Long-Term Effects of Intermittent Fasting on Weight Loss and Body-Weight Maintenance in Adults With Overweight and Obesity: A Systematic Review and Meta-Analysis of Randomized Controlled Trials and Prospective Studies With 12-Month or Longer Follow-Up

Background: Intermittent fasting has gained increasing attention as an alternative to continuous calorie restriction for adults with overweight or obesity. Although short-term weight loss has been widely reported, its long-term effectiveness and ability to maintain weight reduction remain uncertain. This systematic review and meta-analysis evaluated whether intermittent fasting produces sustained weight loss over approximately 12 months or longer and whether it offers an advantage over conventional energy-restriction strategies. Methods: The review followed PRISMA 2020 principles. Twelve bibliographic databases, multidisciplinary platforms, and trial registries were searched from inception through 28 August 2026 without publication-year restriction. Randomized controlled trials and eligible prospective long-term follow-up studies involving adults aged 18 years or older with overweight or obesity were included. Interventions comprised alternate-day fasting, intermittent energy restriction, 5:2 and 4:3 regimens, and time-restricted eating. The primary outcome was change in body weight at approximately 12 months. Random-effects meta-analysis was used, with active calorie-restriction comparators analyzed separately from usual-care and passive controls. Risk of bias was assessed using Cochrane RoB 2. Results: Fifteen studies published between 2012 and 2026 were included, representing diverse fasting strategies, populations, and follow-up periods. Six randomized comparisons contributed directly to the primary comparator-restricted meta-analysis. Intermittent fasting produced a small but statistically significant additional reduction in body weight compared with active calorie restriction, with a pooled mean difference of −1.17 kg (95% CI −2.24 to −0.10; I²=0%). Most individual trials showed no statistically significant superiority of intermittent fasting, while the 4:3 regimen produced the clearest advantage, with approximately 2.9 kg greater weight loss at 12 months. Long-term maintenance studies suggested that weight loss could persist after active intervention, although attrition and adherence were important limitations. Overall methodological quality was characterized mainly by some concerns, particularly missing outcome data and deviations from intended dietary regimens. Conclusion: Intermittent fasting is a valid long-term dietary option for adults with overweight or obesity and may provide a modest additional weight-loss benefit compared with continuous calorie restriction. However, current evidence does not support universal superiority. Its clinical value likely depends on fasting strategy, adherence, intervention intensity, patient preference, and long-term sustainability in routine clinical practice settings.