Recently many researchers revealed that Akkermansia muciniphila, the mucin degrading bacterium belonging to the genus Akkermansia, was negatively related to weight gain and obesity in mice and humans (Everard et al. 2013; Santacruz et al. 2010; Everard et al. 2011). Here, MUC5AC is linked to obesity due to melanocortin 4 receptor deficiency.