Why intermittent fasting isn’t a magic diet trick after all

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Then there was a 2019 paper from the US National Institute on Ageing, featuring results from around 80 studies, which noted that fasting reduced fat stored in the liver as triglycerides (a type of blood fat), with periods of fasting breaking them down and converting them to energy for other tissues in the body to use, especially the brain.

Intermittent fasting has not only been confined to the pursuit of a more svelte figure, however. As obese research participants lost weight, the associated risks – of type 2 diabetes, inflammation and certain lifestyle-induced cancers – were also shown to drop.

All of this may be explained by two factors. The first is that by reducing a window in which a person can eat, they eat less overall, reducing calorie intake by an average of 300-500 daily, according to studies. Then, there are the health benefits of following your circadian rhythm. Limiting the hours in which you eat may mean fewer late nights, for instance, and thus better sleep – something that helps to keep immune function, digestion, cognition and cardiovascular activity in check.

“As a consumption-cutting tool, TRE can work, as caloric restriction will be caloric restriction, whatever time of the day you choose to do it,” says Dr Giles Yeo, principal research associate at the Wellcome-MRC Institute of Metabolic Science at Cambridge University. “But in terms of time restriction itself, universally, as a solution, it probably doesn’t work for everybody.”

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So is it simply a fad worth ditching? This is where things get complex.

“I’m a believer that if you do something and it’s not dangerous, and it actually works for you, then by all means, do it – because the likelihood is, it will work for some people. Just not all,” says Dr Yeo.

The Southern Medical University research is the longest TRE study to date – but Michael Mosley, who runs the popular Fast 800 programme (thefast800.com), which combines intermittent fasting and calorie control, is critical. He claims that those following both TRE and a calorie-controlled diet lost more weight and body fat than those only doing the latter, but that “the study wasn’t big enough for these differences to be considered statistically significant”.

That it was a fairly small trial, and carried out within the Chinese population, whose mealtimes occur over a more compressed timeframe on average – and among volunteers without metabolic issues – may have had an effect on the results, as “you would expect people who do a lot of late-night snacking to get the most benefit from doing TRE”, he adds.

The study did conclude that a combination of low-calorie eating and TRE was “a viable and sustainable approach for obesity management”, possibly because that eating window works as a behavioural tool. “Although the main approach in the Fast 800 is to cut your calories down… we do suggest that people might want to try adding in some time-restricted eating, as they may find this makes doing the diet easier,” says Mosley.

Dr Krista Varady is a professor of nutrition at the University of Illinois Chicago who focuses on the efficacy of intermittent fasting.

“Quite frankly, I’m surprised this study got into such a high-calibre medical journal,” she says, pointing out that it doesn’t have a control group. “It goes to show how popular these diets are and how much people want to know about them.”

She says that “we definitely need more long-term data for TRE”, and that a vital next area for research is to run studies in different population groups, as “we need to find out if these diets can help reverse type 2 diabetes, non-alcoholic fatty liver disease, and help people with thyroid disorders and polycystic ovarian syndrome lose weight and get healthy.”

She is running a year-long study which compares TRE to calorie restriction, as well as to a regular diet control group, to see if the former can “promote long-term adherence because it does not involve calorie counting. I think this is the major question that needs to be answered in this field.”

Tam Fry, of the UK’s National Obesity Forum, is less convinced that TRE is worth further investigation. “I have little time for these dietary fads,” he says. “The way to diet, in our view, is to eat less.” He also warns that following restrictive patterns disturbs “the equilibrium of eating” required for consistent, healthy consumption. “Eat the proper food, eat less, and you will find the weight comes off.”