Taking folic acid was found to lower the risk of stroke. But the finding was largely driven by research from China, where deficiencies of the vitamin are common. The researchers said it was not clear that people who take folic acid in America, where foods are fortified with it, will get the same benefit.

Fish oil, one of the most popular supplements in America, is widely used for its omega-3 fatty acids, which are anti-inflammatory. A recent clinical trial found that high-risk patients who took very large doses of Vascepa, a purified form of omega-3 that is available by prescription only, had a reduction in cardiovascular events. Another study found that supplementing with fish oil could benefit people who eat very little seafood. But the majority of trials involving fish oil have been disappointing. Dr. Khan and his colleagues concluded that there was only weak evidence at best that taking fish oil could prevent heart disease.

“This just reinforces that the supplement story is so shaky,” said Dr. Eric Topol, a cardiologist and the founder of the Scripps Research Translational Institute, who wrote an editorial accompanying the new review. “Not only is there the potential for harm, but there’s also no hard evidence for good.”

When Dr. Khan and his co-authors looked at various diets recommended for cardiovascular prevention, they found a similar lack of solid evidence.

That was certainly the case for low-fat diets, which health authorities have recommended for decades as a way to lower cholesterol and heart disease risk. Dr. Khan and his colleagues found that the most rigorous randomized trials provided no evidence that eating less fat, including saturated fat, had an impact on mortality or cardiovascular outcomes. Low-fat diets have largely fallen out of favor among health authorities in recent years, though the federal government’s dietary guidelines still encourage people to limit their intake of foods rich in saturated fat, such as butter, meat and cheese.

One diet that remains highly touted by health authorities is the Mediterranean diet, with its abundance of whole grains, beans, nuts, fruits and vegetables and olive oil. While clinical trials have found that it reduces cardiovascular risk, some of the major ones have been flawed, and experts who have scrutinized the evidence for the diet have urged caution.

One of the largest and most publicized Mediterranean diet trials, called Predimed and published in 2013, found that it reduced heart attacks and strokes. But last year it was retracted because of methodological problems. The Predimed authors published a new analysis of their data, claiming their conclusions had not changed. But other Mediterranean diet trials have been embroiled in similar controversies. After analyzing data from all the relevant trials, Dr. Khan and his colleagues found that “the totality of evidence did not favor the Mediterranean diet for cardiovascular outcomes.”