Many studies have also shown links between inflammation and depression (Chamberlain et al., 2019; Enache et al., 2019; Hiles et al., 2012; Howren et al., 2009; Lamers et al., 2019; Mikkelsen et al., 2017; Valkanova et al., 2013), which may be linked to vitamin B status since both human and animal studies have shown that B vitamin concentrations are related to immune cell activity and inflammatory mediator concentrations, e.g., C-reactive protein (CRP), interleukin-6 (IL-6) (see Mikkelsen et al., 2017; Ueland et al., 2017). Here, IL6 is linked to major depressive disorder.