June 18, 2025

expert reaction to study comparing evidence on intermittent fasting and traditional calorie reduction diets for weight loss

A study published in The BMJ compares the evidence around intermittent fasting and calorie restriction for weight loss. 

Prof Maik Pietzner, Chair in Health Data Modelling, Queen Mary University of London’s Precision Health University Research Institute; and co-lead of the Computational Medicine Group at Berlin Institute of Health at Charité, said:

“The study is well executed, and results are presented in a balanced way reflecting the results of the analysis.  The press release is also well written and is in line with the evidence in the field, that any strategy reducing calorie intake results in a proportional weight loss, either at each meal (CER) or by skipping meals (intermittent fasting).  The missing additional benefit on cardiovascular risk markers of any intermittent fasting schemes aligns with our study that indicated that much longer periods fasting would be needed to change those.  However, we’ve seen that even those reverse quickly to levels seem before the intervention.

“One point to stress might be the rather moderate level of weight loss achieved with any intervention and the missing long-term follow-up in terms of weight maintenance and reduction in the onset of major diseases.  For example, all dietary regimens, including the different forms of intermittent fasting, are unlikely to be sustainable.  A fact also indicated by the decline in adherence in most studies.

“In brief, eating less leads to weight loss, irrespective on how you do it.  Aspects that are missed but would have been of interest, are any effects on muscle mass, which is a major concern for current pharmacological interventions on obesity.”

Prof Naveed Sattar, Professor of Cardiometabolic Medicine/Honorary Consultant, University of Glasgow, said:

“This meta-analysis of mainly small trials helps to give a general sense of the benefits of intermittent fasting, even if some of the included trials were suboptimal.  Overall, the results do not surprise as there is nothing magical about intermittent fasting for weight loss beyond being another way for people to keep their total calorie intake lower than it would be otherwise – this helps people maintain a lower weight than they would normally.  Hence, it becomes another lifestyle option for weight management.  Whether it is sustainable over the longer term is worth to examine, whereas for those who need to lose much more weight, other options are now clearly available.”

Dr Amanda Avery, Associate Professor in Nutrition and Dietetics, University of Nottingham, said:

“This systematic review has compared the weight loss achieved by people in clinical trials who have undertaken intermittent fasting compared to the more traditional dietary approaches to losing weight which involve a continuous reduction in energy (calorie) intake (CER).  Systematic reviews are considered gold standard in research hierarchy and a meta-analysis allows a deeper understanding of the results – meta-analyses statistically interpret the overall findings for us.  This systematic review involved a novel network meta-analysis which is an advanced statistical technique that enables comparisons of multiple interventions including those that have not been compared in head-to-head trials.

“Given that nearly 100 clinical trials were included in this systematic review, although some with a small number of participants, this research probably provides as good an insight as we are going to find as to whether intermittent fasting (IF) is as effective as traditional dietary approaches to weight management involving a consistent reduction in calories on a daily basis.

“The authors have carefully considered most of the factors that could affect the interpretation of the findings – the first being that there is no definition for what we mean by IF and a number of different approaches to IF such as time restricted eating, alternate day fasting and the 5:2 approach.  The second factor that makes interpreting the findings difficult is that there are different approaches to achieving CER and the support and resources that people are offered to reduce their daily energy intake may affect how successful they are in losing weight and maintaining weight loss.  Compliance to any intervention will make a difference and people are individuals – one approach may work for one person but not for another.

“Some of the studies included in the review had a very short intervention period – that is the time when participants were following the different approaches to losing weight.  The authors did conclude that more emphasis should be put on interventions that have been conducted over a longer period of time.  Perhaps as we may have expected, for the studies that had been conducted for 24 weeks or more, it was found that there was no difference between IF and CER in the weight changes seen – but at least they were both more effective compared to no dietary intervention.

“The pros and cons of IF and CER have been debated for some time now.  This review can hopefully end the debate with the conclusion that if someone choses IF and overall a nutritionally balanced diet is still achieved then it could be used as one of the options to support weight loss with the more traditional dietary approaches still remaining as key strategies – alongside appropriate support.  The majority of the participants in the included studies had higher BMIs and an associated health condition and thus the findings are appropriate for many people who would benefit from weight management.  However I would like to emphasise that IF is not recommended during pregnancy.”

‘Intermittent fasting strategies on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials’ by Zhila Semnani-Azad et al. was published in the BMJ at 23:30 UK time on Wednesday 18 June 2025. 

DOI: 10.1136/bmj-2024-082007

Declared interests

Prof Maik Pietzner: “Professor Pietzner has received funding from industry partners (SomaLogic Inc.) to attend conferences unrelated to this work.  No other conflict of interest.”

Prof Naveed Sattar: “NS has consulted for and/or received speaker honoraria from Abbott Laboratories, AbbVie, Afimmune, Amgen, AstraZeneca, Boehringer Ingelheim, Carmot Therapeutics, Eli Lilly, GlaxoSmithKline, Hanmi Pharmaceuticals, Janssen, Menarini-Ricerche, Merck Sharp & Dohme, Metsera, Novartis, Novo Nordisk, Pfizer, Sanofi, and Roche; and received grant support paid to his University from AstraZeneca, Boehringer Ingelheim, Novartis, and Roche.  No shares in any medical areas.”

Dr Amanda Avery: “Besides my academic position at the University of Nottingham, I also hold a position at Slimming World as Consultant dietitian in the Nutrition, Research & Health Policy team. 

I have no other conflicts of interest to declare.”