Intermittent fasting has become a popular phrase in dieting, promising to boost metabolism, shed body fat, and even help reduce the risk of diabetes, hypertension and obesity.
From the 5:2 diet to the eight-hour feeding window, it has become a hot topic for researchers to study and internet personalities to promote.
But intermittent fasting regimes “fail to match hype” for significant weight loss and reducing health risks in people who are overweight, according to a major review of studies by the Cochrane Collaboration.
Review author and physician Eva Madrid, a professor at the University of Valparaiso, said intermittent fasting was no better than following regular dietary advice — or, in fact, doing nothing at all — for losing weight.
“The key takeaway is that intermittent fasting is not a miracle solution, but it can be one option among several for weight management,” she said.What is intermittent fasting?
The phrase is an overarching term for cycling periods of normal eating and drinking with times during which little to nothing is consumed.
Fasting is promoted by some wellbeing personalities for its ability to encourage the body into “metabolic switching”, where the body relies more heavily on fat stores for energy.
Intermittent fasting is not recommended by health experts for women who are pregnant or breastfeeding, people with eating disorders, people with diabetes or children.
What does science have to say about intermittent fasting?
“Given that obesity is a major public health challenge, we wanted to assess, rigorously and independently, whether intermittent fasting actually offers meaningful benefits compared with more conventional dietary advice,” Professor Madrid said.
The types of fasting looked at in the review included the 5:2 diet, popularised by TV presenter Michael Mosley, which encourages participants to eat their usual calories five days a week, but restricts calorie intake to about 500 calories on the other two days.
Other strategies examined were abstaining from food for 16 hours a day in a method called “time-restricted eating” and fasting on alternate days.
How intermittent fasting studies stack up
The Cochrane team analysed evidence from 22 randomised clinical trials involving 1,995 adults across North America, Europe, China, Australia, and South America.
The review compared the effects of fasting strategies for overweight and obese people with traditional dietary advice that emphasises calorie restriction and eating fruit, vegetables, and whole grains, or with no dietary guidance at all.
A new study has found that intermittent fasting has little to no effect on weight loss in overweight and obese participants. (Unsplash)
According to the analysis, intermittent fasting made little to no difference in weight loss and little to no difference in quality of life compared to the other two approaches.
The authors also raised concerns about the study designs, including how people were randomly assigned to groups and adherence to the diets.
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Most studies were also limited to white populations in high-income countries, so results couldn’t be extrapolated for other groups.
In addition, most trials included in the review lasted fewer than 12 months, limiting conclusions about the long-term sustainability of intermittent fasting.
The authors recommended that future research include information about participant satisfaction, diabetes status, and long-term loss of lean body mass (weight loss that does not include body fat).
Weight loss doesn’t equal other health improvements
Leonie Heilbronn, who leads the Obesity and Metabolism Lab at the University of Adelaide, said intermittent fasting often provided similar outcomes to reducing calorie intake.
“When you do either a fasting diet or a daily reduction in calories, both of them result in about the same amount of body weight loss. So no arguments there,” she said.
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But she said intermittent fasting had some additional benefits that surpassed simple calorie restriction.
“Having that more prolonged fasting period … increases fat burning and allows more rest-and-repair-type mechanisms to start repairing the body.”
However, she thought some methods were less effective than others.
“I think time-restricted eating is a milder intervention than intermittent fasting, in terms of weight loss.”
Luigi Fontana, a professor of medicine and nutrition at University of Sydney, said it could be difficult to interpret studies about intermittent fasting, particularly when managing compliance by participants.
Professor Fontana, who studied diets similar to 5:2, said his research showed that people on intermittent fasting did lose some weight, but health risk factors such as blood pressure, glycaemic control and inflammation did not improve.
“Unlike with moderate daily caloric restriction with optimal nutrition, losing weight through intermittent fasting did not automatically translate into broader metabolic benefits,” he said.
Diets alone not answer to long-term benefit
All diets, whether they involve calorie restriction or intermittent fasting, may only deliver short-term results, Professor Heilbronn said.
“They are really hard to maintain long-term, and it’s not just because you don’t have the willpower.”
She said changes in hunger, hormones and energy expenditure could conspire to make you regain weight.
“It’s your body fighting back.”
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Professor Fontana recommended combining dietary interventions with regular exercise for longevity of results.
“Whenever you lose weight with any type of diet, you are lowering your resting metabolic rate and are more prone to regaining weight.
“If you want good results, combine three days per week of resistance exercise like weightlifting and another two or three days a week of endurance.
“Whether that’s running, biking, swimming, cycling — whatever will increase your resting heart rate and your respiratory rate.”
Ultimately, for someone seeking weight loss, it’s about calorie intake and energy output.
“The broader takeaway is that sustainable results come from how consistently a diet can be followed and from the overall nutritional pattern — not from fasting per se,” Professor Fontana said.