Doctors often recommend certain dietary interventions—such as following a Mediterranean-type diet or cutting salt intake—in the interest of protecting heart health. On top of this, many individuals believe that dietary supplements will help them stay healthy.

Common knowledge has it that diet and lifestyle play an important role in supporting a person's physical health and overall well-being. That is why doctors may advise their patients to modify their diets and lifestyle habits to make them more conducive to good health.

In particular, dietary interventions can allegedly help individuals safeguard their cardiovascular health, preventing heart disease and events such as strokes. Dietary guidelines for people in the United States advise that people adhere to healthful diets, such as a vegetarian diet or the Mediterranean diet, which is rich in vegetables, legumes, and lean meat.

On a related note, many individuals believe that taking dietary supplements can enhance different aspects of their health, including heart health, although recent studies have contradicted this assumption. Now, a meta-analysis by researchers from different collaborating institutions—including The Johns Hopkins School of Medicine in Baltimore, MD; West Virginia University in Morgantown; and Mayo Clinic in Rochester, MN—suggests that many interventions and even more supplements may have no protective effect for the heart, and some may even harm cardiovascular health. The review—the first author of which is Dr. Safi Khan from West Virginia University—appears in Annals of Internal Medicine.

Common interventions may fail the heart

In their research, Dr. Khan and team analyzed the data from 277 randomized, controlled trials that had involved almost 1 million participants between them. They looked at the effects of 16 nutritional supplements and eight dietary interventions on cardiovascular health and mortality.

The supplements that they took into consideration were: selenium; multivitamins; iron; folic acid; calcium; calcium plus vitamin D; beta carotene; antioxidants; omega-3 long-chain polyunsaturated fatty acids; and vitamins A, B complex, B-3, B-6, C, D, and E.

The dietary interventions included: modified dietary fat, reduced salt (in people with normal and high blood pressure), reduced saturated fat, Mediterranean diet, reduced dietary fat, higher intake of omega-6 polyunsaturated fatty acids, and higher intake of omega-3 alpha-linolenic acid.

Dr. Khan and colleagues did find that some of these interventions had a positive effect. For instance, eating less salt reduced the risk of premature death in people with a normal blood pressure. Moreover, they concluded that omega-3 long-chain polyunsaturated fatty acids protected against heart attacks and coronary heart disease and that there was an association between folic acid intake and a slightly lower risk of stroke.

At the same time, however, other supplements and interventions seemed to either have no effect or be downright harmful. The researchers found that taking multivitamins, selenium, vitamin A, vitamin B-6, vitamin C, vitamin D, vitamin E, calcium, folic acid, and iron did not significantly protect against cardiovascular problems and early death. They also noted that following a Mediterranean diet, reducing saturated fat intake, modifying fat intake, reducing dietary fat intake, and increasing the quantity of dietary omega-3 and omega-6 were not beneficial. In fact, people who took calcium and vitamin D supplements together actually had a higher risk of experiencing a stroke.

However, in their paper, the investigators admit that "these findings are limited by suboptimal quality of the evidence." They are referring to the fact that, due to the different methodologies of the studies that they assessed, they "could not analyze interventions according to important subgroups, such as sex, body mass index [BMI], lipid values, blood pressure thresholds, diabetes, and history of [cardiovascular disease]."

Yet, they argue that their current review paves the way to better care and stronger research into the helpfulness and value of different dietary interventions:

"This study can help those who create professional cardiovascular and dietary guidelines modify their recommendations, provide the evidence base for clinicians to discuss dietary supplements with their patients, and guide new studies to fulfill the evidence gap."

The authors of the accompanying editorial, Amitabh Pandey and Eric Topol, both from the Scripps Research Translational Institute in La Jolla, CA, also emphasize that the quality of the data in many studies assessing the effects of dietary interventions and supplements on heart health can be questionable.

"[D]ifferences in geography, dose, and preparation—most studies rely on food diaries, which are based on a person's memory of what they consumed—raise questions about the veracity of the data," they write.

"Perhaps, however, the biggest difference that needs to be considered in the future is the individual," they add, advising that future research should pay more attention to the differences among participants.