A major new study of 20,000 adults could rock the scientific consensus on healthy diets, finding that time-restricted eating – a form of intermittent fasting – could significantly increase the chances of death.
The new research – presented to the American Heart Association – claims restricting eating to a period of fewer than eight hours per day had a staggering 91 per cent higher risk of death due to cardiovascular disease.
The study, which evaluated participants' diets and health outcomes over a maximum of 17 years, failed to identify a single cause of death for which the risk was reduced by restricting eating to an eight-hour window.
Senior study author Prof Victor Wenze Zhong was surprised by the results, telling BBC Science Focus he "had expected that long-term adoption of eight-hour time-restricted eating would be associated with lower risk of cardiovascular death and even all-cause death".
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He added: “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 12–16 hours per day, a shorter eating duration was not associated with living longer."
Despite the compelling statistics, scientists are unsure exactly why time-restricted eating could heighten the risk of cardiovascular death. However, one possible explanation, Zhong said, could be that restricting eating reduces muscle mass.
"We did observe that people who restricted eating to a period less than eight hours per day had less lean muscle mass compared with those with a typical eating duration of 12-16 hours. Loss of lean body mass has been linked to a higher risk of cardiovascular mortality," he said.
In recent years, time-restricted eating has garnered attention for its potential health benefits. Previous studies have suggested, for example, that it could aid weight loss and facilitate falls in blood sugar and cholesterol levels.
Many people applying time-restricted eating adopt a so-called '16:8 schedule', where all food consumption occurs in an eight-hour window, with the remaining 16 constituting a fasting period.
It's thought that this pattern of eating is better aligned with our natural sleep cycle, or circadian rhythm, which promotes better sleep and metabolic function. But even if the theory holds for short-term health, today’s announcement could cast serious doubt over its long-term efficacy.
“This study suggests that time-restricted eating may have short-term benefits but long-term adverse effects,” said Dr Christopher Gardner, Professor of Medicine at Stanford University.
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But Gardner emphasised this research has not yet been peer-reviewed, and details of exactly what foods participants ate have not yet been revealed.
“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,” he said.
Among the study population, of which the average age was 49, a total of 2797 deaths were recorded, 840 of which were from cardiovascular causes.
“It’s crucial for patients, particularly those with existing heart conditions or cancer, to be aware of the association between an eight-hour eating window and increased risk of cardiovascular death,” Zhong said.
"Practising intermittent fasting for a short period such as three months or six months will likely lead to benefits in reducing weight and improving cardiometabolic health based on what we know so far. However, practising intermittent fasting, particularly eight-hour time-restricted eating, for a long time such as years requires extreme caution based on the findings from our study.”
Echoing Gardner's reservations, Zhong highlighted the need for caution before interpreting the results. He pointed out that it's too early to give a specific recommendation on time-restricted eating based on one study alone. Plus, finding an association between time-restricted eating and cardiovascular death doesn't imply that diet causes death.
"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," Zhong said.
About our experts:
Dr Victor Wenze Zhong is professor and chair of the Department of Epidemiology and Biostatistics at the Shanghai Jiao Tong University School of Medicine. He serves as the Chair of the American Society for Nutrition Nutritional Epidemiology Research Interest Section, and on the American Heart Association Council on Epidemiology and Prevention Early Career Committee.
Dr Christopher Gardner is the Rehnborg Farquhar Professor of Medicine at Stanford University. For the past 20 years, most of his research has been focused on investigating the potential health benefits of various dietary components or food patterns using randomized controlled trials. He served as a member of the American Heart Foundation’s (AHA's) Nutrition Committee from 2009 to 2013, and in 2020 he was appointed as a member of the AHA Lifestyle & Metabolic Health Council.
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