Cancer is the leading cause of death globally [1]. Each year, approximate 20 million new cancer cases occur worldwide [2], [3]; of these, 30-50% can be prevented by addressing risk factors or adopting healthy lifestyles (e.g., doing sufficient physical activity) [1], [4], [5].

Metabolic syndrome (MetS) is a cluster of metabolic abnormalities, including visceral obesity, hypertension, dyslipidemia, and hyperglycemia [6]. MetS can induce chronic inflammation with high serum concentrations of pro-inflammatory cytokines (e.g., tumor necrosis factor-α [TNF-α] and monocyte chemoattractant protein-1) [7], which could cause tumor cell proliferation and migration [8]. A prior meta-analysis has shown that MetS was associated with a high risk of incident cancer [9]. However, most of the evidence regarding the impact of MetS on the development of new cancer has only determined the associations of baseline MetS in individuals with the outcome [9]. Given that the MetS status is dynamic [10], the impact of MetS on incident cancer could be altered by changes in MetS status. To the best of our knowledge, only four studies have explored the effect of changes in MetS status on incident cancer [11], [12], [13], [14]. Two previous studies using the Korean National Health Insurance Dataset have found that the risks of pancreatic [12] and laryngeal cancer [13] were higher in the MetS-persisted, MetS-developed, and MetS-recovered groups than those in MetS-free individuals. However, a Korean study using the National Health Insurance Database has revealed that MetS-persisted individuals had a significantly higher risk of cholangiocarcinoma than the MetS-free group, and this high incidence was not observed in MetS-developed and MetS-recovered groups [14]. Another study of 930,055 postmenopausal female participants from the Korean National Health Insurance Database found that, compared with the sustained non-MetS group, those with persistent or newly developed MetS had a significantly higher risk of breast cancer while those who recovered from MetS did not [11]. However, previous studies examining the association between MetS changes and cancer may have misclassified cancer diagnoses by relying on administrative claims data [11], [12], [13], [14] and inadequately controlled for potential confounders such as physical activities [11], [12], [13], [14]. Previous studies have shown that increasing physical activity lowered the level of estrone, estradiol, and insulin resistance [15], [16], [17], which could reduce the risk of cancer development [5].

Understanding the impact of MetS changes on the risk of incident cancer would provide important information for the implementation of future cancer prevention programs. Therefore, we conducted a large cohort study to determine the effects of dynamic changes in MetS on the risk of incident cancer in Taiwanese adults.