The safety of intermittent fasting, a popular strategy to lose weight by limiting food intake to certain times, was called into question by a surprise finding from research presented at a medical meeting. Limiting mealtimes to a period of just eight hours a day was linked to a 91% increase in risk of death from heart disease in the study, which was released on Monday in Chicago. (Also Read | Is intermittent fasting good or bad for your liver? Expert answers)

The American Heart Association published only an abstract, leaving scientists speculating about details of the study protocol. The study was reviewed by other experts prior to its release, according to the AHA.

Lifestyle interventions aimed at weight loss have come under scrutiny as a new generation of drugs help people shed pounds. Some doctors questioned the study’s findings, saying they could have been skewed by differences — such as underlying heart health — between the fasting patients and the comparison group, whose members consumed food over a daily period of 12 to 16 hours.

“Time-restricted eating is popular as a means of reducing calorie intake,” Keith Frayn, emeritus professor of human metabolism at the University of Oxford, said in a statement to the UK Science Media Center. “This work is very important in showing that we need long-term studies on the effects of this practice. But this abstract leaves many questions unanswered.”

The researchers, led by Victor Zhong of the Shanghai Jiao Tong University School of Medicine, analysed data from about 20,000 adults included in the US Centers for Disease Control and Prevention’s National Health and Nutrition Examination Survey.

The study looked at answers to questionnaires along with death data from 2003 through 2019. Because it relied in part on forms that required patients to recall what they ate over two days, scientists said there was room for potential inaccuracies. About half of the patients were men and the mean age was 48.

It wasn’t clear how long the patients continued the intermittent fasting, though the researchers assumed they kept it up, according to Zhong.

The fasting patients were more likely to be younger men with a higher BMI and food insecurity, he said by email. They also had a lower prevalence of hypertension, diabetes and cardiovascular disease based on self-reports. “We controlled for all these variables in the analysis, but the positive association between 8-h time-restricted eating and cardiovascular mortality remained,” Zhong said.

The abstract was presented at the AHA’s Lifestyle Scientific Sessions meeting in Chicago.