Intermittent fasting has become one of the most popular diet trends of recent years, but researchers say proving that it works is harder than many enthusiasts suggest.
The approach involves restricting eating to certain hours of the day or sharply limiting calories on particular days of the week.
Supporters say it can help with weight loss, blood sugar control and other markers of health.
However, like any lifestyle intervention, it is notoriously difficult to study, because people vary widely in what they eat, how they sleep and how closely they stick to a plan.
“For some people it is a helpful structure, but it is not the metabolic miracle it is sometimes made out to be.”
Some trials have found that fasting produces similar results to simply cutting calories, raising questions about whether meal timing itself offers extra benefits.
Scientists say new technologies, such as continuous glucose monitors and wearable devices, could help build a clearer picture by tracking people's responses in detail.
Interest in fasting has been fuelled by books, social media and celebrity endorsements, which some scientists say have run ahead of the evidence. They argue that clearer public messaging is needed about what research has and has not shown.
Experts caution that fasting may not suit everyone, including people with certain medical conditions, pregnant women and those with a history of eating disorders.
Many dietitians say the best eating pattern is simply one that people can maintain over the long term.
Researchers hope larger and longer studies will help settle some of the outstanding questions.
