RATIONALE: Weight loss remains the primary strategy for reducing health risks and societal consequences associated with overweight and obesity. The most common dietary interventions are calorie-restricted diets, including various permutations of energy restriction, macronutrients, foods, and dietary intake patterns, which achieve initial but often unsustained weight loss. Intermittent fasting involves eating patterns during which individuals take little or no energy for extended time periods, alternated with periods of normal food intake. The mechanism for weight loss is related to caloric restriction, increased fat metabolism, enhanced insulin sensitivity, and improved glucose metabolism. Intermittent fasting has been publicised in blogs and news articles but studies show inconsistent effects on health, highlighting the uncertainty faced by physicians and people with overweight or obesity when considering intermittent fasting as a feasible approach for sustained weight loss.
OBJECTIVES: To evaluate the benefits and harms of intermittent fasting versus regular dietary advice, no intervention or waiting list for adults with overweight or obesity.
SEARCH METHODS: We searched CENTRAL, MEDLINE (Ovid), and two trials registers up to 5 November 2024, as well as reference checking, citation searching and contact with study authors to identify additional studies.
ELIGIBILITY CRITERIA: We included randomised controlled trials (RCTs) and cluster-RCTs that compared intermittent fasting (including time-restricted feeding, periodic fasting, alternate-day fasting, and modified alternate-day fasting) with regular dietary advice, no intervention or waiting list in men and women with overweight or obesity, with or without associated comorbid conditions. The minimum duration of the intervention was four weeks, and the minimum duration of follow-up was six months. We excluded cross-over and quasi-RCTs.
OUTCOMES: Our outcomes were weight loss, quality of life, participant satisfaction, diabetes status, and adverse events. We considered outcomes measured up to and including 12 months after randomisation as short-term, and longer than 12 months as long-term.
RISK OF BIAS: We used the Cochrane risk of bias tool (RoB 2) and the RoB2 extension for cluster-RCTs.
SYNTHESIS METHODS: We synthesised results for each outcome using meta-analysis where possible, using random-effects models to calculate risk ratios (RR) and 95% confidence intervals (CI) for dichotomous outcomes, and mean differences (MD) or standardised mean differences (SMD) for continuous outcomes. Where this was not possible due to the nature of the data, we would have synthesised results using narrative synthesis, including the summary of effect estimates. We used GRADE to assess the certainty of evidence for each outcome.
INCLUDED STUDIES: We included 22 studies with 1995 participants. All studies were conducted in an outpatient setting in North America, Australia, China, Denmark, Germany, Norway, and Brazil and were published between 2016 and 2024.
SYNTHESIS OF RESULTS: Compared to regular dietary advice, intermittent fasting may result in little to no difference in percentage from baseline weight loss (MD -0.33, 95% CI -0.92 to 0.26; 21 studies, 1430 participants; low-certainty evidence due to risk of bias). Intermittent fasting may have little to no effect on achieving a 5% reduction in body weight, but the evidence is very uncertain (RR 0.98, 95% CI 0.82 to 1.18; 4 studies, 472 participants; very low-certainty evidence due to risk of bias and imprecision). Intermittent fasting may result in little to no difference in quality of life (SMD 0.11, 95% CI -0.27 to 0.49; 3 studies, 106 participants; low-certainty evidence due to risk of bias and imprecision). Intermittent fasting may have little to no effect on adverse events but the evidence is very uncertain (RR 1.45, 95% CI 0.64 to 3.28; 7 studies, 619 participants; very low-certainty evidence due to risk of bias, inconsistency and imprecision). Compared to no intervention or waiting list, intermittent fasting likely results in little to no difference in percentage weight loss from baseline (MD -3.42, 95% CI -4.95 to -1.90; 6 studies, 427 participants; moderate-certainty evidence due to risk of bias). Intermittent fasting may result in little to no difference in quality of life, but the evidence is very uncertain (MD 3.49, 95% CI -49.35 to 56.33, 1 study, 60 participants; very low-certainty evidence due to extreme concerns about imprecision). Intermittent fasting may result in little to no difference in adverse events, but the evidence is very uncertain (RR 1.84, 95% CI 0.88 to 3.85; 2 studies, 189 participants; very low-certainty evidence due to risk of bias and imprecision). None of the included studies reported participant satisfaction, diabetes status or overall measure of comorbidity.
AUTHORS' CONCLUSIONS: Compared to regular dietary advice, intermittent fasting may result in little to no difference in weight loss or quality of life. Intermittent fasting may result in little to no difference in adverse events, but the evidence is very uncertain. These approaches did not differ in achieving weight loss, producing no clinically meaningful changes in most of the outcomes considered in this review. Compared to no intervention or waiting list, intermittent fasting likely results in little to no difference in weight loss and may result in little to no difference in quality of life or adverse events, but the evidence is very uncertain. Physicians and patients may need to evaluate willingness and readiness to implement intermittent fasting as a treatment strategy, based on individual practicality and sustainability. The included studies focused on short-term effects of the intervention (up to 12 months), limiting the applicability of the evidence in this review to inform decision-making for longer durations. It would be beneficial for future studies to extend follow-up periods beyond 12 months to build a stronger evidence base for the long-term effects. Further research is needed to address the effect of intermittent fasting on several outcomes, including participant satisfaction, diabetes status and overall measures of comorbidities. These studies must consider different populations where obesity and overweight have different burdens, like those from low- and middle-income countries and high-income countries, men or women separately, and different body mass index categories.
FUNDING: This Cochrane review had no dedicated funding.
REGISTRATION: Protocol (2023): doi.org/10.1002/14651858.CD015610.
