Preliminary research presented at an American Heart Association conference in Chicago suggests that individuals who practice time-restricted eating, otherwise popularly known as intermittent fasting, which means consuming calories within an eight-hour window each day, nearly double their risk of cardiovascular disease-related mortality compared to those with a typical 12 to 16-hour eating window.
Senior study author, who also happens to be a professor and chair of the Department of Epidemiology and Biostatistics at the Shanghai Jiao Tong University School of Medicine in Shanghai, China, Victor Wenze Zhong, PhD, said in a press release, “We were surprised” by these findings.
The findings are undeniably attention-grabbing. However, it’s crucial to recognize that this is an observational study, so it doesn’t establish a causal relationship between time – restricted eating and cardiovascular death but rather indicates an association.
Christopher D. Gardner, PhD, a professor of medicine at Stanford University in California, and a prominent nutrition researcher not involved in the study, highlights additional caveats that cast doubt on the study’s results.
For example, Dr. Gardner questions the types of foods consumed by the participants. Since the analysis has not yet been published in a peer-reviewed medical journal, essential details are still unavailable.
Dr. Gardner says, “I find the concept of time-restricted dieting problematic in general because the focus remains on when foods are consumed rather than the quality of what is being consumed. As a nutrition scientist, I am more concerned with the quality of what people eat.”
Previous Studies Demonstrate Short-Term Improvement in Heart Health with Time-Restricted Eating
Time-restricted eating involves consuming calories within a specific time window each day, typically ranging from 4 to 12 hours.
For example, individuals may adopt a 16:8 schedule, fasting for 16 hours and consuming all their calories within an eight-hour window. This could be from 7 a.m. to 3 p.m. or from 11 a.m. to 7 p.m.
Previous research, which has mainly been limited to relatively short follow-ups ranging from one month to one year, has shown that time-restricted eating can improve various cardiometabolic health measures. These include blood pressure, blood glucose levels, cholesterol levels, and being overweight or obese.
The Latest Study Tracked 20,000 Adults for a Median for 8 Years
The new study utilized data collected over several years, with a median follow-up period of eight years and a maximum of 17 years, from approximately 20,000 U.S. adults participating in the annual 2003-2018 National Health and Nutrition Examination Surveys.
Upon enrollment, participants completed two questionnaires recalling their dietary intake over the past 24 hours. Based on the length of their eating window, participants were categorized into one of five groups, ranging from less than 8 hours to 16 hours.
The researchers then followed up with the participants over the years to monitor their health.
After adjusting for various factors, including age, race, ethnicity, and medical history, the researchers concluded that:
- People who restricted their eating window to less than eight hours per day had a 91 percent higher risk of death due to cardiovascular disease (mainly heart disease and stroke) compared with people with a window of 12 to 16 hours.
- Among people with existing heart disease, an eating window of 8 to 10 hours per day was associated with a 66 percent higher risk of death from cardiovascular disease.
- Time-restricted eating did not reduce the overall risk of death from any cause.
Study Limitation Raise Questions About the Findings
Nutrition experts consulted about the new research highlight limitation in the study design that may cast doubt on the results.
Professor of Kinesiology and Nutrition at the University of Illinois Chicago, Krista Varady, PhD, says, “I think the conclusions are extremely overstated considering that the investigators only have two days of dietary intake data over a 20-year period — what were the subjects eating on the other 7,300 days of the study?”
Dr. Varady, who was not part of the study, has conducted extensive research on intermittent fasting for weight loss and reducing metabolic diseases in individuals with obesity.
Dr. Garnder also raises concerns about grouping individuals into eating window categories based on only two days of data.
“Better than one day, but it is still likely that many people in the study were misclassified. Better would have been multiple days in the first year, and then additional diet data from subsequent years,” Dr. Gardner says.
Dr. Zhong, on the other hand, acknowledges this as a significant limitation of the study but points out that the analysis excluded individuals who reported an atypical diet on either of the two days.
Stress Level and Access to Food May Influence Outcomes
Dr. Gardner suggests that there are numerous factors that might have impacted the risk of heart disease among the study subjects, although these may be clarified once the full analysis is published.
“For example, what if those eating in a shorter time period had less access to food, worked more work shifts, and experienced more life stress compared with those in the 12 to 16 hour category?” Dr. Gardner says.
This implies that the findings regarding cardiovascular deaths could potentially be attributed to factors other than the shorter eating window.
Consensus Among Researchers: More Studies on Time-Restriced Eating Are Necessary
“It’s too early to give a specific recommendation on time-restricted eating based on our study alone,” says Dr. Zhong.
However, he emphasized that his findings suggest caution should be exercised when adopting this eating pattern for an extended period.
Instead of opting for a time-restricted diet, he advocates for following diets with established health benefits, such as the DASH diet and Mediterranean diet.
“Based on the evidence as of now, focusing on what people eat appears to be more important than focusing on the time when they eat,” Dr. Zhong says.
Dr. Gardner echoes this sentiment, but he acknowledges that time-restricted eating may be beneficial for some individuals who find it psychologically helpful.
He points out that many people struggle to make predominantly healthy food choices in the current food environment of the United States, where low-quality foods are cheap, readily accessible, highly palatable, and very convenient.
“I’m convinced that for some people a time-restricted diet proves helpful — from a behavioral psychology perspective,” says Dr. Gardner.
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