Age-Specific Analysis of the Effects of Intermittent Fasting on Body Composition and Cardiometabolic Markers in Healthy Adults and Individuals with Overweight or Obesity: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.

<b>Background</b>: Intermittent fasting (IF) is a popular dietary strategy for improving weight and cardiometabolic health. However, its effectiveness and potential risks across different adult age trajectories remain unclear. This systematic review and meta-analysis evaluated the age-specific effects of IF on body composition and cardiometabolic markers. <b>Methods</b>: Following PRISMA 2020 guidelines, PubMed, Scopus, and Web of Science were searched for randomized controlled trials (RCTs) up to September 2025. Participants were stratified into three cohorts: <30 years, 30-44 years, and ≥45 years. Random-effects meta-analyses and leave-one-out sensitivity analyses were conducted on body composition, lipid profiles, glycemic markers, and blood pressure. Additionally, a conservative methodological sensitivity analysis (imputed correlation r = 0.5) and subgroup analyses by fasting modality (TRF vs. intermittent energy restriction) were performed. Risk of bias was assessed using the RoB 2 tool. <b>Results</b>: Analysis of 28 RCTs (N = 1833) demonstrated that IF significantly reduced body weight and BMI across all age groups. Notably, subgroup analyses revealed comparable physiological responses between TRF and intermittent energy restriction modalities. Cardiometabolic adaptations were highly age-dependent. Young adults exhibited significant reductions in fasting insulin and HOMA-IR, alongside a robust reduction in fat mass. However, a significant loss of fat-free mass (FFM) was observed in both young and older cohorts. While middle-aged and older adults experienced the most pronounced improvements in triglycerides, systolic blood pressure, and insulin sensitivity, our conservative sensitivity analysis unmasked a significant elevation in low-density lipoprotein cholesterol (LDL-C) in this group, mirroring the robust LDL-C increase observed in young adults. Early middle-aged adults exhibited highly variable responses with no significant overall improvements in cardiometabolic parameters. <b>Conclusions</b>: IF is an effective weight-management tool, but elicits distinct, age-specific metabolic trajectories. While middle-aged and older adults derive pronounced cardiometabolic benefits, they face critical risks of lean mass depletion, necessitating a combined "IF+" strategy (adequate protein and resistance training). Crucially, the age-specific risk of LDL-C elevation dictates a mandate for vigilant lipid monitoring. Given that the certainty of evidence was rated as low to very low per GRADE criteria, these age-specific patterns should be interpreted as hypothesis-generating, warranting validation in future large-scale trials.