Intermittent fasting is no better for shedding the pounds than conventional diets and is barely more effective than doing nothing, according to a major review of the scientific evidence.
Researchers analysed data from 22 global studies and found people who are overweight or living with obesity lost as much weight by following traditional dietary advice as when they tried fasting regimes such as the 5:2 diet popularised by the late Michael Mosley.
The approach was hardly better for weight loss than not dieting at all, the review adds, with people losing only about 3% of their body weight through fasting, far below the 5% that doctors consider clinically meaningful. The studies were all short term, looking at improvements over 12 months at most.
“Intermittent fasting is not a miracle solution, but it can be one option among several for weight management,” said Dr Luis Garegnani, the lead author and director of the Cochrane Associate Centre at the Italian hospital of Buenos Aires in Argentina. “Intermittent fasting likely yields results similar to traditional dietary approaches for weight loss. It doesn’t appear clearly better, but it’s not worse either.”
Intermittent fasting, where people restrict their eating to set hours, or fast on certain days, has soared in popularity amid claims it can help people lose weight, boost their physical and cognitive health and even slow ageing.
The Cochrane review used gold-standard techniques to analyse evidence from randomised clinical trials involving 1,995 adults across Europe, North America, China, Australia and South America. The trials examined different kinds of intermittent fasting, such as fasting every other day, the 5:2 diet where people fast for two days a week, and time-restricted eating.
Beyond the minimal benefits for weight loss, the researchers found no strong evidence that intermittent fasting improved people’s quality of life more than other diets.
Garegnani said it was important to clarify the evidence around fasting diets given their enormous popularity and widespread promotion in the media. Many of the studies are short-term and of poor quality, making it hard to reach firm conclusions on the possible benefits. Surprisingly, none of the 22 studies asked people how satisfied they were with intermittent fasting, Garegnani added.
Dr Zhila Semnani-Azad at the National University of Singapore, said the benefits of intermittent fasting may be affected by timing, since the body’s circadian rhythms are so deeply connected to metabolism. Studies in animals suggest fasting can change how fat reserves are used, improve insulin sensitivity – which is important for diabetes – and reduce inflammation and oxidative stress. It may also be good for ageing and longevity, she said, by triggering a process called autophagy, the body’s recycling mechanism. One problem is that there is no universal definition of intermittent fasting, making it hard to understand its effects, she added.
Maik Pietzner, a professor of health data modelling at the Berlin Institute of Health at Charité, was surprised that weight loss from fasting was so small compared with doing nothing. But he said the finding fits with evidence that people are less physically active when fasting and that weight loss is hard without drugs.
The findings also line up with his own work that shows short periods of complete fasting, even up to two days, has little effect on our bodies and that people need to fast for much longer to see changes that may drive benefits later on. In one of his studies, people consumed only water for seven days, but widespread changes to proteins in their blood only occurred after three days.
“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,” Pietzner said. “Our bodies have evolved under constant scarcity of food, and can deal really well with prolonged periods without it, but that does not mean that we perform any better once these evolutionary conserved programmes kick in.”
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