Intermittent fasting shows similar benefits to traditional dieting for weight loss, study finds

Intermittent fasting is as effective as traditional calorie-restriction diets for weight loss, according to a new review of evidence from a University of Toronto-led team of researchers.

Published in The BMJ, the study showed that intermittent fasting also improved cardiometabolic factors like blood pressure and cholesterol levels when compared to an unrestricted diet. Among intermittent fasting approaches, alternate day fasting, which involves a 24-hour fast on alternate days, offered greater benefits in the short term, but the researchers caution that longer studies are needed to confirm the results.

“When we started this project, intermittent fasting was all over the news,” says Zhila Semnani-Azad, who completed her PhD in nutritional sciences at U of T’s Temerty Faculty of Medicine and led the work in collaboration with professor John Sievenpiper.

During intermittent fasting, which has gained popularity over the past decade, periods of unrestricted eating are regularly interspersed with periods of fasting. There are three main methods: time restricted eating (for example, a 16-hour fasting period followed by an eight-hour eating window), whole day fasting (for example, five days of unrestricted eating followed by two days of fasting) and alternate day fasting.

“We were interested in looking at whether this particular dietary strategy had any additional benefit compared to the typical reduced-calorie diets that were more common at the time,” says Semnani-Azad, now a postdoctoral fellow at the Harvard T.H. Chan School of Public Health.

To answer this question, the researchers analyzed results from 99 randomised clinical trials involving over 6,500 participants.

They found that intermittent fasting led to similar amounts of weight loss relative to a traditional calorie-restriction diet. People who followed either of these diets lost an average of 1.7 to 2.5 kg more than people who ate without restrictions.

Semnani-Azad notes that their systematic review was unique and comprehensive because it examined both weight loss and cardiometabolic factors and compared different types of intermittent fasting.

While all three fasting approaches were similar in the long term, alternate day fasting was the only method that offered greater weight loss benefits than calorie counting in the short term, with participants losing roughly 1.3 kg more than those following a reduced-calorie diet.

Participants who followed alternate day fasting also saw the greatest number of changes across a range of cardiometabolic risk factors including blood pressure, waist circumference and total cholesterol and triglyceride levels. A reduction in these risk factors can lower a person’s chances of developing serious chronic conditions like type 2 diabetes and cardiovascular disease.  

Semnani-Azad stresses that the findings highlight intermittent fasting as another effective tool for people to consider when thinking about losing weight.

“These different weight loss strategies seem to perform similarly,” she says. “It gives individuals a choice to pick the one that is easiest for their lifestyle and for them to maintain in the long term.”

As GLP-1 drugs like Ozempic become increasingly popular for weight loss, Semnani-Azad hopes that this work will provide evidence-based options for people who want to manage their weight without medications.

"Clinical practice guidelines for obesity have moved away from a 'one size fits all' approach, placing an important emphasis on values, preferences, and treatment goals of the individual,” says Sievenpiper, who is also a member of the Banting & Best Diabetes Centre at U of T.

“This issue is especially important in nutrition where we have a number of approaches with similar evidence of advantages. In showing that intermittent fasting produces similar weight loss and related risk factor reductions as other traditional approaches to calorie-restriction, our findings provide an important opportunity for personalization to achieve the greatest adherence and benefits over the long term.”

This study was funded by the Diabetes and Nutrition Study Group of the European Association of the Study of Diabetes and the Canadian Institutes of Health Research.