In 2021, Harvard Medical School published a bulletin with the title, “Intermittent fasting: The positive news continues.” According to the authors, in a mounting number of studies conducted on rats, intermittent fasting — a type of time-restricted diet which involves only eating during a set number of hours each day — seems to improve the rodents’ health. They lose weight, their blood sugar improves, their cholesterol drops.
Humans understandably struggle considerably more with consistent fasting than a subset of the rat population whose diet is entirely overseen by caretakers in lab coats, but the bulletin pointed to research that suggests “the circadian rhythm fasting approach, where meals are restricted to an eight to 10-hour period of the daytime, is effective” for many people, according to metabolic expert Dr. Deborah Wexler, Director of the Massachusetts General Hospital Diabetes Center and associate professor at Harvard Medical School.
Especially for people at risk for diabetes, there is some solid scientific evidence that suggests circadian rhythm fasting, when combined with a healthy diet, can be an effective approach to weight loss and can help improve several cardiometabolic health measures, such as blood pressure, blood glucose and cholesterol levels (though scientists are still debating whether it actually provides different outcomes than simply reducing one’s calorie intake).
But then, at a late March American Heart Association conference, a new poster was presented that actually linked intermittent fasting to cardiac death.
Soon thereafter, the organization published a release about the research with the headline “8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death,” which quickly garnered a lot of attention online. As the study’s virality increased, and as mainstream outlets began running headlines of their own like “Intermittent fasting may raise the risk of cardiovascular death” and “Shocking dangers of intermittent fasting diet revealed,” the nuance of its actual contents was flattened, including some crucial limitations of the study — which is to say, adherents of the diet shouldn’t worry just yet.
What is intermittent fasting and why is it so popular?
Fasting to lose weight isn’t a novel approach, but the idea of regimented, intermittent fasting first gained mainstream popularity after the 2012 broadcast of “Eat Fast, Live Longer,” a television documentary by BBC broadcast journalist Dr. Michael Mosley. This was quickly followed by the best-selling books “The 5:2 Diet,” which recommends patients eat normally five days a week and in a more intense calorie-deficit the other two days, and “The Obesity Code,” which has the subtitle: “Why Intermittent Fasting Is the Key to Controlling Your Weight.”
“IF makes intuitive sense,” the Harvard Health bulletin read. “The food we eat is broken down by enzymes in our gut and eventually ends up as molecules in our bloodstream. Carbohydrates, particularly sugars and refined grains (think white flours and rice), are quickly broken down into sugar, which our cells use for energy. If our cells don't use it all, we store it in our fat cells as, well, fat. But sugar can only enter our cells with insulin, a hormone made in the pancreas. Insulin brings sugar into the fat cells and keeps it there.”
It continues: “Between meals, as long as we don't snack, our insulin levels will go down and our fat cells can then release their stored sugar, to be used as energy. We lose weight if we let our insulin levels go down. The entire idea of IF is to allow the insulin levels to go down far enough and for long enough that we burn off our fat.”
However, Victor Wenze Zhong, one of the senior authors of the intermittent fasting study presented at the American Heart Association conference, remarked that the “long-term effects of time-restricted eating, including risk of death from any cause or cardiovascular disease, are unknown.”
“We were surprised to find that people who followed an 8-hour, time-restricted eating schedule were more likely to die from cardiovascular disease,” Zhong said in a statement to the American Heart Association. “Even though this type of diet has been popular due to its potential short-term benefits, our research clearly shows that, compared with a typical eating time range of 12 [to]16 hours per day, a shorter eating duration was not associated with living longer.”
How was the study conducted?
In “Association of 8-Hour Time-Restricted Eating with All-Cause and Cause-Specific Mortality,” researchers from Shanghai Jiao Tong University, Northwestern University, Harvard University, University of Massachusetts Lowell and Wuhan University began with a hypothesis: Eight-hour time-restricted eating, or consuming one’s daily calories across a span of just eight hours, would be associated with a lower-risk of all-cause and cause-specific mortality.
To assess the long-term health impact of intermittent fasting, researchers 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 U.S., from 2003 through December 2019, from the Centers for Disease Control and Prevention’s National Death Index database.
What were the findings?
The analysis found a few key pieces of information that seemed antithetical to prior research about intermittent fasting. According to their data, people who followed a pattern of eating all their food across less than eight hours per day had a 91% higher risk of death due to cardiovascular disease. The increased risk of cardiovascular death was also seen in people living with heart disease or cancer and, among people with existing cardiovascular disease, an eating duration of no less than 8 but less than 10 hours per day “was also associated with a 66% higher risk of death from heart disease or stroke.”
“We were surprised to find that people who followed an 8-hour, time-restricted eating schedule were more likely to die from cardiovascular disease,” Zhong told the American Heart Association. “Even though this type of diet has been popular due to its potential short-term benefits, our research clearly shows that, compared with a typical eating time range of 12 [to]16 hours per day, a shorter eating duration was not associated with living longer.”
In the end, the researchers concluded that time-restricted eating did not reduce the overall risk of death from any cause.
What are the study’s limitations?
The study had some key limitations, which weren’t really reflected in the headlines about its bombastic findings, namely that this analysis has yet to be peer reviewed or published in full (and, as pointed out by the British Heart Federation, includes different numbers in the study summary and press release). The observational study also relied on self-reported dietary information, which may include errors or distortions, and it’s unclear whether the subjects continued time-restricted eating beyond the two days they reported.
Additionally, factors that may play a role in health — such as activity level, tobacco and drug use, or socioeconomic status — were not included in the analysis.
“Overall, this study suggests that time-restricted eating may have short-term benefits but long-term adverse effects. When the study is presented in its entirety, it will be interesting and helpful to learn more of the details of the analysis,” Dr. Christopher D. Gardner, the Rehnborg Farquhar Professor of Medicine at Stanford University in Stanford, California, told the American Heart Association.
“One of those details involves the nutrient quality of the diets typical of the different subsets of participants. Without this information, it cannot be determined if nutrient density might be an alternate explanation to the findings that currently focus on the window of time for eating. Second, it needs to be emphasized that categorization into the different windows of time-restricted eating was determined on the basis of just two days of dietary intake,” he said.
“It will also be critical to see a comparison of demographics and baseline characteristics across the groups that were classified into the different time-restricted eating windows – for example, was the group with the shortest time-restricted eating window unique compared to people who followed other eating schedules, in terms of weight, stress, traditional cardiometabolic risk factors or other factors associated with adverse cardiovascular outcomes?” Gardner asked. “This additional information will help to better understand the potential independent contribution of the short time-restricted eating pattern reported in this interesting and provocative abstract.”
What now?
The researchers will continue working on their full analysis to prepare it for peer-review. In the meantime, senior researcher Victor Wenze Zhong has a message for patients concerned about their health.
“It’s crucial for patients, particularly those with existing heart conditions or cancer, to be aware of the association between an 8-hour eating window and increased risk of cardiovascular death,” Zhong told the American Heart Association. “Our study’s findings encourage a more cautious, personalized approach to dietary recommendations, ensuring that they are aligned with an individual’s health status and the latest scientific evidence.”
He continued: “Although the study identified an association between an 8-hour eating window and cardiovascular death, this does not mean that time-restricted eating caused cardiovascular death.”
