Fasting regimes deliver results no better than conventional dieting approaches, according to a major Cochrane review examining 22 studies from around the world.
The 5:2 diet, which became hugely popular thanks to the late Michael Mosley, was among the methods analysed.
People following fasting plans shed roughly three per cent of their body weight on average.
That's barely more effective than not dieting at all, and falls well short of the five per cent threshold that doctors consider clinically meaningful for health improvements.
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Intermittent fasting may be barely more effective than not dieting at all
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The review drew on gold-standard analysis techniques, looking at randomised clinical trials involving nearly 2,000 adults across Europe, North America, China, Australia and South America.
Researchers examined several fasting approaches, from alternate-day fasting to the 5:2 method and time-restricted eating windows.
Dr Luis Garegnani, who led the study as director of the Cochrane Associate Centre at the Italian hospital of Buenos Aires, was clear about what the evidence shows.
"Intermittent fasting is not a miracle solution, but it can be one option among several for weight management," he said. "It doesn't appear clearly better, but it's not worse either".
Professor Maik Pietzner, who specialises in health data modelling at the Berlin Institute of Health at Charité, admitted he was taken aback by just how little weight people lost compared to those who did nothing at all.
However, he noted that the results align with our understanding of fasting, which can lead to reduced physical activity, and the challenges of losing weight without medication.
In one study, participants drank only water for a full week, yet significant changes to blood proteins didn't kick in until after three days of fasting.
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Professor Pietzner offered a balanced take on the whole debate.
"If people feel better on such diet regimens, I wouldn't stop them, but this work, along with others in the field, clearly shows that there's no robust evidence for positive effects beyond a possible moderate weight loss," he said.
The body clock is closely linked to metabolism
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Dr Zhila Semnani-Azad from the National University of Singapore pointed out that timing could matter, given how closely our body clocks are linked to metabolism.
One notable gap in the research was that none of the 22 trials actually asked participants whether they were happy with their fasting experience.
The studies were also all short-term, tracking results over 12 months at most.
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