Although fasting – voluntary abstinence from food for a determined period – is traditionally associated with religious purposes, it has become popular today as a nutritional strategy, and as a way to reduce calorie intake.
In a commentary in The BMJ, Mauricio Medina Rodríguez, Angélica María Muñoz and Katherine Franco Hoyos – from CES University, Columbia – discuss an analysis, published in the same issue of The BMJ, looking at various forms of fasting.
They suggest that positioning alternate day fasting as an additional option offers the best results, particularly for obese individuals.
They write:
Fasting is frequently an alternative in weight-loss attempts, especially given the difficulties many individuals face in adhering to diets structured under strict parameters, they wrote of the analysis findings by a global team of scientists, including from the US, Germany, Canada and Croatia.
This study shows adherence to a 30% caloric restriction is maintained during the first three months of the intervention, however, it progressively declines, reaching only a 9.5% restriction after 12 months, which compromises the sustainability of the clinical effects.
In this context, intermittent fasting has emerged as a popular dietary intervention, in which popularity is spread mainly through social media, where its potential benefits for weight loss are emphasised.
Yet questions remain about its long term efficacy, feasibility in terms of adherence, and effects on cardiometabolic variables.
A scarcity of rigorous comparative syntheses mean that uncertainty exists around whether any forms of intermittent fasting are superior, inferior, or equivalent to continuous caloric restriction in terms of clinical efficacy.
To address this gap, a linked network meta-analysis by Semnani-Azad and colleagues provides relevant evidence by synthesising findings from 99 randomised clinical trials that compared continuous energy restriction and ad libitum diets with any of the three main modalities of intermittent fasting: alternate day fasting, time restricted eating, and whole day fasting.
The results show that all strategies produced significant weight reductions compared with ad libitum diets.
Alternate day fasting was the only intervention with additional reductions in body weight (−1.29kg), body mass index, and certain lipid parameters compared with continuous energy restriction, although with small effects and moderate certainty according to GRADE.
Importantly, these differences did not reach the prespecified clinical relevance threshold of at least 2kg defined for individuals with obesity.
Nonetheless, randomised trials have shown that alternate day fasting can induce more substantial weight losses (around 4kg-6kg in 8-12 weeks), accompanied by reductions in visceral fat and cardiometabolic improvements, particularly in obese adults or those with metabolic dysfunction associated steatotic liver disease (previously known as non-alcoholic fatty liver disease).
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.
Pursuit of an ideal diet applicable to the entire population is a reductionist approach that overlooks the necessity of personalised interventions. The choice of dietary scheme should consider medical history, food preferences, psychosocial context, and the feasibility of sustained adherence.
The population included in Semnani-Azad and colleagues’ analysis encompassed adults with overweight, obesity, type 1 and 2 diabetes, metabolic syndrome, and metabolic dysfunction-associated steatotic liver disease, resulting in a wide clinical scope of intermittent fasting.
This strategy gains further relevance considering that, according to the WHO in 2022, 2.5bn adults – and 43% of the global adult population – were overweight, and about 890m (16%) obese.
A relevant methodological limitation is that many comparisons, especially those including alternate day fasting, were conducted against ad libitum diets, which, although without explicit energy restriction, may include general nutritional recommendations.
Within this framework, 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.
Diet quality during free eating days could affect alternate day fasting outcomes, however, this association has not been systematically evaluated in clinical trials.
Studies specifically designed to isolate this component and understand its impact on metabolic outcomes are required.
Likewise, studies shorter than 24 weeks reported adherence above 80%, while trials with follow-ups longer than 52 weeks showed a marked decline in adherence, especially in the whole day fasting group, with levels below 22% after one year.
In this regard, the clinical goal should not focus solely on weight loss or punctual metabolic improvements but on fostering sustainable changes over time.
Intermittent fasting does not aim to replace other dietary strategies but to integrate and complement them within a comprehensive, patient centred nutritional care model.
Mauricio Medina Rodríguez (researcher); Professor /researcher Angélica María Muñoz; Professor/researcher Katherine Franco Hoyos – all from Faculty of Nutrition and Food Sciences, CES University, Medellín, Colombia.
Study details
ntermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials
Zhila Semnani-Azad, Tauseef A Khan, Laura Chiavaroli, et al.
Published in The BMJ on 18 June 2025
Abstract
Objective
To assess the effect of intermittent fasting diets, with continuous energy restriction or unrestricted (ad-libitum) diets on intermediate cardiometabolic outcomes from randomised clinical trials.
Design
Systematic review and network meta-analysis.
Data sources
Medline, Embase, and central databases from inception to 14 November 2024.
Eligibility criteria for selecting studies
Randomised clinical trials comparing the association of intermittent fasting diets (alternate day fasting, time restricted eating, and whole day fasting), continuous energy restriction, and ad-libitum diets were included.
Main outcomes
Outcomes included body weight (primary) and measures of anthropometry, glucose metabolism, lipid profiles, blood pressure, C-reactive protein, and markers of liver disease.
Data synthesis
A network meta-analysis based on a frequentist framework was performed with data expressed as mean difference with 95% confidence intervals (CIs). The certainty of the evidence was assessed using grading of recommendations assessment, development, and evaluation (GRADE).
Results
A total of 99 randomised clinical trials involving 6582 adults of varying health conditions (720 healthy, 5862 existing health conditions) were identified. All intermittent fasting and continuous energy restriction diet strategies reduced body weight when compared with ad-libitum diet. Compared with continuous energy restriction, alternate day fasting was the only form of intermittent fasting diet strategy to show benefit in body weight reduction (mean difference −1.29 kg (95% CI −1.99 to −0.59), moderate certainty of evidence). Additionally, alternate day fasting showed a trivial reduction in body weight compared with both time restricted eating and whole day fasting (mean difference −1.69 kg (−2.49 to −0.88) and −1.05 kg (−1.90 to −0.19), respectively, both with moderate certainty of evidence). Estimates were similar among trials with less than 24 weeks follow-up (n=76); however, moderate-to-long-term trials (≥24 weeks, n=17) only showed benefits in weight reduction in diet strategies compared with ad-libitum. Furthermore, in comparisons between intermittent fasting strategies, alternate day fasting lowered total cholesterol, triglycerides, and non-high density lipoprotein compared with time restricted eating. Compared with whole day fasting, however, time restricted eating resulted in a small increase in total cholesterol, low density lipoprotein cholesterol, and non-high density lipoprotein cholesterol. No differences were noted between intermittent fasting, continuous energy restriction, and ad-libitum diets for HbA1c and high density lipoprotein.
Conclusions
Minor differences were noted between some intermittent fasting diets and continuous energy restriction, with some benefit of weight loss with alternate day fasting in shorter duration trials. 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 BMJ article – Intermittent fasting as a nutritional tool (Open access)
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