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Intermittent fasting or dieting: Both are about the same for weight loss

Embargoed until: Publicly released:
Peer-reviewed: This work was reviewed and scrutinised by relevant independent experts.

Meta-analysis: This type of study involves using statistics to combine the data from multiple previous studies to give an overall result. The reliability of a meta-analysis depends on both the quality and similarity of the individual studies being grouped together.

Systematic review: This type of study is a structured approach to reviewing all the evidence to answer a specific question. It can include a meta-analysis which is a statistical method of combining the data from multiple studies to get an overall result.

People: This is a study based on research using people.

Intermittent fasting - such as the 16:8 diet involving a 16 hour fasting period followed by an 8 hour eating window - appears to have the same benefits as regular calorie-restricted diets for weight loss, say international researchers, though the former may come with other health benefits. To work this out, the researchers reviewed the results of 99 randomised clinical trials on over 6500 people. They found all forms of intermittent fasting and energy restricting diets to potentially lead to small reductions in body weight, compared to no diet at all, but alternate day fasting was the only form of intermittent fasting that led to a larger weight loss than standard calorie restricted diets. The team also found alternate day fasting to be linked to lower levels of total and 'bad' cholesterol when compared with time restricted eating, though they say more research is needed to draw any solid conclusions on this.

Journal/conference: The BMJ

Research: Paper

Organisation/s: Harvard TH Chan School of Public Health, USA

Funder: The Diabetes and Nutrition Study Group (DNSG) of the European Association of the Study of Diabetes (EASD) commissioned this systematic review and meta-analysis and provided funding and logistical support for meetings as part of the development of the EASD Clinical Practice Guidelines for Nutrition Therapy. This work was also supported by the Canadian Institutes of Health Research (funding reference number, 129920) through the Canada-wide Human Nutrition Trialists’ Network (NTN). The Diet, Digestive tract, and Disease (3-D) Centre, funded through the Canada Foundation for Innovation (CFI) and the Ministry of Research and Innovation’s Ontario Research Fund (ORF), provided the infrastructure for the conduct of this project. The Clinical Practice Guidelines Committee of the DNSG of the EASD had input on all aspects of the work. ZSA is supported by the Canadian Institutes of Health Research (CIHR) Fellowship. VC was supported by the CIHR Canada Graduate Scholarship-Masters (CGS-M) Research Award and a University of Toronto Department of Nutritional Sciences Fellowship. SK is primarily funded by the German Center for Diabetes Research (DZD).

Media release

From: BMJ Group

Intermittent fasting comparable to traditional diets for weight loss

May also offer other health benefits, but longer trials are needed to confirm this

Intermittent fasting diets appear to have similar benefits to traditional calorie-restricted diets for weight loss, suggests an analysis of trial evidence published by The BMJ today.

Alternate day fasting also demonstrates greater benefits compared with both calorie restriction and other intermittent fasting approaches, but the researchers say longer trials are needed to substantiate these findings.

According to the World Health Organization in 2022, approximately 2.5 billion adults, 43% of the global adult population, were overweight, and about 890 million (16%) lived with obesity.

Weight loss can reduce cardiometabolic risk factors, such as high blood pressure, cholesterol and blood sugar levels, and consequently lower the burden of serious chronic conditions like type 2 diabetes and cardiovascular disease.

Intermittent fasting is an eating pattern that cycles between periods of eating and fasting on a regular schedule and is becoming a popular alternative to traditional calorie-restricted diets, which are often unsustainable in the long term.

While no clear definition exists for intermittent fasting, its various methods can fall under three broad categories: time restricted eating (eg, the 16:8 diet involving a 16 hour fasting period followed by an 8 hour eating window), alternate day fasting (a 24 hour fast on alternate days), and whole day fasting (eg, a 5:2 diet involving five days of unrestricted eating and two days of fasting).

But the health effects of intermittent fasting compared with continuous caloric restriction or an unrestricted (ad-libitum) diet remain unclear.

To address this, researchers analysed the results of 99 randomised clinical trials involving 6,582 adults (average age 45; 66% female) to compare the effect of intermittent fasting diets with continuous energy restriction or unrestricted diets on body weight and cardiometabolic risk factors.

Participants had an average body mass index (BMI) of 31 and almost 90% had existing health conditions.

The trials ranged from 3-52 weeks (average 12 weeks) and were of varying quality, but the researchers were able to assess their risk of bias and the certainty of evidence using recognised tools.

All intermittent fasting strategies and continuous energy restriction diets may lead to small reductions in body weight when compared with an unrestricted diet.

Alternate day fasting was the only intermittent fasting diet strategy to show a small benefit in body weight reduction compared with continuous energy restriction (mean difference -1.29 kg).

Alternate day fasting also showed a small reduction in body weight compared with both time restricted eating and whole day fasting (mean difference -1.69 kg and -1.05 kg respectively).

However, these differences did not reach the minimally important clinical threshold of at least 2 kg weight loss for individuals with obesity, as defined by the study authors.

Alternate day fasting was also linked to lower levels of total and “bad” cholesterol compared with time restricted eating. Compared with whole day fasting, however, time restricted eating resulted in a small increase in cholesterol levels. No benefit was found for blood sugar or "good" cholesterol levels in any diet strategy comparison.

Estimates were similar among trials with less than 24 weeks follow-up. But longer trials of 24 weeks or more only showed weight loss benefits in diet strategies compared with an unrestricted diet.

The researchers point to several limitations, such as high variation (heterogeneity) among the diet strategy comparisons, small sample sizes of many included trials, and low to moderate certainty of evidence in most of the investigated outcomes.

Even so, this is highlighted as one of the first systematic reviews to combine direct and indirect comparisons across all dietary strategies, allowing for more precise estimates.

As such, they conclude: “The current evidence provides some indication that intermittent fasting diets have similar benefits to continuous energy restriction for weight loss and cardiometabolic risk factors. Longer duration trials are needed to further substantiate these findings.”

The value of this study is not in establishing a universally superior strategy but in positioning alternate day fasting as an additional option within the therapeutic repertoire, say researchers from Colombia in a linked editorial.

They point out that any structured intervention - including continuous energy restriction - could show benefits derived not only from the dietary pattern but also from professional support, planning, and nutritional education, while diet quality during free eating days could also affect alternate day fasting outcomes.

The focus should be on fostering sustainable changes over time, they say. “Intermittent fasting does not aim to replace other dietary strategies but to integrate and complement them within a comprehensive, patient centred nutritional care model.”

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