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Does intermittent fasting live up to the hype?

Charli Shield
December 10, 2019

Hailed as a health hack, intermittent fasting is widely promoted as a reliable way to lose weight, lower disease risk and increase lifespan. But despite some positive study results, does it really deserve so much kudos?

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Image: Imago Images/photothek

Almost everyone, it seems, is intermittently fasting. Celebrities, my boss, my friend's boyfriend, my mother. 

Twitter's CEO, Jack Dorsey, only eats once a day. He doesn't have breakfast or lunch, but consumes his sole meal between 6:30 p.m. and 8:30 p.m.

While Dorsey's approach is extreme (and not recommended), intermittent fasting is widely promoted, with a plethora of book titles that promise it'll "supercharge energy," "optimize weight," "activate stem cells" and "slow aging." There's also intermittent fasting apps that track the fasting window and weight loss. It is, after all, part of a booming global diet industry worth $189 billion (€175,45 billion). 

While fasting has long been a cleansing ritual for many cultures and religions (albeit more so in a spiritual sense), its arrival on the modern wellness scene raises some serious health questions, not least about why we should follow a diet many proponents say is good for us because it's based on what our primitive ancestors did. Foremostly, what exactly is intermittent fasting, and why do it?

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What's intermittent fasting?

An eating pattern that restricts calorie intake to certain hours of the day or days of the week, intermittent fasting has several different iterations. 

One of the most common examples, popularized by science journalist Michael Mosley, is the 5:2 diet. You eat unrestricted for five days of the week and limit calorie intake on the other two days of the week, usually to around 500 kcal. Mosley says it reversed his type 2 diabetes.

Time restricted eating, on the other hand, limits intake to a block of time in the day (usually between eight and 10 hours), and alternate day fasting, as the name suggests, limits calorie intake every other day. There are also more extreme versions, including the so-called "warrior diet," which alternates between 20 hours of undereating and four hours of unlimited eating. But because no health professional would recommend doing that, it hasn't been studied. 

The main idea behind intermittent fasting is to give the body's insulin levels enough time to go down between meals so our fat cells can then release their stored sugar to be used as energy. Positive buzz about the diet spiked after a series of successful studies on rats, which lost weight, improved their blood pressure, cholesterol and blood sugars, and even reduced their risk of type 2 diabetes and cancer. 

But unlike lab-bound rats, it's much more difficult for humans to follow rigid dietary rules and, in part, that's made it difficult to carry out long term studies on humans.

What's the latest?

One of the newest studies on the topic, published in Cell Metabolism, found restricting eating to a 10-hour window during the day had positive health benefits for overweight people with high cholesterol levels, elevated blood sugar and high blood pressure. After three months, the participants — who were not asked to change what they ate — lost around 3% of body fat and 4% of abdominal visceral fat.  

This could suggest a 10:14 circadian rhythm-fasting approach (eating between 8 a.m. to 6 p.m., for example) is effective for people with metabolic syndrome, except that the study was only carried out with 19 participants over three months. 

That's simply "too small and too short," according to Tilman Kühn, a nutritional epidemiologist from the German Cancer Research Center. The study was also — crucially — missing a comparison to a conventional calorie reduction diet, Kühn said.

"My interpretation is that people just benefitted from professional dietary support," Kühn told DW. 

Satchidananda Panda, a professor at the Salk Institute and co-author of the study, said while "more research is needed" to establish whether time restricted eating or intermittent fasting can really help reverse the issues created by an unhealthy diet, he added they have found that "people sleep well and feel more rested in the morning" while on a 10:14 diet. Panda is also an author of one of those book titles, the subheading of which reads: "lose weight, supercharge your energy and transform your health from morning to midnight."

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No better than conventional dieting

Previous research has found intermittent fasting to be no more effective than conventional calorie reduction diets for weight loss or improved insulin levels.

A recent study co-authored by Kühn, carried out over a 50-week period, compared a 5:2 intermittent fasting diet with a continuous calorie reduction diet, whereby participants reduced their daily calorie intake by 20%. A third control group didn't change their diet.

Researchers found that both intermittent fasting and calorie reduction led to weight and fat loss in comparison to the control group. Some health markers, like insulin levels and lipids, also improved. But these results were almost identical for both diets.

"We found no evidence at all for a stronger effect or greater benefit of intermittent fasting," Kühn told DW. "It was equally as effective compared to a traditional moderate daily calorie reduction diet."

While Kühn says he "would never trust just one study," two other comparative, year-long studies from Norway and Australia conducted around the same time found exactly the same result — 5:2 intermittent fasting is no more effective than calorie reduction.

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Tough to follow

Panda, though, says this is one of the main appeals of the 10:14 fast, as he thinks it is much easier to follow than other forms of intermittent fasting or conventional dieting. 

"Even if two methods lead to similar outcomes, why would you prefer something that is difficult to follow?" Panda said. 

The issue with the claims made by small-scale studies and the intermittent fasting book titles, though, is that, like many fad diets, it's presented as a miracle solution. When in actual fact, the science is not so clear.

"If someone finds that intermittent fasting is working, then I would say it's a good method for them. But it's not a universal solution to any health problem for everyone," Kühn said.

In light of the evidence we have so far, he said the scientific community may be better off looking to policy measures (like the sugar tax in the UK) as a way of preventing obesity and unhealthy eating habits, as opposed to appealing to individuals to change their eating behavior.