The enzyme 5β-reductase (AKR1D1) catalyses an essential step in bile acid synthesis, but in addition, controls intra-cellular steroid hormone availability by generating inactive 5β-reduced dihydrosteroid metabolites. As disturbances in steroid hormone and bile acid metabolism have potent effects on metabolic health and lipid homeostasis, we hypothesize that AKR1D1 may play a role hepatic lipid accumulation and contribute to the development of metabolic disease. We generated global AKR1D1 knockout mice (KO, n=18) alongside littermate wildtype controls (WT, n=18). Mice were fed either normal chow (NC, n=9) or the American lifestyle induced obesity syndrome diet (ALIOS; 45% fat, 55% fructose:45% glucose in H2O, n=9), which replicates the clinical features of non-alcoholic fatty liver disease (NAFLD), for 52 weeks. Data (mean±SEM) were assessed using Students t-test. Whilst weight gain was similar in WT normal chow and ALIOS treated animals, in WT mice, the ALIOS diet induced liver inflammation and fibrosis in comparison with NC. AKR1D1 KO mice fed the ALIOS diet had increased hepatic steatosis in comparison with WT mice (WT: 16.7±3.3, KO: 21.7±3.6mg/g, P<0.005). In addition, there was evidence of increased hepatic inflammation on H&E stained sections in male AKR1D1 KO mice on a NC, but not ALIOS diet (1.6 vs. 1.1, P<0.01). However, at 37 weeks of age, liver biochemistry was significantly elevated in AKR1D1 KO mice fed the ALIOS diet in comparison with WT mice (ALT; WT: 140.7±51.9, KO: 404.7±171.4U/l, P<0.05. AST; WT: 136.7±39.0, KO: 360.7±121.7U/l, P<0.05). Endorsing observations in our rodent model, AKR1D1 knockdown experiments in human hepatoma cells (Huh7 and HepG2) increased mRNA expression and secretion of the pro-inflammatory cytokines IL1β, IL-6 and IL-8 as well as inducible nitric oxide synthase (iNOS). Hepatic inflammation is a key driver of fibrosis. AKR1D1 KO mice fed the ALIOS diet for 52 weeks had increased hepatic fibrosis as quantified by sirius red staining (WT: 5.4±2.6%, KO: 10.0±4.9%, P<0.01). Furthermore, it is well-established that advanced fibrotic metabolic liver disease increases the risk for the development of hepatocellular carcinoma (HCC) and AKR1D1 mice were more prone to tumour development in comparison with WT mice (WT: 11.5%, KO: 42.1%, P<0.05). Deletion of AKR1D1 in combination with a dietary stress evokes increased hepatic triacylglycerol content and fibrosis, which could exacerbate the progression of NAFLD to NASH and potentially fuel the development HCC. These data suggest that AKR1D1 may have a pivotal role in the regulation of the NAFLD progression.