New Research Questions Heart-Healthy Diets
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Yet another study that seemingly contradicts much of what we thought we knew about the best way to eat for heart health was released this week. The study suggests that there is a lack of evidence to support taking vitamin supplements or following a healthy diet to help prevent heart disease.
But does this mean that you can go ahead and binge on french fries and soda? Not so fast.
The paper, published in the Annals of Internal Medicine, was not actually a new study, but a meta-analysis—which means it looked at previous research. In this case, researchers looked at 277 studies that involved nearly 1 million people to see if vitamin supplements or diets reduced the risk of death, heart attack, or stroke. They looked only at randomized clinical trials, in which people are assigned to a certain intervention (in this case either diet or vitamins) and compared with a control group.
With just a few exceptions, the researchers concluded, neither diet nor vitamin supplementation seemed to help.
While there are some strengths to the study, nutrition researchers that CR spoke to unanimously agreed that there’s nothing in this study to suggest that we should throw out current recommendations to follow heart-healthy diets.
“Studies like this one contribute to the perception that nutrition science is always flip-flopping,” says Charlotte Vallaeys, M.S., Consumer Reports’ senior policy analyst for food and nutrition, “but diet absolutely matters for good health.”
So why didn’t this study find evidence that taking vitamins and eating a healthy diet matter? And why do new studies like this one often seem to debunk what we knew before? The answer has a lot to do with how the study was designed and what the researchers were looking for.
Here’s what you need to understand about what we learned from this research and why current recommendations for healthy eating—such as incorporating plenty of vegetables into your diet—still stand.
What Did the Study Find?
Researchers found limited evidence that a reduced sodium diet, taking omega-3 supplements, and taking folic acid supplements could reduce the risk of heart attack and stroke in adults. They also found that taking calcium and vitamin D may actually increase the risk of stroke.
The analysis did not find a benefit to taking other vitamin supplements or to following seven different diets that they analyzed, including the Mediterranean diet, a low-fat diet, and others touted to improve heart health.
Is It True That Not Much Evidence Supports a Heart-Healthy Diet?
No. There is a preponderance of evidence that eating a diet with fruits and vegetables, whole grains, low-fat dairy, and heart-healthy fats like olive and canola oil protects against heart disease. All of the nutrition experts CR spoke with agreed on this.
If There’s so Much Evidence, Why Didn’t This Study Find It?
This meta-analysis looked at how different diets and vitamin supplements affect the risk of heart attack, stroke, and death. The problem is that the longest studies included in the analysis lasted five years, and it can take decades for the effects of a bad diet to lead to something like a heart attack—or for the protective benefits of a good diet to become apparent.
“You’re not going to see the impact of a diet on an outcome like dying in a couple of years,” says Edgar R. Pete Miller, M.D., Ph.D., a professor of medicine at the Johns Hopkins University School of Medicine.
The other major issue was the quality and consistency of the studies assessing diet. Each diet was not clearly defined, so what constituted a “Mediterranean” diet in one study might have been quite different from a Mediterranean diet in another study. “And that might explain why they didn’t see the benefits of a Mediterranean diet,” says Penny Kris-Etherton Ph.D., R.D., a distinguished professor of nutrition at Pennsylvania State University.
The researchers also did not account for how well study participants followed the diets they were assigned—a common problem in dietary studies, explains Eric Topol, M.D., a cardiologist and director of the Scripps Research Translational Institute, who wrote an editorial accompanying the new paper. Sometimes, he says, participants “are going to eat what they want to eat.”
The researchers may have fallen short in taking the possibility of non-adherence—as well as other variables—into account, experts told CR. “This meta-analysis just mindlessly threw all the studies in one pot without consideration of adherence, duration, or background diet, which is very likely to miss important benefits that might exist,” says Walter Willett M.D., Dr. P.H., a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health.
Finally, both Miller and Kris-Etherton say some of the diet studies included in the meta-analysis may have had issues with poor design, and one of the studies was suspected of fraud.
Safi U. Khan, M.D., the lead author of the meta-analysis, did not respond to a request for comment.
Should You Stop Taking Vitamins?
Probably—unless your doctor specifically recommends it (if you’re pregnant, for example).
While a study like this has certain limitations, experts CR spoke to said the meta-analysis of randomized controlled trials made more sense for drawing conclusions about vitamins than diets. This is because with other types of research that aren’t well-controlled, it can be very difficult to capture the effects of vitamin supplements.
The kind of people who take vitamins tend to be people more concerned with their health in general, which means they may also engage in other healthy behaviors (such as eating well, exercising, and getting enough sleep). By only including randomized controlled trials in the study, researchers were able to separate the effects of the vitamin from the kind of person taking it.
This is where this sort of methodology can be the most helpful. By combining studies, researchers were able to demonstrate “that supplements are [largely] ineffective, and may put you at harm in the short term,” says Miller.
Still, some researchers told CR that the analysis may have also masked the benefits of vitamins in certain cases.
The analysis generally did not take into account the nutritional status of the person taking the vitamins, and if people were already well-nourished they would not have received any benefit from the vitamin, says Kris-Etherton. The differing doses of vitamins also were not taken into account, so even people who were in fact vitamin deficient may not have received enough of the supplement to show a benefit.
Even the conclusions that showed a benefit from certain vitamin supplements may be misleading for certain audiences. The research found a benefit of folic acid supplementation, for example, but the data came from China—where foods are not fortified with folic acid, Topol points out in his editorial. For people living in the U.S., where many foods are fortified with folic acid, they would likely not see any benefit from taking a folic acid supplement as well.
What Can You Learn About Healthy Eating From This Study?
Meta-analyses can be helpful in uncovering effects that you wouldn’t normally be able to see in smaller studies, but they have limitations. Willett described a meta-analysis like this one as akin to “looking at the world through a narrow and dirty window.” It can give you some insights into the effect that interventions are having, but when you mix so many things together, you won’t get the full story.
The meta-analysis backed up what previous studies have pointed to as a general lack of evidence supporting the benefits of most vitamin supplements. It was not able to show any cardiovascular benefit of a healthy diet, but that was likely due to the short duration of the studies that were included as well as other issues with the consistency and quality of the data.
And some of the new study’s conclusions are directly contradicted by other robust research. Willet noted, for example, the 400-page evidence review produced by the last Dietary Guidelines Advisory Committee.
While there is indeed some disagreement in nutrition research, studies have consistently shown that a holistic approach to eating rather than simply looking at macronutrients or narrow interventions is what improves health.
“Because of the serious limitations of the available randomized trials, especially for dietary interventions, it is important to consider all the evidence,” says Willett. For example, “a huge body of evidence—including controlled feeding studies with risk factors as outcomes, long-term cohort studies, and several randomized trials with good adherence—supports benefits for a Mediterranean-type diet.”
Evidence of the benefits of eating healthy foods like fruits and vegetables has been well established. “It is simply wrong, and a disservice, to say we know nothing about the consequences of diet,” says Willett. “We actually do know a lot about diets and health, and our food choices make a major difference in our long term well-being.”
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