getting the recommended levels of nutrients essential for overall health and wellness," Mister wrote in a statement to NBC News.
Nutrition experts maintained that a diet full of vegetables and fruit is sufficient to meet recommended nutrient levels for most people.
"There's no easy answer," said Dr. Alice Lichtenstein, director of the Cardiovascular Nutrition Laboratory at Tufts University.
"You can't just eat a crummy diet and then pop a pill and assume that's going to solve all the problems," she said.
What about diets for heart health?
Supplements aside, the new research also found very little high-quality evidence that diets widely touted as heart healthy — including the Mediterranean diet — had any impact on heart health. In fact, the only dietary intervention that seemed to be beneficial was reducing salt intake, but only in people with normal blood pressure.
Cardiologists readily acknowledge that the science behind how food may impact heart disease is lacking, and suggest this is where researchers should focus their attention, rather than on supplements.
"There's a little bit of data on blood pressure reduction. But the studies don't measure the effect on stroke and myocardial infarction (heart attack)," Dr. Steven Nissen, chair of cardiovascular medicine at the Cleveland Clinic, said.
"How can you make good scientific decisions in the absence of high-quality, randomized controlled evidence? You can't do it," Nissen said. "I wish we had better studies."
The most robust study of diet's potential effect on cardiovascular disease was a 2013 study called Predimed. It made international headlines at the time, claiming the Mediterranean diet — rich in olive oil, nuts, fruit and vegetables — cut the risk for heart disease by a third. The benefit was found to be so huge that the study was stopped early so everyone could be made aware of the dramatic findings.
But the study was later retracted because of minor flaws in the way it was conducted. Last year, the researchers released a watered down version of their findings, writing that "the incidence of major cardiovascular events was lower among those assigned to a Mediterranean diet supplemented with extra-virgin olive oil or nuts than among those assigned to a reduced-fat diet." In other words, they still found a link between the Mediterranean diet and a reduced risk of cardiovascular outcomes.
But because of the flaws in the study, the researchers could not prove cause-and-effect.
Still, cardiologists generally do recommend diets that mostly resemble the Mediterranean diet.
"I tell my patients the best evidence we have is for the Mediterranean diet. And that limiting calorie consumption is useful because you want to keep your weight down," Nissen said.
"The bottom line is we just need more evidence."