Highlights: 

  • A recent study indicates that intermittent fasting may increase cardiovascular disease mortality risk by 91%, but experts criticize the methodology and reliance on self-reported data. 
  • Critics, including noted nutritionists and cardiologists, argue the study’s conclusions are unreliable due to its use of minimal, non-peer-reviewed survey data and failure to account for confounding lifestyle variables. 
  • Experts stress that the findings do not prove causation between intermittent fasting and heart disease risk, highlighting the need for more comprehensive research before drawing conclusions.

This past week, a new study was released indicating that intermittent fasting can lead to a higher risk of death from cardiovascular disease. The findings sent shockwaves through the press and across social media worldwide.

After all, it wasn’t like it was some small uptick in risk—the study suggested that people who restricted food consumption to less than eight hours per day had a 91% higher risk of dying from cardiovascular disease over a median period of eight years, relative to people who ate across 12 to 16 hours.

But what was not covered widely by the media was that the study has caused an uproar amongst the scientific and medical communities, who don’t buy into the new research that challenges the notion that restricting eating to a limited window of time is good for heart health.

What Is Intermittent Fasting?

In recent years, time-restricted eating—sometimes called intermittent fasting—has exploded in popularity as a means of losing weight. The main idea behind this plan is to cut down on the amount of time you eat each day.

But there hasn’t been a lot of research on the effects of intermittent fasting on human health. Research on humans has shown that it can lead to weight loss, reduced levels of “bad” cholesterol, reduced blood pressure, and improved insulin sensitivity. Plus, there has been a growing body of evidence suggesting that intermittent fasting may have short-term beneficial effects on heart health, such as improved blood pressure and resting heart rates as well as other heart-related measurements.

Almost all of our understanding of intermittent fasting’s health benefits comes from studies conducted on animals. Animals can live longer if they restrict their caloric intake throughout their lives, like in intermittent fasting. Fasting improves blood glucose regulation, increases stress resistance, decreases inflammation, and reduces the production of harmful free radicals, according to these studies. While fasting, cells eliminate or fix molecules that have been damaged, a process known as autophagy. These benefits may forestall the onset of degenerative diseases like Alzheimer’s, diabetes, heart disease, cancer, and obesity. Other beneficial effects of intermittent fasting in animals include improved balance and coordination, and improved cognition, specifically with memory.

What Do We Know About the Study?

In this study, researchers claimed to have investigated the potential long-term health impact of following an “8-hour time-restricted eating plan.” They reviewed information about dietary patterns for participants in the annual 2003–2018 National Health and Nutrition Examination Surveys (NHANES) in comparison to data about people who died in the US from 2003 through December 2019 from the Centers for Disease Control and Prevention’s National Death Index database.

This is how they discovered that people who consumed all of their food in less than 8 hours per day had a 91% higher risk of dying from cardiovascular disease. They also showed that people with pre-existing cardiovascular disease who ate for at least 8 but not more than 10 hours per day had a 66% higher risk of dying from heart disease or stroke.

The study even went on to look at the risk of death from other causes, reporting that an eating duration of more than 16 hours per day was associated with a lower risk of cancer mortality among people with cancer. They even said that the data showed that time-restricted eating did not lower the overall risk of death from any cause.

Fasting Experts Fire Back

Dr. Christopher Rhodes, a PhD in Nutritional Biochemistry from UC Davis and expert on nutrition, biohacking, and human fasting, said that the main issue with the study is that it wasn’t actually looking at people who were on intermittent fasting plans or time-restricted protocols. Instead, the authors pulled their information from the massive repository of self-reported data from surveys given to participants across the country about their general health and lifestyle. 

“These were not people, as the authors claim, who were following an “8-hour time-restricted eating plan,” but rather people who had reported eating their meals on a specific day in less than 8 total hours, which there could be a number of reasons for; high-stress jobs may only have time to eat infrequently throughout the day or those experiencing economic hardship may only have the resources to buy one meal a day,” said Dr. Rhodes. “These populations may already be at higher risk for cardiovascular disease due to lifestyle factors from stress, income, food quality, environment, and dozens of others that have nothing to do with fasting.”

Dr. Rhodes wasn’t alone in this criticism. Dr. Dariush Mozaffarian, a cardiologist and professor of medicine at Tufts University, called the study “very problematic” and questioned whether the eight-hour eating group may have included many people who were very busy or faced other challenges that forced them to miss meals or eat erratically.

Krista Varady, a professor of nutrition at the University of Illinois at Chicago, picks the study apart from another angle, saying that there’s no data to actually look at seriously because the study analyzed participants’ diets based on just two days worth of surveys on their eating habits.

According to Varady, “Two days of diet record data are not at all reflective of an individual’s regular eating pattern—this is a major limitation to the study.” Varady also noted that the study didn’t account for lifestyle factors like exercise, socioeconomic status, alcohol and tobacco use, and other variables that can make a major difference in heart disease risk.

For Victoria Taylor, Senior Dietitian at the British Heart Foundation, the main issue with the claims that have gone viral is that the results are not from a peer-reviewed published paper—they’re from a poster being presented at a conference. It is unclear how reliable any of the work actually is because the study has not undergone extensive scrutiny and examination by experts.

The Take-home Message

While the data and study may be shaky, one thing you can be certain of is that the study does not show any kind of causation. These kinds of studies, which don’t actually test the effect of a drug, treatment, or lifestyle change, can at best show that two things are correlated or linked—they have a pattern that matches or is the exact opposite.

The problem with those kinds of studies is that you can find correlations for almost anything, like that “Google searches for ‘zombies’” correlates with the number of real estate agents in North Dakota.

Additionally, there was very little testing to see if any other factors that might affect health could affect the relationship between intermittent fasting and risk of death from heart disease. For all we know, these people could be doing more exercise, eating healthier, or having more money, which has definitely been linked to better heart health outcomes (as explained by this article from Harvard).

So, the jury is still out on whether intermittent fasting has long-term effects, whether positive or negative.