More than half of adults report using dietary supplements, but new research suggests that most vitamins and minerals don't do anything for heart health. Some products, according to a review of hundreds of existing studies, may even increase the risk of stroke.

"This research further shows that despite extensive sales and use of different dietary supplements, there is a lack of scientific evidence supporting the use of many supplements," said Dr. Bruce Y. Lee, an associate professor of international health at the Johns Hopkins Bloomberg School of Public Health, who was not involved in the research.

The research did find that omega-3 fatty acids -- commonly found in fish oil -- reduced the risk of heart attacks and coronary heart disease. Folic acid also reduced the risk of stroke, while taking calcium and vitamin D together made strokes more likely.

But almost everything else, including niacin, iron and a veritable alphabet soup of vitamins, "had no significant effect on mortality or cardiovascular disease outcomes," according to the review, published Monday in the medical journal Annals of Internal Medicine.

The findings were unsurprising, said Susan Jebb, a professor of diet and population health at the University of Oxford. "Except to prevent or correct specific deficiencies" such as low vitamin D levels, or in specific circumstances such as pregnancy, "there is generally good agreement that dietary supplements should not be recommended to the general population."

Low-salt diets may reduce the risk of death

The new research, which included almost a million patients overall, also found limited evidence that a low-salt diet may reduce the risk of death, but in an editorial accompanying the review, experts called that a "peculiar and controversial finding."

The research team found that low-salt diets reduced the risk of death overall in those with normal blood pressure, but not in those with high blood pressure. In those with high blood pressure, though, low-salt diets did make deaths due to cardiovascular problems less likely -- even though the diet had no significant effect on deaths overall.

Those confusing results may be because of limitations in study design, according to Lee, and not because low-salt diets are ineffective. "There are so many factors that affect mortality," he said, "and it is difficult to account for all of these in randomized controlled trials and population cohort studies."

Still, Lee said that "Americans are consuming too much salt" and said people should "check to see how much salt is in foods and ask restaurants to not add salt." Reduced salt intake seems to lower blood pressure, according to a 2013 review, and "the science behind sodium reduction is clear," according to the American Heart Association.

The association said in 2018 that "significant evidence links excess sodium intake with high blood pressure, which increases the risk of heart attack, stroke and heart failure."

Researchers looked at other popular diets, such as the low-meat Mediterranean diet, and found that most did little to improve heart health. The study was a so-called review of reviews that analyzed hundreds of published studies, but Jebb argued that deciding what research to include can create a biased outcome. "Different reviews reach different conclusions," she said.

For example, Jebb noted that a separate review, published earlier this year in the Journal of the American Heart Association, found that conforming to a Mediterranean diet was associated with "better cardiovascular health outcomes, including clinically meaningful reductions in rates of coronary heart disease, ischemic stroke, and total cardiovascular disease" -- contradicting what the most recent review found.

"Neither of these two papers reports new research," Jebb said. Instead, "they each provide their own interpretation of what has already been published."

'Not a single magical nutrient'

The new research analyzed 277 randomized controlled trials, which often involve splitting patients into different groups and comparing those who received an intervention -- for example, folic acid supplements -- to those who received a placebo.

That method may not be the best way to study diet and heart disease, according to Lee. "Randomized controlled trials are not enough to study the relationship between diet and outcomes," he said. "Their timelines are often too short to see longer-term effects."

Other ways to study diet include "population cohort studies" that track populations of people over long periods of time, looking for connections between behavior and health. But these can be problematic too, Lee said, because "they show only associations and correlations, and not cause and effect."

More complex research is needed, Lee said, but his advice for now is simple: Watch how much salt you consume and eat a range of natural foods, not supplements, to get the nourishment you need.

"There is not a single magical nutrient that can help improve health and reduce the risk of early death," he said.