Depression is one of the most common illnesses worldwide, affecting approximately 280 million individuals. This includes 5 % of adults, and the prevalence increases further to 5.7 % in those aged >60 years. Depression not only harms health and causes a loss of function but also leads to fatal consequences, such as suicide, in severe cases (World Health Organization (WHO), 2020). Most cases of depression are idiopathic, and the causes have not been entirely identified. In addition to psychological stressors and maternal depression (Hammen, 2018), traumatic experiences, low-quality diet, overweight and obesity, smoking, lack of physical activity, and loss of sleep are risk factors for depression (Berk et al., 2013).

Depression may precede cardiovascular disease (CVD), and vice versa. CVD and depression have common risk factors, such as inflammation, hypercoagulability, rhythm disturbances, and neurohormonal activation, which can cause both pathological conditions (Joynt et al., 2003). Therefore, it may be natural to observe harmful associations between cardiovascular risk factors, such as overweight or obesity (Luppino et al., 2010), and abdominal obesity (Xu et al., 2011), and the risk of depression.

Metabolic syndrome (MetS) is an assemblage of various CVD risk factors, including central obesity, high triglyceride status, reduced high-density lipoprotein (HDL) cholesterol, elevated blood pressure, and hyperglycemia (Grundy et al., 2005). Although various international criteria for MetS differ slightly, a combination of components is commonly approved (Amihăesei and Chelaru, 2014). In the past 30 years, the prevalence of MetS has rapidly increased from 12 % to 37 % in Asia and from 12 % to 26 % in Europe (Engin, 2017; Sigit et al., 2020). The prevalence of MetS in the United States increased from 33 % in 2011 to 37 % in 2016, although the increase was not statistically significant. The prevalence of MetS tends to increase as age increases, with the prevalence among those aged 20–39 years being 20 %, whereas the prevalence among those aged ≥60 years being as high as 49 % (Hirode and Wong, 2020).

Several meta-analyses demonstrating the association between depression and MetS have been recently performed (Vancampfort et al., 2014; Mordi et al., 2021). However, these meta-analyses examined people who reported depressive symptoms or major depressive disorder prior to MetS. It is important to consider whether MetS is a risk factor for depression in people who did not have depressive symptoms at baseline. A meta-analysis by Pan et al. (2012) investigated the association between MetS and depression in 12 cross-sectional studies. The adjusted odds ratio with depression as the outcome was 1.27 (95 % confidence interval [CI]: 1.07–1.57). However, this meta-analysis included only one study conducted in Asia and more recent studies in this region have been described in the literature and should be considered. Additionally, MetS is composed of five components and it is plausible that individual components defining MetS are more strongly associated with risk of depressive symptoms. Thus, the current study aimed to investigate the association between MetS and between the component of MetS and the risk of depression or depressive symptoms by conducting a thorough systematic review and meta-analysis of cohort studies in the literature.