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Cardiologists Tell Us What You Really Need to Know About Intermittent Fasting and Heart Health

Photographs: Getty Images; Collage: Gabe Conte

New research on intermittent fasting made splashy headlines last week. The abstract, presented at an American Heart Association conference, found that intermittent fasting—specifically, the restriction of food consumption to an 8-hour period each day—was associated with a 91% increase in risk of cardiovascular death. (Insert needle-scratch.)

This is shocking intel, not just because the statistic is so staggering, but because it flies in the face of what we’ve previously known to be true about the benefits of intermittent fasting, which would suggest it has the opposite impact on heart health. The practice, in fact, is supposed to be good for your heart.

However, experts say that prior research shouldn’t necessarily be discarded in favor of these new findings and that you don’t need to panic if you get your calories within a restricted window of time each day. Here, they unpack everything we know to date on intermittent fasting’s impact on heart health.

What is intermittent fasting?

Generally speaking, intermittent fasting is a dietary approach where you are in a fasted state for some period of time, says Julia Zumpano, RD, a preventive cardiology nutritionist at Cleveland Clinic. The precise period of time depends on the type of intermittent fasting you’re doing—there are several different types—but one popular approach involves restricting calories to an eight-hour window each day. For example, 12 p.m. to 8 p.m. It doesn’t involve any dietary restrictions beyond eating within a specific time window, and some people find this easier to adhere to than trying to cut out specific foods or counting calories. However, it can result in an overall reduction in caloric intake—some research indicates that people end up cutting out about 300 to 500 calories by intermittent fasting.

What has prior research said about intermittent fasting’s impact on heart health?

Prior, well-controlled, short-term studies have shown that intermittent fasting can result in improvements in body weight, insulin resistance, blood pressure, and some markers of inflammation, a.k.a. “a number of surrogates for heart health that tend to be associated with improved cardiovascular outcomes in the long term,” says Sean Heffron, MD, preventive cardiologist and director of fitness-focused cardiology at the Center for the Prevention of Cardiovascular Disease at NYU Langone Heart.

Longer-term data is scarce in humans due to the general limitations of diet and nutrition studies (more on that to follow), but we do have longer-term data based on animal studies. “Lots of these studies show that life-long adherence to limited energy intake is associated with greater lifespan,” Dr. Heffron says.

What did this new research find?

This new study aimed to look at long-term data in humans to fill the gaps in our understanding of intermittent fasting’s impact on health. It included data collected as part of the National Health And Nutrition Examination Survey, which asked 20,000 adults of varying ages, sexes, and ethnicities what they ate during an average period of eight years.

During that time period, participants reported what they had eaten in two separate interviews less than two weeks apart. If they reported having eaten within a restricted eight-hour window, researchers generalized that eating pattern to their lives overall, essentially labeling them as intermittent fasts.

Researchers then looked at who had died subsequent to filling out the surveys to see whether or not their engagement in this form of intermittent fasting was associated with overall death, cardiovascular death, and death from cancer, says Dr. Heffron. “They found that there were associations, meaning that restricting food consumption over a very short period was associated with worse outcomes,” he says.

In other words, the study noted that those who consumed food in windows of time shorter than 8 hours had a greater risk of cardiovascular death than those who ate food in windows of time longer than 8 hours—hence the shocking headline.

What are the limitations of this new research?

There are many, which even the study’s authors acknowledge. To begin with, this research was presented in an abstract rather than as a full study. It has not yet been published in a peer-reviewed journal.

An abstract, Zumpano explains, is just the preliminary part of a study in which researchers share their methods and hypotheses or conclusions. Still, they don’t provide any of the details or specifics of the study. “It’s basically a one-sheet page describing what they wanted to do,” she says. “So it’s kind of like looking at the front of a food label without being able to look at the back. You don’t know the nutrition facts or have the ingredients—you’re just looking at the marketing on the front.” Generally, she says, this format is one of the biggest limitations of these findings. “We can't come to conclusions until we have all of the information when the full study comes out,” she says.

Another limitation is that the abstract was based on an observational study, which means its details were self-reported. “We know observational studies aren’t necessarily always accurate,” she says. People can’t always recall what they ate, for example, or they might even fib about it. And given that participants only contributed a 24-hour dietary recall twice throughout the entire survey period, this analysis does not include a lot of data from which to draw conclusions, says Dana Hunnes, RD, PhD, senior dietitian at UCLA Medical Center and author of Recipe for Survival: What You Can Do to Live a Healthier and More Environmentally Friendly Life.

Additionally, several factors are not accounted for in the analysis. Perhaps most glaringly, it does not seem to include any details whatsoever on the content of participants’ diets. “We don’t know what these 20,000 individuals are eating as it’s not described anywhere,” says Dr. Hunnes. “They could be eating nothing but hamburgers, hot dogs, fries, ice cream, and other highly processed, high-calorie foods.” Other lifestyle factors that impact health, such as exercise habits, were not included in the analysis either.

And finally, the study did not demonstrate a causal effect, says Dr. Hunnes, meaning that you can’t conclude from it that intermittent fasting caused the increased risk of cardiovascular death but rather that the two were linked. It also offers no insight into why this might be the case.

For all of these reasons, Dr. Heffron ultimately says, rather definitively, that this is “not a noteworthy study.”

What research needs to be done to investigate this finding further?

Perhaps some of the above gaps in knowledge will be filled in when the full study is released. Still, even then, Dr. Heffron says a considerable amount of further work would be required to validate the results of this analysis. “You’d need to do the appropriate statistical analyses from the cohort that these data are derived from and then look for validation within other independent cohorts,” he says. “And if that pans out, then you would think about potential short-term-duration interventions, which are possible in nutrition.”

Generally speaking, Dr. Hunnes says a significant amount of additional research is needed on intermittent fasting beyond this analysis, especially regarding its impacts on heart health. “In fact, much more information in general is needed with regard to lifestyle and heart health,” she says.

She says this intel can be difficult to collect as there are limitations to nearly all nutrition studies. “There are so many variables and factors that are difficult to account for and are confounding,” she says. “If we were to conduct a long-term nutrition study where all variables could be accounted for—ie. 1,000 people live in a hotel or dormitory and are given access only to foods provided by the study, and were monitored in their daily activities 100 percent of the time, and all other variables are known—we could make grand conclusions.”

Dr. Heffron agrees. “The gold standard in health outcomes research is a randomized controlled trial, and it’s very difficult to randomize anybody to a nutritional intervention,” he says. “Modifying even one small aspect of a diet for a long-term study, which is what’s necessary to find out if someone dies or not as a result of that modification, is very difficult.”

TL;DR: What conclusions should we draw about the impacts of intermittent fasting on heart health? In the short term, we know that intermittent fasting can help people lose weight and reduce certain risk factors for heart disease. We don’t yet know how intermittent fasting affects human health in the long run.

If the data on intermittent fasting is inconclusive, what diets are shown to be beneficial to heart health?

Even though Dr. Heffron believes this abstract’s data should more or less be taken with a grain of salt, he says it’s okay to abandon intermittent fasting if you’re troubled by the new intel. In fact, he rarely recommends it per se anyway, despite the fact that there is evidence to suggest that intermittent fasting is an effective way to restrict overall calories and, in turn, mitigate major risk factors for heart disease. This is because he prefers to focus his counseling on encouraging behaviors patients will be able to maintain for decades, as those are better associated with improved heart health outcomes.

Such behaviors include better food choices, and Zumpano agrees that emphasis should be placed more on what you eat than when you eat it. “Focus on including heart-healthy foods and decreasing the things that we know can negatively impact your heart, like processed meats, processed foods, fast foods, junk foods, and sugary foods,” she says.

If you are looking for specific dietary guidelines, Dr. Hunnes says we know that some diets are generally good for heart health. “For example, we know from long-term epidemiological studies that the Mediterranean diet is, generally speaking, a heart-protective diet and that populations that eat this type of diet live longer with fewer cardiovascular problems, tend to have lower body weight, lower body-fat percentage, and die less from heart disease,” she says. “We similarly know that the DASH diet (dietary approaches to stop hypertension), which is similar to a Mediterranean diet, also helps protect against heart disease and death from heart disease. And we know that a whole-food, plant-based diet that limits processed foods is protective against heart disease and death from heart disease.”

This doesn’t mean that intermittent fasting has no role in a heart-healthy diet, but rather that more research is needed to understand its impact, says Dr. Hunnes. “We would need to tweak those diets to find out if intermittent fasting makes any of them better or worse for heart disease outcomes,” she says.

Zumpano also notes that, in any case, there isn’t a one-size-fits-all approach to diet. So, even if it is determined that intermittent fasting is harmful or helpful to heart health in the long term, those findings may not be applicable to all demographics. “If you’re questioning what the best diet is for you, I would recommend seeing a dietician,” she says. “We’re able to look at your risk factors, medical history, family history, labs, etcetera, and can pinpoint the best plan for you.”

Originally Appeared on GQ