AQP9 is hypothesized to be involved in the hepatic synthesis of TAG in NAFLD as indicated by (i) the amelioration of high fat diet-induced NAFLD in rats with knockdown of hepatic AQP9 (Cai et al., 2013), (ii) the increase in hepatocyte AQP9 expression accompanying the development of fatty liver in diet-induced obese (DIO) mice (Hirako et al., 2016), (iii) the impairment of hepatocyte AQP9 and glycerol permeability found in an animal model of NAFLD [leptin-deficient mice (ob/ob mice)] (Gena et al., 2013) and in patients with obesity, insulin-resistance and NAFLD (Rodríguez et al., 2014). Here, AQP9 is linked to obesity due to melanocortin 4 receptor deficiency.