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CNN
 — 

Intermittent fasting is one of the many trendy ways people try to lose or maintain their weight.

Also known as time-restricted eating, the practice is a method of weight loss that confines a person’s eating window to set times — typically eight hours during a 24-hour period — with only clear liquids consumed during the remaining 16 hours. Other methods include two or three days of fasting during a week or month.

How well does intermittent fasting work?

Prior research has shown advantages to time restriction. A December 2019 review of human and animal studies had found benefits to restricting calories to a shortened period of the day, including improved longevity, a reduction in blood pressure and weight loss. (However, a number of those studies were in mice and those in humans were of much shorter duration, mere months.)

However, a yearlong study published in April 2022 that followed 139 Chinese adults ranging from overweight to significantly obese found no benefit over calorie counting for weight loss or improved cardiovascular health.

Research presented this week immediately drew doubt and critiques from experts by suggesting that eating within an eight-hour window or less was significantly associated with a 91% increased risk of dying from cardiovascular disease, when compared with eating over a 12- to 16-hour period.

An abstract of the preliminary research, which is not yet peer reviewed or published, was presented Monday in Chicago at a conference of the American Heart Association.

“We were surprised to find that people who followed an 8-hour, time-restricted eating schedule were more likely to die from cardiovascular disease,” said senior study author Victor Wenze Zhong, a professor and chair of the department of epidemiology and biostatistics at the Shanghai Jiao Tong University School of Medicine in China.

“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,” Zhong said in a statement.

The new study analyzed data on 20,000 people who answered questions about their 24-hour eating habits on two days during the first year of enrollment in a long-term analysis of the health of US adults, then looked back at death records in the years afterward.

The analysis showed a link between an eight-hour eating window and death from cardiovascular disease, but the study could not determine if this eating pattern caused the deaths, the authors said.

Many experts expressed concerns about the new research.

“There’s just about enough in the conference abstract to throw huge doubts on whether the study can show what it purports to show,” said Kevin McConway, professor emeritus of applied statistics at The Open University in the United Kingdom, who was not involved in the study.

“The researchers classified people into different dietary patterns on the basis of what and when they reported they ate in just two days, over a study period averaging 8 years,” McConway said in a statement. “To relate those patterns to a deliberate long-term time-restricted eating intervention seems to be going far beyond the data.”

Nor does the abstract disclose whether the people practicing time-restricted eating worked “antisocial” hours, as truck drivers, night workers and health professionals often do, said Tom Sanders, professor emeritus of nutrition and dietetics at King’s College London, who was not involved in the study.

“This is important because there is evidence that this type of working practice is associated with increased risk of type 2 diabetes and CVD (cardiovascular disease),” Sanders said in a statement.

There is also no information in the abstract about tobacco and alcohol use, physical activity, or poverty level among those who said they practiced intermittent fasting, all of which are risk factors for heart disease, said Duane Mellor, a registered dietitian and senior teaching fellow at Aston Medical School in Birmingham, United Kingdom. Mellor was not involved in the study.

“We need to be very careful not to generate concerning headlines and stories based on such limited information,” Mellor said in a statement. “It is perhaps what you eat and your overall lifestyle that is more important than if you ate all your food in less than 8 hours on two days in the last decade.”

As with many investigations in science, research can elicit conflicting results, often dependent on the quality of the study and whether the studies have all measured the same thing in the same way.

In the case of fasting, experts say studies are all over the map, with some studying fasting for two or more days during the week, some studying fasting between 8 a.m. and 4 p.m., and others from noon to 8 p.m. or other times.

“The data are not very compelling, in my opinion, for intermittent fasting. It’s a hard thing to study and publish with clean results,” nutrition researcher Christopher Gardner told CNN’s Chief Medical Correspondent Dr. Sanjay Gupta.

“And there’s no emphasis on quality, right?” said Gardner, a research professor of medicine at the Stanford Prevention Research Center in Palo Alto, California. “I fear that people say, ‘It’s the window, so I can have the pint of ice cream or I can have the cookies, or I can have whatever, because the most important thing is the window.”

What and how much you eat is more important than anything else, experts say.

“Bottom line, the determinant of weight loss, as well as reductions in body fat, visceral fat, blood pressure, and glucose and lipid levels, is dependent on reducing calorie intake, regardless of the distribution of food and beverages consumed throughout the day,” Alice Lichtenstein, director and senior scientist at Tufts University’s Cardiovascular Nutrition Laboratory, told CNN in a prior interview. She was not involved in that study.

In addition, a September 2020 randomized clinical trial — considered the gold standard of research — that looked at 116 people found no significant difference in weight loss between people who restricted eating from 8 p.m. and noon the next day and those who did not.

Nor did a January observational study of 547 people — it too found no real difference between restricting-eating times and weight loss.

CNN’s Madeline Holcombe, Scottie Andrew and Grace Walker contributed to this report.