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Objectives This study aimed to determine whether certain lifestyle factors, specifically alcohol consumption, smoking, physical activity, sleep duration and sleep quality, are associated with an increased risk of metabolic syndrome (MetS) and to assess how changes in lifestyle behaviours over time influence MetS prevalence in the middle-aged Korean population.Design Community-based, prospective cohort study.Setting South Korea, baseline in 2017–2019 and follow-up in 2020–2022.Participants The study included 1436 adult individuals aged ≥30–59 years.Outcome measures The primary outcome of this study was the prevalence of MetS at follow-up, determined according to established diagnostic criteria. Secondary outcomes included the association between MetS and five key lifestyle factors: alcohol consumption, smoking, physical activity, sleep quality and sleep duration. Lifestyle changes were categorised into four groups based on stability between baseline and follow-up assessments. Outcome measures remained consistent with the study’s initial protocol, with no modifications to the planned variables or measured criteria.Results The prevalence of MetS increased from 15.3% at baseline to 19.6% at follow-up. At follow-up, insufficient physical activity (OR=1.42, 95% CI 1.06 to 1.92) and poor sleep quality (OR=1.37, 95% CI 1.02 to 1.84) were significantly associated with MetS. Among lifestyle change patterns, long-term unhealthy physical activity (OR=1.57, 95% CI 1.03 to 2.39) and sleep quality (OR=1.50, 95% CI 1.01 to 2.24) behaviours were associated with an increased risk of MetS. Poisson regression analyses of incident cases showed that only unfavourable changes in sleep duration were significantly associated with incident MetS (rate ratios=1.74, 95% CI 1.02 to 2.95). However, no significant associations were observed for other lifestyle change patterns, including behaviours that improved or worsened over time, and no meaningful associations were identified for non-significant lifestyle factors, such as smoking and alcohol consumption.Conclusions The prevalence of MetS was associated not only with individual lifestyle factors but also with long-term patterns of unhealthy behaviours, with persistent shortage of physical activity, poor sleep quality and inadequate sleep duration identified as significant contributors to increased MetS risk. Future research should explore a broader range of risk factors over longer follow-up periods to better understand the long-term impact of lifestyle modifications on MetS development.